Dong Hn, Jin Zhng,?, Jinfeng Hung, Yong Lin, Gungyu He

a University of Science and Technology, 30 Xueyuan Road, Haidian District, Beijing 100083, PR China

b Science and Technology on Plasma Dynamics Laboratory, Air Force Engineering University, Xi’an, Shanxi 710038, PR China

Abstract Magnesium alloys have become more attractive because of their low density and electromagnetic shielding effectiveness in the aerospace industry.However, some unpredictable situation may lead to the ignition of magnesium alloys.In this review, the thermodynamic conditions and transfer processes of magnesium alloys ignition are analyzed from the point of mechanisms.The criteria of ignition are emphasized.In addition, ignitability and flammabilit test systems are compared.And a more suitable method to assess the potential ignition and flammabilit risks of magnesium alloys in extreme environments is recommended.Furthermore, the ignition characteristics of magnesium alloys are discussed in detail.It was found that the ignition of magnesium alloys is a complex process determined by internal properties such as thermo-physical properties, oxide fil properties, chemical compositions and geometrical parameters, as well as the external environment such as gas species, oxygen concentration and oxygen pressure.Ignition temperature is not physical constants of materials.It is not simply assumed that ignition may occur when the temperature of Mg alloys reaches a certain ignition point.Finally, the unsolved issues in the ignition of magnesium alloys are pointed out and the future investigation are suggested for improving the safety and reliability of magnesium alloys in the aerospace applications.

Keywords: Magnesium alloys; Ignition mechanisms; Ignition characteristics.

1.Introduction

Mg alloys are potential lightweight structure materials in aerospace because of their low density, high specifi strength and specifi stiffness,as well as their vibration absorption and electromagnetic shielding effectiveness [1].The applications of Mg alloys in aerospace industry can not only bring great benefit in weight loss, but also improve mobility, transport capacity and reduce transport cost [2].However, the application of Mg alloys in aircrafts is limited by the following factors: (i) poor corrosion resistance [3]; (ii) poor plastic deformability [4]; (iii) Mg alloys in powder or scrap form are easily ignited by sparks generated by friction and even explode at room temperature [5], furthermore, once Mg alloys are ignited, the flam spreads rapidly and cannot be extinguished with the available extinguishers in aircraft [6].While the Federal Aviation Administration (FAA) lifted bans on the use of Mg alloys in aircraft cabins after assessing some nonflammabl Mg alloys [7], it remains to be verifie whether existing ignitability tests and flammabilit tests can ensure the safety of Mg alloys in other aircraft components.In order to broaden the application of Mg alloys in aerospace field it is necessary to have a comprehensive understanding of ignition and flammabilit of Mg alloys.

The present article reviews the research on ignition mechanisms,ignition and flammabilit tests and ignition characteristics of Mg alloys.The purpose of this article is that review the ignition mechanisms of Mg alloys,comparing current ignition and flammabilit test systems in order to explore experimental approaches which can meet the safety requirements for Mg alloys usage in aircrafts.In addition, the ignition characteristics of Mg alloys are summarized in order to identify the ignition critical parameters and more effective methods to improve the ignition resistance of Mg alloys.It is remarkable that the ignition of Mg alloy is determined by the internal properties(thermo-physical properties, oxide fil properties, chemical composition, geometrical parameters) and the external environment (gas species, oxygen concentration, oxygen pressure,etc.), and the ignition temperature is not physical constants of materials, but a comprehensive function with chemical kinetic and hydrodynamic parameters.

2.Ignition mechanisms of magnesium alloys

Metal ignition is essentially a violent oxidation process[8].The heat generated by oxidation reaction exceeds the heat lost to the surroundings, resulting in a sharp rise in temperature.Therefore, the research method of oxidation is also suitable for ignition of alloys.

2.1.Thermodynamic analysis of ignition

The ignition of Mg alloys is related to the oxide layer.When the protective oxide layer breaks and loses its protective properties at elevated temperature,ignition may occur[9].A thermodynamic factor affecting the formation condition and stability of oxide fil on the surface of metal at high temperature is the Gibbs free energy of formation of the oxide.The reaction of an alloying element X with MgO is expressed as[10]:

wherea, bpresent the stoichiometric numbers.Previous research indicated that if the change of Gibbs free energy of the reaction was negative, theXbOaoxide can form prior to MgO on the outer surface and retarded oxidation[9,11].However, recent studies showed that few rare earths like Ce and alkaline-earth elements like Ca, Sr with more affinit to oxygen in MgO, although the Gibbs free energy was positive,these oxide film also protected the matrix from oxidation[12-14].

The possible reactions of Mg-Ca-Ce system are as follows[13]:

The change in the Gibbs free energy of reactions can be expressed as [13]:

whereΔGθis the change of Gibbs free energy in the standard state (25°C, 1atm), T is the reaction temperature, R is the gas constant andαrepersents the activity.The change of Gibbs free energy value were positive for Ca and Ce oxidation at 700°C.It indicated that MgO could form in preference to CaO and Ce2O3if the concentrations of Ce and Ca were lower than the equilibrium concentration.However, the concentration of Mg2+cations decreased as the outer layer of MgO grew.Therefore, Ca and Ce with more affinit to oxygen were continuously enriched under the surface of the outer MgO to form stable protective layers of CaO and Ce2O3that increased ignition resistance.Similarly, the change of Gibbs free energy value was positive for Mg95.9Zn1Y2Al1Be0.1(at%)alloy at 923K [12], according to the results of elements depth profile of the surface oxide film and the thermodynamic calculation, the Be2+cations diffused to MgO grain boundaries and inhibited the outward diffusion of Mg2+cations,retarding the matrix from oxidation.

