Tungsten combustion in impact initiated W-Al composite based on W(Al) super-saturated solid solution

Kong-xun Zhao,Xiao-hong Zhang,Xiao-ran Gu,Yu Tang,Shun Li ,Yi-cong Ye,Li'an Zhu,Shu-xin Bai

College of Aerospace Science and Engineering, National University of Defense Technology, Changsha, 410073, China

Keywords:Tungsten combustion Reactive materials Super-saturated solid solution Shock-induced reactions W-Al composite

ABSTRACT Element W can effectively improve the density of energetic structural materials.However,W is an inert element and does not combust in air.To change the reaction characteristics of W,60 at.% Al was introduced into W through mechanical alloying.XRD analysis shows that after 50 h of ball milling,the diffraction peak of Al completely disappears and W(Al60) super-saturated solid solution powder is obtained.Further observation by HAADF and HRTEM reveals that the W(Al60)super-saturated solid solution powder is a mixture of solid solution and amorphous phase.Based on the good thermal stability of W(Al60) alloy powder below 1000 °C,W(Al60)-Al composite was synthesized by hot pressing process.Impact initiation experiments suggest that the W(Al60)-Al composite has excellent reaction characteristics,and multiple types of tungsten oxides are detected in the reaction products,showing that the modified W is combustible in air.Due to the combustion of tungsten,the energy release rate of the W(Al60)-Al composite at speed of 1362 m/s reaches 2.71 kJ/g.

1.Introduction

Energetic structural materials (ESMs) are a new kind of energetic material with high strength.They have high stability under normal conditions,and can maintain structural integrity under detonation loading[1].However,when interacting with the target object,chemical reactions will occur between different components of the ESMs or between components and the air,resulting in combustion or even explosion reactions [2-5].Using ESMs to replace the inert parts of existing warheads,such as kinetic penetrator,shaped charge liners and reactive fragments,can not only impose traditional kinetic energy penetration on the target,but also bring additional chemical damage,such as high temperature and overpressure [6-8].Therefore,the development of ESMs has become an important technical way to enhance the damage effect of warheads.

In order to obtain excellent energy release characteristics,most ESMs are composed of active metals,including Al,Zr,Ni,Mg,B,etc.[9,10].Al-based ESMs have high energy density and excellent energy release characteristics,so it has attracted much attention in the field of ESMs [11-13].Typical Al-based ESMs,such as Al-Ni[14,15],Al-Ta [16],Al-PTFE [17-19],Al-Fe2O3 [20,21],have been studied extensively.For example,drop hammer test shows that the Al/PTFE composite has a strong temperature destruction effect,the highest flame temperature of PTFE/Al/Si/CuO containing 20%PTFE/Si can reach 2589 K.Ren [18] fabricated Al/PTFE composites by mold pressing and sintering method and investigated the dynamic mechanical properties and ignition behavior of the materials under extreme conditions.The results show that Al/PTFE composite is an low threshold ESMs,and the reaction sensitivity increases with the increase of temperature.Xiong [22] studied the influence of the introduction of Cu and PTFE on the shock-induced chemical reaction properties of Al/Ni Composites.The results show that PTFE can improve the sensitivity of Al/Ni Composite under impact,while Cu can prolong the duration of the reaction.

Al-based ESMs have good energy release characteristics,but its application is limited by low density,low strength and low permeability.Taking the Al/Ni composite as an example,its tensile strength is generally lower than 200 MPa,and its compressive strength is between 250 MPa and 500 MPa [23-25].For Al/PTFE ESMs,the quasi-static compressive and tensile strength of Al/PFTE is even less than 100 MPa,which makes it difficult to provide effective penetration capability [26-29].At present,the density and penetration ability of Al-based alloys are mainly improved by adding high density elements such as W[30,31].The experimental results of split Hopkinson bar indicate that the dynamic compressive strength of the W-PTFE-Al composites increases significantly with the content of W increased from 50 to 80 wt%.However,impact initiated experiments show that the reaction energy of WPTFE-Al composites decreases with the increase of W in oxygen and argon atmosphere[32].This is mainly because W has high thermal inertia and low adiabatic flame temperature in air,which impede its ignition and self-sustaining combustion [33,34].But in view of thermodynamics,the oxidation heat release per unit volume of W is 88.5 kJ/cm3,second only to boron and tantalum[35].