CALPHAD method is also applicable to the analysis of oxide layer [15].The selective oxidation model of WE43 alloy was established by CALPHAD method, and the structural transformation of oxide layer and oxygen activity gradient were described in detail, as shown in Fig.1 [16], the outer oxide layer mainly consisted of Y2O3and a small amount of ZrO2and MgO, the middle layer was a mixture of MgO and Y2O3, and the inner oxide layer consisted of Y2O3-rich oxide, which was consistent with observed microstructures.

In addition, the possibility of Mg reacting with oxygen can be measured by activation energy.However, due to the different reaction conditions and the selection of parameters in related literature [17,18], the activation energy varied from 60kJ/mol to 600kJ/mol, and the reaction rate was even fi e orders of magnitude different [19].

2.2.Ignition process

Metal ignition is a severe oxidation reaction occurring in high temperature, including heat transfer, mass transfer and oxygen transfer.The analysis of transfer process is an important means to understand the ignition mechanism of Mg alloys.

2.2.1.Heat transfer

The ignition energy of Mg alloys mainly come from the exothermic oxidation and the external energy input.With the intense exothermic oxidation of Mg alloys at elevated temperature, the temperature continuously increases without limit until a critical valueTcritis reached and burning.The critical ignition temperatureTcritis define as the lowest initial temperature from which the sample may self-heat to reach the steady-state burning [20].The critical ignition temperatureTcritof Mg alloys can be obtained by isothermal test as in Fig.2, where isothermal holding time from 10s to 1600s the critical ignition temperatureTcritof AZ91D alloy was not a constant value, which decreased from 650°C (representing the liquid state) to 520°C(representing the solid state with liquid pools).Similar results are also found in pure Mg [20].Such behavior is related to the time dependent phenomena of exothermic oxidation, the ignition temperature changes with the isothermal holding time.It reveals that melting is not a prerequisite for ignition of Mg alloys, and ignition temperature may occur below or above the liquidus temperature.The reason is that exothermic oxidation of Mg alloys breaks the thermal equilibrium between the heat input through the surface layer and the heat loss to the surroundings.The thermal equilibrium at the critical ignition temperature without considering the latent heat of melting and evaporation is as follows[22]:

Fig.1.Thermodynamic predictions of equilibrium activities of oxygen for different oxides and the schematic for the formation of three-layered structure of oxide scale [16].

Fig.2.Ignition delay time against ignition temperature of AZ91D magnesium alloy [21].

WhereCPis the heat capacity,ρis the density, V is the volume,is the ignition energy,QOis the exothermic oxidation,Qconvis the heat convection,Qradis the thermal radiation.

In addition, it is worth noting that the critical ignition temperature is distinct from the experimental ignition temperature by continuous heating test (as shown in chapter 3.2.1).This difference between the critical ignition temperature and the experimental ignition temperature is consider to the possibility of self-heating and ignition delay time.The ignition energy mainly come from the exothermic oxidationQOfor the critical ignition temperature.However, the ignition energy of continuous heating test not only comes from the exothermic oxidation but also from the external heat inputQconvandQrad.As a result, the external conduction and radiation heat overcomes the oxidation reaction contributions to heat generation that generally determine ignition of Mg alloys.

The formation of oxidation products on the surface of Mg alloys are also important factors affecting the ignition temperature.The oxide products of Mg alloys become porous and cracked and non-protective at higher temperatures [22].The temperature at which the surface oxides become nonprotective due to cracking, peeling, or evaporation is called the transition temperatureTtrans.The transition temperatureTtransof Mg alloys is generally considered to be related to the mechanical properties of the oxide products and vapor pressure,which acts to break the oxide layer by inducing generating sufficien pressure difference across the radial thickness.The transition temperatureTtransof Mg alloys is difficul to measure by conventional ignition temperature-time curve.However, the transition temperatureTtransat which the surface oxide lose protection can be recorded by combining the high-speed camera with thermocouple,and it is found that theTtransis more closely corresponds to the ignition temperature[23,24].As shown in Fig.3 [24], a typical example of the bulk Mg from solid phase to the full burning, and the light mission intensity of the sample is also shown.A white layer of MgO is rapidly formed on the outer surface of bulk Mg sample at high temperature before melting (Fig.3a).When the temperature reaches the melting point (near 650°C), the internal Mg melts and collapses, but the outer MgO still remains in the solid state and continues to grow(Fig.3b).After that, the vapor pressure of the internal Mg increases and the specimen stretches upward until the outer MgO layer ruptures(Fig.3c).As soon as the MgO layer breaks down, a large amount of Mg and Mg vapor react with oxygen to generate a visible flam coincided with the appearance of a luminous reaction front (Fig.3d).It should be noted that the transition temperature is close to the ignition temperature of Mg sample.Some results show that Mg-Al alloys have similar variation in ignition behavior, which their critical temperature and ignition temperature are relatively close [23].

In addition, the quantitative theory of magnesium ignition based on thermal equilibrium and reaction kinetics was established and the theoretical ignition temperature was calculated[25].In an adiabatic system, if the heat release through the oxide layer is insufficien to remove the heat generated due to oxidation reaction, the magnesium will be ignited, and the ignition temperatureTiis given by [26]:

Fig.3.Schematic diagram of temperature and light emission intensity of Mg with time.

whereTiis the ignition temperature at the outer surface,Tis the temperature at the metal-oxide interface,χis the thermal conductivity,dis the thickness of the oxide filmCois the number of adsorption sites on metal atoms per unit area,kis Boltzmann’s constant,his Planck’s constant, N is Avogadro’s number,F is surface roughness factor,ΔHis the heat of MgO formation,Eais the activation energy.

The current ignition temperature models of Mg are mainly based on the exothermic oxidation reaction in adiabatic system as the source of heat generation without considering the external heat source, the size and shape of the sample, airfl w velocity and oxygen pressure, which cannot be ignored in the experiment, and these parameters have important reference value for the application of Mg alloys in aerospace.Furthermore, only the ignition temperature quantitative theory models of pure Mg is available, while the models of Mg alloys have not been reported.

2.2.2.Mass transfer

Mg reacts with oxygen mainly in two ways:(i)the outward lattice diffusion of Mg2+cations and(ii)Mg vapor evaporates through porous oxide layer at high temperature creating conditions that favor ignition.

The high-temperature oxidation curves of Mg alloys are generally separated into two stages:initial parabolic stage and acceleration line stage.The initial parabolic stage is considered to be the incubation period, during which the oxide fil has protective effect, and the lattice diffusion of Mg2+cations is the control step of reaction [27].The thermo-gravimetric measurement of weight change versus time for AZ91 in air at different temperature as shown in Fig.4 [28].The oxidation weight gain of Mg alloys followed parabolic kinetics at 197°C.The MgO formed in this stage acted as a protective layer, so the overall weight gain was small and the oxidation rate was slow.However, the kinetics of oxidation changed from parabolic to the linear reaction after the parabolic stage above 400°C.It revealed that oxide layer could no longer prevent matrix from oxidation.With increasing of temperature and time, the incubation period reduced from 25 to 13min.Similarly, the incubation period of AE42 at 450°C was 2h,which decreased to 1.5h at 500°C, indicating a degradation of oxidation resistance[29].Some results show that Mg-Zn-Y,Mg-Ca-Y,Mg-Gd-Y alloys have similar variation of oxidation weight gain at high temperature [30-32].

The previous research results showed that the oxidation rate of Mg increased abruptly when the temperature of Mg reaches the melting point compared with that before melting[23,33,34].G.K.Zhovskii [35,36] proposed the kinetic parameters of surface oxidation reaction of Mg at significantl lower and higher melting points respectively,in which the activation energy of solid state was E1/R=34,000K, and the activation energy after melting was E2/R=15,000K.V.N.Bloshenko[37] attributed the difference in reaction rates to the melting of Mg and the breakdown of oxide films

Fig.4.Thermo-gravimetric measurements of weight change versus time for AZ91D alloy in air [28].

Furthermore, Mg vaporization also accelerates the oxidation reaction and reduces ignition temperature.The vapor pressure of Mg at 650°C is eight orders of magnitude higher than that of Al and seven orders of magnitude greater than that of Zn[38].Czerwinski gave a functional relation between evaporation rate and temperature when high-temperature oxidation reaction of Mg-Al alloy was controlled by Mg evaporation rate, the equation can be expressed as [28]:

For the Mg-Al alloy, the evaporation rate of Mg is over three orders of magnitude higher than the diffusion rate at 773K [28].The high vapor pressure of Mg alloy indicates that Mg has strong volatility at high temperature, which leads to the faster diffusion of Mg to air.It leads to the formation of loose, porous and caulifl wer-like surface morphology of Mg alloys.Fig.5 exhibited the microscopic surface morphology of Mg-Gd-Y-Zn-Zr alloy at 773K-873K [32].A dense and compact protective layer formed on the surface at 773K and 823K for 360min, which retarded the transfer of oxygen to the matrix.However, with the increase of temperature,although the phase composition of the oxide layer remained unchanged, the surface morphology changed obviously.When the temperature reached 873K, some cracks and holes appeared at the surface.These cracks and pores acted as transport channels for oxygen and Mg2+cations.It accelerated the reaction of Mg with oxygen [29].The continuous supply of Mg vapor exacerbates the oxidation reaction, releasing large amounts of heat and increasing the risk of ignition.

In general, the ignition of Mg alloys includes heat, mass and oxygen transfer.If the heat generation is not enough and the oxide fil is complete and dense, transport capacity of mass and oxygen decreases, ignition may not occur.

Fig.5.Surface morphology of Mg-Gd-Y-Zn-Zr alloy oxidized at (a, b)773K, 360min; (c, d) 823K, 360min; and (e, f) 873K, 180min [32].

3.Experimental methods to evaluate ignition and flammabilit properties

Ignition and flammabilit are two different properties which represent the possibility of a material to burn.Ignition is define as the irreversible process of material from slow reaction or non-reaction state to violent reaction state accompanied by luminous, exothermic and mass change [39].Ignition temperature is usually used as an evaluation index in ignition test, while ignition time or extinguishing time is emphasized in flammabilit testing [40,41].The main difference between ignition and flammabilit is that ignition does not necessarily lead to burn, if the heat source is removed after ignition[41].Ignition and flammabilit both are controlled by the accumulation of heat, which is the evolution process of reaction system from quantitative to qualitative change.Therefore, there is a certain correlation between temperature and time, long-time heating or lower heating rate can reduce the ignition temperature in the ignition test, while in the flamma bility test, the lower heat source temperature delays the ignition time.