Element Al is easy to burn and has a high combustion heat(84.0 kJ/cm3)[36].If a large number of Al atoms are dissolved into the W lattice and form a metastable W(Al) alloy powder,the high activity and high heat release of Al may induce the combustion of modified W.According to the Al-W binary phase diagram,the solid solubility limit of Al in W is about 15.9 at% at 1255°C,which may not be enough to realize the ignition of W.To achieve the dissolution of more Al atoms,mechanical alloying (MA) is a feasible technical means[37].Many scholars have pointed out that the solid solubility expansion of binary systems can be achieved through MA,such as W-Ag [38],Cu-Nb [39] Fe-Zr [40] and W-Al.Ouyang [41]first proposed that the solubility limit of Al in W could reach 50 at.%by MA.Rafiei[42]prepared and studied the formation mechanism of W-20 wt% Al,the result reveals that the formation of the fine layered microstructure play a great role in promoting the diffusion of Al atoms into W.Recent studies on W(Al) super-saturated solid solution powders mainly focus on the microstructure [43,44],oxidation resistance [45] and sinterability [46],and there is no relevant report on the combustion characteristics of W.

To improve the reaction characteristics of W,this work attempts to introduce Al atoms into the W lattice and prepare W(Al) supersaturated solid solution powder by the MA process.The milling process,crystal structure and thermal stability of the W(Al) alloy powder are also studied.The reaction characteristics of the W(Al)-Al composite and the combustion characteristics of modified W(Al)alloy powder are tested through impact initiation experiments.

2.Materials and methods

In this study,W-60 at% Al (W(Al60)) super-saturated solid solution powder was prepared via mechanical alloying.Fig.1 shows the morphology of original materials.W particles (Tijo metal materials,3-4 μm,99.9%purity)and Al particles(Tijo metal materials,5-6 μm,99.5% purity) were blended to obtain the desired composition.This experiment used a YXQM-4L planetary ball mill(Changsha Miqi Instrument Equipment Co,Ltd) equipped with 500 mL stainless steel milling jar.The milling balls are stainless steel balls with diameters of 5 mm,8 mm and 10 mm,which are mixed in the proportion of 5:3:2.The experiments were conducted under an argon atmosphere at 250 rpm with a ball-to-powder mass ratio of 10:1.Alcohol was used as the process control agent(PCA)to prevent powder agglomeration.After each 10 min of milling,the planetary ball mill stopped for 10 min to avoid the volatilization of PCA induced by excessive temperature.The density of obtained alloy powder was measured by pycnometer method.The phase evolution during ball milling was detected by D/max2550 VB X-ray diffraction (XRD) with CuKα (λ=0.1542 nm) radiation in the 2θ range of 30-90°.The grain size and lattice strain of the alloy powder were calculated using the Scherrer equation and the Williamson-Hall method,respectively.The morphology evolution and elemental distribution of particles were characterized by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy(EDS).The crystal structures were analyzed by high-resolution transmission electron microscopy (HRTEM).In addition,STA449-F3 thermo-gravimetry differential scanning calorimeter (DSC) was used to study the thermal stability of alloy powder between 20 and 1000°C.

Fig.1.Morphology of original powders: (a) W and (b) Al.

Using 40 vol% pure Al powder as a binder,W(Al60)-Al composite was fabricated by hot pressing at 400°C for 2 h under 400 MPa.The quasi-static compression tests were conducted on a universal testing machine (WDW-100 B) under velocity control with a nominal strain rate of 10-3s-1.The impact initiation reaction of the W(Al60)-Al composite was verified by a ballistic gun test system.Fig.2 shows the schematic diagram of the impact initiation experimental device.14.5 mm ballistic gun,velocity measurement and 27 L test chamber were set in a straight line.The energetic projectiles were driven by ballistic gun.After penetrating the front steel plate and impacting the steel anvil,they fractured and began to react.By adjusting the mass of black powder,the initial velocity of the projectiles can be controlled.The reaction phenomenon in the chamber was recorded by a high speed camera,and the energy release characteristics were evaluated by pressure sensors.

Fig.2.The schematic diagram of the experiment setup.