3.1.The criteria of ignition

The ignition temperatures of Mg alloys reported in various literatures are inconsistent.One reason is the lack of a precise definitio of ignition.It is worth to notice that melting point or violent oxidation temperature cannot be equated with ignition temperature.Previous work indicated that when pure magnesium (99.93 wt%) was heated at 300°C/min, and its ignition temperature was generally 630～640°C [42].Some results also showed that the ignition temperature of Mg alloys were lower than the melting point [43-45].It indicated that melting point of Mg alloys cannot be equated with ignition temperature,and melting is not a prerequisite for ignition.In addition, the violent oxidation point is different from that of ignition of Mg alloys, for example, the ignition temperature of AZ91D alloy is 100°C higher than violent oxidation temperature [46].The following ignition criteria are mainly adopted:

Fig.6.Schematic of continuous heating test [55].

(1) The ignition criterion is define as the temperature rises sharply [47].It represents the critical temperature at which the heat released by oxidation exceeds the heat lost to surroundings.

(2) The ignition criterion is also define as a sudden change in mass corresponding to the temperature [48,49].It reveals the critical point at which the reaction state changes from chemical kinetic control to diffusion control or evaporation control.

(3) The ignition criterion can be determined by the intensity of flam radiation spectrum, which is intuitive and accurate [50-52].The determination of ignition by flam radiation spectrum is commonly used in recent years[53,54].

3.2.Ignition test

Another reason for the different ignition temperatures of Mg alloys reported in the literature is that the absence of a standardized test.The ignition temperature obtained by different test methods and criteria.In order to acquire comparable ignition and flammabilit data, the consistent testing method and ignition criterion must be agreed upon.

3.2.1.Continuous heating test

Schematic diagram of continuous heating test is shown in Fig.6 [55].This testing uses DTA(Differential Thermal Analysis)or TGA(Thermo-Gravimetric Analysis)or induction furnace to control the heating rate to determine the ignition temperature [56,57].The sample was heated at a predetermined rate until ignited and the ignition temperature was recorded,as shown in Fig.7.The heating rate control precision of DTA and TGA is higher than induction furnace.And the ignition temperature is determined by the mass and temperature sudden change points of the sample, so the result is relatively more accurate.However, DTA and TGA are mainly suitable for small size and powder sample, but not for larger size sample.

Fig.7.Typical temperature-time curve of continuous test.

It is remarkable that the ignition temperature of Mg alloys obtained by different heating rate, sample size and measuring methods are also different.The ignition temperature of WE43 alloy specimen(15mm×15mm×15mm) was 644°C[58], while WE43 alloy specimen (20mm×20mm×20mm)did not ignite at 730°C in the induction heating furnace [42].Liu [51] used infrared thermal radiation and thermocouple to measure the ignition temperature of mixed powder of Mg and calcium iodate, and found that the response rate and sensitivity of infrared thermal radiation measurement was better than thermocouple.

3.2.2.Isothermal test

Isothermal test device is similar to the continuous heating test by induction furnace [21].The sample was heated at a set ambient temperature to obtain the ignition-delay time and the ignition temperature.The key difference between continuous heating test and isothermal test is that the former only measures the ignition temperature, while the latter is to determine the inter-dependence between ignition temperature and ignition delay time, and the higher ambient temperature,the shorter the ignition delay time.Furthermore, when the isothermal holding temperatures is lower, that is, below the liquidus, the temperature-time curve of the sample is shown in Fig.8(a).The sample temperature gradually approaches the set ambient temperature, where the thermal equilibrium was established between the exothermic oxidation and the heat loss to the surroundings.With the increase of oxidation rate,more heat is generated, which breaks the thermal equilibrium and leads to a sharp temperature increase and ignite.This is based on a prerequisite that Mg kept at a lower temperature(below liquidus temperature) will spontaneous ignition without an external source of ignition.The ignition energy mainly come from the exothermic oxidation in this condition.On the other hands, when the isothermal holding temperatures is higher (above the liquidus), the temperature-time curve of the sample is shown in Fig.8(b), the sample temperature continuously increased until ignition temperature by external source of heat convection and thermal radiation.The ignition occurs during the heating stage by external sources, which masks the temperature rise caused by the increase of oxidation rate.The ignition energy not only derives from exothermic oxidation,but also from the input energy by heat convection and thermal radiation.In addition, there is also no unifie standard for isothermal testing, the ignition temperature of Mg-Al alloy measured at 1173K in furnace is about 100K higher than that measured at 1073K in furnace [59].

Fig.8.(a) TsetTig. Typical temperature-time curve of isothermal test.

Fig.9.Schematic of cone calorimeter test.

3.3.Flammability test

3.3.1.Cone calorimeter test

The cone calorimeter can measure the flammabilit parameters of Mg alloy [60], such as heat release rate, peak heat release rate and total heat release, the schematic of cone calorimeter is shown in Fig.9.The flammabilit of material is evaluated by fir performance index and fir growth index.The fir performance index is define as the ignition time divided by the peak heat release rate, which is related to the fir risk of the material, the smaller the value, the greater the possibility of ignition [61,62].The fir growth index is define as the heat release rate divided by the time to reach the peak heat release rate, which represents the fir growth rate of the material, the larger the value, the faster the fir develops [61,62].

Fig.10.Schematic of oil-fire burn test [7].

Fig.11.Schematic of full scale aircraft fir test [7].

3.3.2.Oil-fi ed burner test

The Federal Aviation Administration (FAA) designed oilfire burner test for evaluating the flammabilit of Mg alloys(Fig.10).The sample size was 0.63×3.8×50.5cm.The oilfire burner was used to continuously heat the sample in the sealed cabin.If the sample cannot ignite within 2 min,and the burning sample self-extinguished within 3 min after turning off the burner, and the maximum weight loss of the sample cannot exceed 10% during testing, then the flammabilit of simple can be qualifie [7].In addition, horizontal specimen seemed to burn faster than vertical specimen when exposed to the flam ejector [63].

3.3.3.Full scale aircraft fi e test

Considering the ignition and flammabilit of aircraft components are related to the material itself, the geometric size and location.Full scale aircraft fir test can simulate the potential threat of electrical arc, oxygen canister and external flam during the flight as shown in Fig.11 [7].The test subjects covered almost all the components in the cabin, such as seats, paneling and carpet.

Table 1Partial comparison of environmental conditions and sample configuration of the con calorimeter [60], the oil-fire burner Test [7], full scale aircraft fir test[7] and ignition and flam spread test [65].

Fig.12.Schematic diagram of ignition and flam spread test [65].

3.4.Ignition and flam spread test

Mg alloy as an excellent lightweight material has great potential to be used in aerospace, such as shell, engine accessory etc.However, no standard specification are established to evaluate the ignition and flammabilit of Mg alloys under high/low pressure, gas species, oxygen concentrations and air fl w.The ignition and flam spread test is widely used to evaluate the ignition and flammabilit of metals under extreme conditions [64].Schematic diagram of the device is shown in Fig.12 [65].The sample with a diameter of 3.2mm was suspended under the test bench at one end and ignited by flammabl metal wires (such as aluminum, magnesium,etc.) at the other end.If the sample burns completely, lower pressure until the sample does not burn fi e times under the pressure.The lowest ignition pressure is the ignition threshold pressure of the metal.In addition, regression rate of the melting interface, which represents the burning rate, is obtained by dividing the burning length by the burning time.

The ignition temperature and ignition delay time of Mg alloys are not only related to material characteristics, but also affected by sample size and geometry, test conditions.Partial flammabilit tests are compared in Table 1.It is necessary to establish the precise criteria of ignition and standardized test to evaluate the risk of Mg alloys application in aircrafts.

4.Ignition characteristics

The main factors influencin ignition characteristics of Mg alloys can be divided into two categories:internal and external properties.The internal properties are not affected by environment, including the micro-characteristics (i.e.thermo-physical properties, oxide fil properties, chemical compositions) and macro-characteristics (i.e.shape, size, orientation) of materials.These internal properties significantl affect energy requirement for ignition.The external properties are related to the environment, such as gas species, oxygen concentration and oxygen pressure.These external factors primarily influ ence the extent of exothermic reaction and heat loss.The ignition of Mg alloys needs to be considered from both internal and external properties.

Table 2Thermo-physical comparison [66,67].

4.1.Thermo-physical properties

4.1.1.Burning ratio

It is suggested that the possibility of ignition can be measured by the burning rate at the melting point (BR)mpand the burning rate at the boiling point (BR)bp.(BR)mpand (BR)bpcan be expressed by the following formula respectively [66]:

whereΔHfis the heat of formation of the metal oxide;ΔHrt?mpis the heat required to melt a material from room temperature;ΔHmpis the heat of fusion;ΔHmp?bpis the heat required to rise from the melting point to the boiling point;ΔHbpis the heat of vaporization.The burning rates of some metals were summarized in Table 2[66,67].The burning ratio represents the contribution of heat generated during the oxidation of metal to the maintenance of burning.If the burning ratio value is small, it means that the heat generated during the burning is difficul to melt the metal matrix.It is difficul to keep burning even if the material is ignited.On the contrary, if the burning ratio value is high, it means that there is enough heat to melt or even vaporize the metal matrix, and once ignited, the metal will burn violently.

4.1.2.Thermal inertia

The surface temperature of metal can be calculated according to the heat fl w equation.And the surface temperatureτis given by [67]:

whereh’ is environmental factor; t is ignition time; k is thermal conductivity;ρis density; c is specifi heat.In addition,one characteristic of the solid, kpc, define as thermal inertia.Thermal inertia as a control step in the thermal transfer process, can be used to evaluate the possibility of ignition[67,68].The thermal inertia of some metals were compared in Table 2 [66,67], obviously, the thermal inertia and ignition temperature of pure magnesium were both minimal in Table 2.It is generally believed that the greater the thermal inertia,the more difficul for the metal to be ignited.However, the thermal inertia of various Mg alloys has not been reported.

4.2.Oxide fil properties

The ignition characteristics of Mg alloys are related to the oxide fil properties.Oxide film are usually classifie into two types according to whether they are protective or not.One is porous and loose non-protective oxide fil and the other is compact protective oxide film The dense and continuous protective oxide fil reduces reaction rate and limits heat transfer to metal matrix.As previously mentioned (as seen in 2.2.1), once the surface temperature of the metal exceeds the transition temperature of the oxide film the oxide fil lose protective effect.

4.2.1.P-B ratio

Pilling and Bedworth [69] firs proposed the Pilling-Bedworth ratio (PBR) to evaluate the protective of oxides formed on metal surfaces, the PBR is expressed as:

whereVoxrelates to the volume of metal oxides generated;VMrelates to the volume of metal consumed to form oxides;M is the molecular weight of metal oxides; m is the mass of metal used to form metal oxides.ρMandρOXare the densities of metals and metal oxides, respectively.