3.Results and discussion

3.1.Characterization of W(Al60) alloy powder

The XRD patterns of W(Al60) mixed powders with different milling time are shown in Fig.3(a).After milling for 10 h,only three major diffraction peaks of Al remained,while the rest of Al peaks disappeared.By further increasing the milling time to 40 h,the diffraction peak of Al only left a faint (111) peak,and the Al (111)peak disappeared completely after 50 h of milling.Then,the diffraction peaks no longer changed with the extension of milling time,but the FWHM was slightly increased.In addition,the diffraction peak of W moved slightly to the higher angle during the MA process,which implies that Al atoms may dissolve into the W lattice and resulting in a decrease in the W lattice constant.Fig.3(b)illustrates the evolution trend of the grain size and lattice strain.The grain size of the W(Al60) mixed powder decreased monotonically with the extension of milling time,and the grain size decreased to 35 nm after 70 h ball milling.The reduction in the grain size will greatly increase the grain boundary area.The grain boundary is the bridge of atomic diffusion and the place where solute atoms gather first,the increase of the grain boundary area is beneficial to the expansion of solid solubility[38,47].Different from the grain size,the lattice strain first increased and then decreased with milling time.This means that amorphous phase may appear after ball milling for 40 h.Generally,amorphization will reduce interfaces and defects,which helps to release lattice strain [48].Further characterization of the crystal structure will be shown in HRTEM analysis.

Fig.3.(a) XRD patterns of W(Al60) mixed powder with different milling times;(b) Evolution trend of grain size and lattice strain during ball milling.

Fig.4(a)-Fig.4(c)provide the morphology evolution during the MA process.In the early stage,the powders undergo plastic deformation and become flakes (Fig.4(a)).Subsequently,the flake particles were cold welded and stacked together.Continuous plastic deformation led to severe work hardening.When the degree of work hardening exceeds a certain level,the particles begin to refine.Fig.4(b)and Fig.4(c)shows the morphology and EDS results of the powder milled for 50 h and 70 h,respectively.Considering the possible differences in the element ratio of small particle samples,we selected multiple small particles for EDS test to ensure the representativeness and repeatability of the date shown in Fig.4(b) and Fig.4(c).Although the diffraction peak of Al disappeared after ball milling for 50 h,the EDS results reveal that the element distribution was nonuniform.As shown in Fig.4(b),there was still a significant difference in the element ratio between region II and region III.However,when the milling time reached 70 h,the uniformity of the element distribution was obviously improved.In Fig.4(c),the W/Al atomic ratio in region I,II and III was close to 2:3,which is consistent with the design chemical ratio.

Fig.4.SEM images of W(Al60)mixed powders with milling time of(a)10 h;(b)50 h and(c)70 h;(d)HAADF image and elemental maps of obtained W(Al60)alloy powder;(e)SAED;(f) HRTEM and (g) Corresponding IFFT image of obtained W(Al60) alloy powder.

HRTEM observation was utilized to study the crystal structure of the alloy powder obtained by ball milling for 70 h.As shown in Fig.4(d),the HAADF image shows that the solvent and solute atoms have good dispersibility at the nanoscale,and the elemental maps of W and Al also demonstrated a good matching degree.Although the element distribution is uniform on the whole,it should be pointed out that the distribution of W still reflects certain local aggregation characteristics,indicating that the obtained W(Al60)alloy powder is not composed of a single-phase solid solution structure.Fig.4(e) gives the selected area electron diffraction(SAED)image of the prepared W(Al60)alloy powder.The diffraction rings correspond to different crystal plane groups of W,and the diffraction features of Al completely disappeared.In addition,the SAED image shows faint amorphous features.Through HRTEM image observation (Fig.4(f)),it can be confirmed that a small amount of amorphous phase existed around the crystal grains.Therefore,the prepared alloy powder with ball milling for 70 h was actually a mixture of solid solution and amorphous phase.The existence of amorphous phase also explains why the lattice strain in Fig.3(b) decreased after 40 h of ball milling.Under repeated impact and extrusion,the mixed powder experienced continuous deformation,and the lattice defect accumulated during this process.When the lattice defect exceeded the critical value,the lattice tended to collapse and transform into amorphous structure,and lattice strain in the crystalline region decreased.Besides,we also provide an inverse fast Fourier transform(IFFT)image to clarify the lattice defects in W(Al60)super-saturated solid solution powders.In the IFFT image,the lattice defects are reflected as the bending of the stripes.As shown in Fig.4(g),numerous lattice defects can be observed in the crystalline region.The lattice defects will accumulate with the extension of milling time;excessive lattice defects can lead to the formation of amorphous phase.Therefore,If the milling time is too long,the amorphous phase may gradually become the dominant phase of the alloy powder.