The metal oxides with PBR<1 and PBR>2, which are non-protective and non-adherent to metal.The PBR value of protective metal oxides ranges from 1 to 2.The PBR values of some metal oxides were summarized in Table 3 [33].The PBR value of MgO is 0.81, which can not provide adequate protection to the matrix from oxidation at high temperature.Although the PBR value of CaO/Ca is 0.64, the substrate in Table 3 is Ca rather than Mg alloys.Hence, it is not appropriate to use the PBR value of 0.64 to evaluate the protective of a CaO fil on Mg alloys.In addition to oxide/alloy interfaces, the oxide/oxide interfaces should be considered.Inoue et al.[70] calculated that the PBR value of CaO/Mg-Al-Ca was 1.17 and estimated the anionic volume ratio of the CaO/MgO was 1.48.Intensifying the surface oxide densificatio can improve the ignition resistance of Mg alloys.As validated by experiments, the ignition resistance of Mg alloys increased with the addition of Ca [71-73].The surface morphology and microstructure of unburned Mg-Al-Ca-Mn alloys samples after the oil-fire burner test were shown in Fig.13 [73].These samples were melted without ignited.Although there were wrinkles on the surface of the oxide film no cracks and other defects were found.It indicated that the oxide fil consisting of CaO and MgO was stable and protective, which could completely cover the molten matrix and reduce the risk of ignition.

Table 3PBR values of some metal oxides [33].

4.2.2.Mechanical properties of oxide fil

The oxide fil on the alloy surface may crack under the internal stress, the cracked oxide fil makes the metal more easily ignited.The oxide fil with higher mechanical properties can withstand higher internal stress without cracking,which greatly increases the ignition resistance [74].The mechanical properties of the oxide layer of AZ91D alloy were significantl improved by adding Be, as shown in Fig.14[75].The hardness and lateral force of (Mg,Be)O were obviously higher than that of MgO, which were attributed to the formation of a fine-graine (Mg,Be)O solid solution,resulting in the solid solution strengthening of Be and the grain refine ment strengthening.As a result, the (Mg,Be)O layer exhibited higher resistance to cracking and fracture, ignition temperature of AZ91D was improved.

Furthermore, Shin-ichi Inoue et al.[76] reported that when Ca and Be were added to Mg-Zn-Y alloys, the oxide fil not only was refined but also the thickness and residual stress of the oxide fil were reduced.It was inferred that the suppression of oxide fil thickness might be beneficia to prevent the increase of residual stress in the oxide film These characteristics help to improve the protection and ignition resistance of the oxide film Besides, the ignition temperature of the Mg alloy may be positively correlated with the critical thickness of the oxide fil rupture [77].Czerwinski et al.[78] proposed that the oxide fil had a certain critical thickness, if the thickness of oxide fil did not exceed the critical thickness, the residual stress would not lead to the formation of oxide cracks.Moreover,heat treatment may enhance the adhesion of metal/oxide and tightness of metallic oxide, resulting in increasing the ignition resistance of Mg alloys [79,80].

Fig.13.The macroscopic and microscopic surface morphology of unburned specimens: (a, d, g) Mg-2.5Al-2.5Ca-Mn; (b, e, h) Mg-3.5Al-3.5Ca-Mn; (c, f, i)Mg-4.5Al-4.5Ca-Mn [73].

Fig.14.The values of the hardness and lateral force of oxide: (a) hardness-relative indentation depth curve; (b) lateral force-lateral displacement curve [75].

Fig.15.Two kinds of oxide fil structures formed on Mg alloys with RE addition: (a) REO was located in the outermost layer; (b) REO was located in the inner layer of the oxide fil [85].

4.3.Chemical composition

At present, alloying is the common method to improve the ignition resistance of Mg alloys.However, not all elements can enhance the ignition resistance of alloys.In this section,the research progresses on improving the ignition resistance of Mg alloys by adding reactive elements,nano-sized particles and reducing the low melting phases are reported.

4.3.1.Reactive elements

Be [75], Ca [70,81] and some rare earth elements [82-84] have been reported to have reactive element effect.The reactive element effect has two mechanisms for enhancing the ignition resistance of Mg alloys: (i) reactive elements react with oxygen to form a dense oxide film which retards the diffusion of Mg2+cations and oxygen ions, thus restricting the growth of MgO; (ii) reactive elements with high solid solubility concentrate in the voids or cracks of MgO layer and react preferentially with oxygen to compensate for the diffusion paths formed in MgO layer.

Furthermore,recent studies found that the dense oxide fil formed by rare earth elements had two kinds of structures[85].One was the rare earth oxides REO distributed in the outermost layer of the oxide film as shown in Fig.15(a).Most of REO are anionic defective oxides, cations in the oxide fil are difficul to diffuse and oxygen ions diffuse inward.At the same time, Mg2+cations in the inner layer of MgO diffuses outward, so the oxidation reaction mainly concentrates on the interface between REO and MgO layer.The other was the outermost layer composed of MgO and the inner layer made up of REO, as shown in Fig.15(b).

Above structures have been confirme by experiments.The oxide layer of Mg-10Gd-3Y alloys in 723～873K pure oxygen for 90 min was a double layer structure, the outermost layer was mainly Gd2O3and Y2O3, and the inner layer was Gd2O3, Y2O3and MgO [86], which was consistent with the oxide structure in Fig.15(a).The ignition temperature of Mg-10Al-5Ca alloys can reach 1436K, the microstructure was seen in Fig.16 [70].The protective oxide fil consisted of three layers, the outer layer was fin CaO, the middle layer was refine MgO, and the inner layer was roughness MgO.The dense CaO layer inhibited the reaction between oxygen ions and metal cations and protected the matrix from ignition.On the other hand, the oxide outer layer of Mg-0.5Nd alloys formed at 773K was a relatively thin MgO, and the inner layer was a mixed of MgO and Nd2O3[87], which was similar to Fig.15(b).