An important prerequisite for the application of W(Al60) alloy powder is good thermal stability.Therefore,the mixed powders with different milling times were heated to 1000°C in an Ar atmosphere to test their stability at high temperature.As shown in Fig.5,for the original powder,there was an endothermic peak P1 and an exothermic peak P2,which corresponded to the melting of Al and the formation of Al4W,respectively.Extending the milling time to 30 h,the intensity of peak P1 was significantly weakened,while peak P2 disappeared completely.After 70 h of milling,there was no obvious endothermic or exothermic peak in the temperature range from room temperature to 1000°C,indicating that Al atoms were completely dissolved into the W lattice and could stay stable below 1000°C.The good thermal stability of W(Al60)supersaturated solid solution powder provides more options for the preparation of W(Al60)-based composites.

Fig.5.DSC curves of W(Al60) mixed powder with milling time of 0 h,30 h and 70 h.

3.2.Mechanical properties of the W(Al60)-Al composite

The W(Al60)-Al composite based on W(Al60) super-saturated solid solution powder was fabricated by hot pressing.The density of W(Al60) alloy powder measured by pycnometer method is 9.30 g/cm3.According to the law of volume invariance,the theoretical maximum density (TMD) of the W(Al60)-Al composite is calculated to be 6.66 g/cm3.The actual density of the sample measured by Archimedes method was 6.44 g/cm3,which exceeded 96% of the TMD.XRD analysis shows that the W(Al60)-Al composite was a two-phase mixture of W(Al60) solid solution and Al matrix,and no intermetallic compounds were formed during the HP process (Fig.6(a)).The corresponding backscattered electron(BSE) image is given in Fig.6(b).It can be clearly seen that the W(Al60)-Al composite consists of two phases: the darker phase is the Al matrix,and the white phase is the W(Al) solid solution.Significantly,the fine W(Al60) particles exhibited obvious aggregation,and the split effect of W(Al60)particles on the Al matrix may affect the plasticity and strength of the W(Al60)-Al composite.

Fig.6.(a)XRD pattern and(b)BSE image of the W(Al60)-Al composite;(c)True stress-strain curves and macroscopic cracks under quasi-static loading;(d)and(e)Microstructure of tensile fracture.

Fig.6(c) shows the true stress-strain curves of the W(Al60)-Al composite under quasi-static loading.The quasi-static compressive and tensile strength of the composite are 320.4 MPa and 161.4 MPa,respectively.The macroscopic compression cracks propagate along the direction of 45°to the axial direction,indicating that the compression fracture mode is shear fracture.The cohesion strength of the weak interface between W(Al60) and Al matrix cannot effectively resist the shear load,which is the main reason for the lower compressive strength.According to Meyers's research on Al-Ta,Al-W,Al-Ni and other Al-based composite materials [12],the mechanical strength of composite materials with a “soft” matrix is generally low due to the lack of restriction on deformation and weak interfacial bonding strength.Similar conclusions were also obtained by Williamson and coworkers [49].The tensile properties of the materials are more sensitive to the interfacial bonding strength and original defects.As shown in Fig.6(d),numerous cracks occurred at the interface between the Al matrix and W(Al60).The separation of W(Al60) alloy powder from Al matrix is also the main inducer of tensile fracture.In addition,it was mentioned that some W(Al60) particles are gathered together by mechanical combination.The limited bonding strength makes these areas prone to cracks(Fig.6(e)).However,sufficient crushing is beneficial for the contact of the reaction components with air.The brittleness property may improve the sensitivity of the W(Al60)-Al composite under high speed impact [14].