Fig.16.(a) The microstructure of cross section of Mg-10Al-5Ca alloy oxide layer; (b) Schematic of layered structure of oxide layer [70].

In addition, some active elements (Y, Gd, etc.) have high solid solubility at high temperatures, which can repair oxide cracks and patching diffusion channels.Kumar et al.[42] reported that Y could fil in the cracks of MgO.As shown in Fig.17(a), cracks appeared on the oxide surface of WE43 alloy at 750°C, but no oxide nodules.EDS analysis results(Fig.17(b)) showed that the cracked region with high content of Y.It indicated that Y in the melt can rapidly form a dense Y2O3layer at the fracture region of MgO, preventing the violent oxidation.

Shin-ichi Inoue et al.[76] further investigated the effect of the reactive elements Ca and Be on the formation of oxide layer of non-flammabl Mg-Zn-Y alloy, the microstructure diagram was shown in Fig.18 [76].The native oxide of non-flammabl Mg-Zn-Y alloy was Y2O3instead of MgO,which was affected by the reactive element Y.However, nonflammabl Mg-Zn-Y alloy have severe internal oxidation feature.The addition of Ca contributed to the formation of continuous and dense CaO in the outermost layer and protected the inner oxide layer and matrix, while the addition of Be could refin and densify the MgO and Y2O3and inhibited the oxidation of inner layer and matrix.In addition, Be aggregated at the grain boundary of Y2O3layer, which retarded the diffusion of metal cations and oxygen ions and inhibited oxidation.After adding Ca or Al and Be to Mg-Zn-Y alloy,the ignition temperature increased from 1150K to 1320K,respectively.

4.3.2.Intermetallic phases

The ignition of Mg alloys cannot be expressed by a single quantitative property since alloys are consisted of different phases.Ignition temperature of Mg alloys is associated with melting point of the phase and microstructure, the low melting phases may lower ignition temperature.Because the low melting phases melt before the matrix form the liquid phase,which accelerate the oxidation rate and the evaporation of Mg [88].Besides, higher vapor pressure from the low melting phases promotes the growth of the porous oxides and cracks, makes the surface protective fil rupturing [89].

Fig.17.(a) Cracks on WE43 oxide layer by SEM; (b) EDX analysis of the oxides on WE43 [42].

Fig.18.Schematic diagrams of multilayer oxide structures formed on the Mg-Zn-Y alloys after adding Ca or Be [76].

Al, Zn, Cu and other elements are easy to form low melting phases with Mg at grain boundaries [90].In order to improve the ignition resistance of Mg alloys, low melting phase should be avoided as far as possible.Alloying and heat treatment can homogenize the microstructure, eliminate the low melting phases and improve ignition resistance.It has been reported that adding 2wt%Ca to AZ91 alloy can not only refin the microstructure, but also form a stable high melting point Al2Ca phase at the grain boundaries to replace the original low meltingβ-Mg17Al12phase, avoiding selective oxidation induced byβ-Mg17Al12phases [81].In addition, Zn element may evaporate during the dissolution ofβ-Mg17Al12phases in AZ91D alloy, creating conditions for the generation of oxide fil cracks [90].However, the ignition temperature of AM60 alloy did not increase by adding Be element [74].It might be due to the preferential reaction between Be and Al-Mn phase to form Al-Mn-Be intermetallic compound, resulting in the decrease of Be content and the reactive element effect.

4.3.3.Nano-sized particles

Some nano-sized particles addition can improve Mg alloys ignition resistance.The ignition temperature of Mg containing nanoparticles was measured by thermo-gravimetric analysis, as summarized in Fig.19 [91-95].On the same test condition, the ignition temperature of Mg containing nanosized particles was higher than that of pure Mg, the Mg with 1.5wt% nano-CaO particles was the highest.However, the ignition mechanism of Mg alloys containing nanoparticles is still not clear.Some guesses are as follows: (i) nanoparticles and Mg formed agglomerated phases, which had high affinit with oxygen and were stable at high temperatures.Moreover,the densities of these agglomerated phases were lighter than that of matrix, mainly distributed on the surface of matrix and fille in the native porous MgO, reducing the specifi areas for oxidation [94]; (ii) the addition of nanoparticles reduced the thermal conductivity of Mg-based composites and improved thermal stability, which were beneficia to enhance the ignition resistance [94-96].Although the role of nanoparticles in the ignition process of Mg alloys and the ignition mechanism of Mg-based composites still need to be further studied and verified these mechanisms can be used for the development of new non-flammabl Mg alloys.

Fig.19.Ignition temperature of alloys by Thermo Gravimetric [91-95].

4.4.Geometrical parameters

To a large extent, ignition temperature is controlled by the specimen size.However,the size of Mg alloy samples is often inconsistent in the present ignition test, the ignition temperature of 2ⅹ2ⅹ1mm pure Mg sample was 590°C when heated at 10°C/min, while the ignition temperature of 2ⅹ2ⅹ2mm pure Mg sample was 609°C at the same heating rate [93].On the other hand, the size of Mg alloy specimen also affects the ignition time.Taking WE43 alloy as an example, the sample with 15mm thickness was ignited, and the burning stopped immediately, but the sample with 10mm thickness was ignited,continued burning for 90 s after the burner was removed in full scale aircraft fir test[63].In the ignition test,the ignition delay time of strip and column specimens of WE43 alloy with similar volume was obviously different, the ignition delay time of the strip sample with size of 2.2ⅹ38ⅹ500mm was 214s, while the column sample with size of?15mmⅹ200mm was 236s [73].

The ignition resistance of samples with different geometrical is related to the heat transfer and heat loss during the ignition.The components with high specifi surface area (surface area to volume ratio) are easier to ignite due to less heat loss [97].For example a metal tube and a metal rod of the same material and diameter, the tube is ignited more easily[98].In addition, powdered metals are more susceptible to ignition due to their lower thermal conductivity than solid metals and their increased higher surface area to volume ratio.The literature [99] indicated that the heat generation was proportional to the volume of the particles by the third powder of the radius, the heat loss was proportional to the surface area of the particles by the second powder of the radius.

4.5.External environment

4.5.1.Gas species

Mg can react with O2and N2as well as with CO2.Different gases react with Mg at the burning interface and release heat, which influenc the ignition and burning process.It is generally believed that Mg ignites more easily in O2than in N2.The ignition temperature of Mg-Ca alloys in N2was about 250°C higher than that in O2[100].G.Li et al.[101] found that the inertia effect of N2was the best by comparing the maximum explosion pressure and the maximum rate of pressure rise of coarse Mg dust in Ar, N2and CO2.It was speculated that nitrogen could effectively reduce the explosion pressure of metal powders, which might be related to the low reaction heat of N2and metal [102,103].In addition, the activation energies of reactions between Mg alloys and gasses are also different.The activation energies of ZE10, AZ31 and WE43 alloys in N2were higher than that in O2[60].It indicated that the nitridation of ZE10 alloy need more energy, and Mg alloys were relatively hard to ignite in N2than in O2.The reference [60] also pointed out that the protective nitride fil kept growing and densifying during the nitridation of Mg alloys.

4.5.2.Oxygen concentration

Parameters such as oxygen concentration, oxygen pressure and gas fl w velocity can control the extent of reaction and rate of heat loss to the surroundings [104].In the stages of metal oxidation, the reaction is shown as follows [99]:

The rate of the reaction v can be expressed as [99]:

where: Q is the heat of reaction (kJ/g); [O2] is the oxygen concentration (kmol/m3).It can be inferred that low oxygen concentration reduces the rate of reaction and heat generation which leads to ignite difficultl .In addition, there was a minimum critical oxygen concentration maintaining the sample burning,which was different for different alloys and increased with sample size [105].

4.5.3.Oxygen pressure

The heat flu is related to the oxygen pressure.The heat flu q generated by metal oxidation reaction can be expressed as [106]:

Fig.20.The relationship between oxide fil thickness and oxygen pressure of Mg-Al alloys after 1h of oxidation at 304K [107].

where Q is the heat of metal oxidation; w is oxidation rate; T is oxidation temperature; p0is standard atmospheric pressure;p is oxygen pressure; k0is pre-exponential factor; R is the gas constant;E is activation energy;It indicated that if oxygen pressure and oxidation temperature were increased simultaneously, the heat generated by oxidation would increase sharply and the metal might ignite more easily.In addition, oxygen pressure may affect the thickness of the oxide film The relationship between oxide fil thickness and oxygen pressure of Mg-Al alloys after 1h of oxidation at 304K was summarized in Fig.20 [107].The results showed that the oxide fil thickened with the increase of oxygen pressure.

Ignition temperature is not physical constant of material,but a comprehensive function with chemical kinetic and hydrodynamic parameters.Current studies only focus on the qualitative analysis of these factors affecting ignition, but the correlation among them have not yet been established.In addition, the bulk Mg alloys are not easy to ignite, it is not simply assumed that ignition is bound to happen when the temperature of Mg alloy sample reaches a certain ignition point.

5.Summaries and future development

In recent decades, ignition resistance of Mg alloys had become a focus of attention in the airspace.Although some progresses have been made, there are still several unresolved issues:

(1) Current studies suggested that the ignition resistance of Mg alloys was related to the protection of surface oxide film the melting point of phase and solid solubility limit.However, it cannot explain the ignition resistance of magnesium matrix composites.Hence, it is meaningful to update the ignition resistance mechanisms.

(2) Mg alloy as an excellent lightweight material has great potential to be used in aerospace, such as shell, engine accessory etc.However, there are few research focus on the ignition thermodynamic parameters of Mg alloys at low or high pressure.It is necessary to accumulate the ignition properties data for the designing ignition resistance of magnesium alloys or fir resistance coating.

(3) The surface modificatio techniques of Mg alloys have been applied to improve their surface hardness, corrosion resistance and wear resistance or other functional characteristics, while few data can be found about their ignition resistance.It is necessary to evaluate the coating’s ignition-resistant characteristic and develop new coatings of Mg alloys with comprehensive properties.

(4) Various experimental devices have been designed to study the ignition and flammabilit of Mg alloys.The ignition temperature and ignition-delay time of Mg alloys are not only related to material characteristics, but also affected by sample geometry, test conditions.In order to acquire comparable ignition and flammabilit of Mg alloys, it is necessary to establish the precise criteria of ignition and standardized test to evaluate the risk of Mg alloys application in aircrafts.

Acknowledgments

This project is supported by Fundamental Research Funds for the Central Universities (Grant no.FRF-GF-18-024B).