3.3.Impact initiated experiment results

The impact initiated experiments were conducted through cylindrical projectiles against the test chamber.To study the response characteristics under different impact speeds,the tests were carried out at design speeds.According to the high-speed camera records,the impact induced initiation process of the W(Al60)-Al composite can be divided into four stages: perforate front plate,impact steel anvil,projectile fracture into debris clouds,chemical reaction and reaction product venting from the chamber.Fig.7(a)shows the reaction phenomenon of the W(Al60)-Al composite under an impact velocity of 1362 m/s.The violent flame vented from the chamber and the dazzling lights lasted for more than 200 ms.

Loving Gordon doesn t mean in any way that we love you any less. There is plenty of room for two wonderful fathers in our lives. Gordon always encouraged contact with you, never spoke6 a word against you or undermined our feelings for you. We respect the fact that you never voiced negative feelings about Gordon.

Fig.7.(a) Overpressure curves of W(Al60)-Al composite at different impact velocities;(b) Statistical diagram of fragment recovery rate and particle size distribution.

Chemical reaction will produce an instantaneous blast wave and decline rapidly.The continuous reaction of remaining debris clouds continue to replenish the pressure and produce a global quasistatic pressure in the chamber.Generally,the overpressure value is used as a standard to measure the reaction characteristics of ESMs.Normally,the overpressure value of an inert projectile (20 steel) impacting the target plate at a speed of about 1200 m/s is about 0.03 MPa (Fig.7(a)).In contrast,the W(Al60)-Al composite exhibits a peak overpressure of 0.13 MPa at impact velocity of 988 m/s,indicating that it is a low-threshold ESMs.In addition,the overpressure values corresponding to impact velocities of 1181 m/s and 1362 m/s are 0.16 MPa and 0.28 MPa,respectively,showing a significant positive correlation with the impact velocity.This is mainly related to the fracture degree of the projectiles.In the impact initiated experiment,the projectile will break into debris clouds after impacting the steel anvil.Severe plastic deformation caused a rapid temperature rise.When small debris clouds with high temperature contact with air,deflagration will occur and release chemical energy.Therefore,the degree of fragmentation has a significant impact on the reaction characteristics.Fig.7(b)shows statistical data on the size of particles recovered after the reaction.The average size of the recovered particles was significantly smaller at an impact speed of 1362 m/s,and the particles below 150 μm accounted for more than 70%,indicating that the projectile was broken sufficiently.Sufficient fragmentation induced more violent chemical reactions.And the fine dust produced by combustion will vent from the test chamber with the gas.Therefore,the fragment recovery rate is negatively correlated with the impact velocity,and the fragment recovery rate at 1362 m/s is only 18.6%.

3.4.Analysis of reaction products and mechanism

To determine the reaction process of the W(Al60)-Al composite under high speed impact,the reaction products were analyzed by XRD and SEM.Fig.8 shows the XRD pattern of recovered fragments.Compared with the original XRD pattern of the W(Al60)-Al composite in Fig.6(a),the phase composition of the reaction product is significantly more complex,including not only W,Al2O3and Al4W,but also WO3,W3O and other atypical tungsten oxides WOx(e.g.WO2.9and W19O55).Among them,the appearance of Al2O3and Al4W phases can be expected,which is the typical reaction product of most W-Al composite materials.Surprisingly,the existence of a variety of tungsten oxides indicates that W is involved in the oxidation reaction.

Fig.8.XRD patterns of reaction product.

SEM observations provide direct evidence for this conclusion.Fig.9(a) shows the cross-sectional morphology of a larger recovered fragment.The dark region corresponds to the Al2O3phase formed by the Al matrix reaction.In addition,there are a large number of lath phases and dispersed spot phases.Fig.9(b) is the enlarged view of this area.EDS analysis suggests that the lath phase is Al4W,while the dispersed spots are W particles.The formation and distribution of Al4W indicate that Al has been precipitated during the impact reaction,and the dispersed fine W particles are evidence of Al precipitation.Because the size of this kind of particle is large and wrapped by the Al matrix,it cannot contact enough oxygen.Therefore,the precipitated Al reacted with W in situ to form Al4W.In addition,considering that W(Al60) super-saturated solid solution powder is stable below 1000°C,the precipitation of Al indicates that the impact initiation experiment may be accompanied by a high-temperature field.

Fig.9(c) provides a representative morphology of the reaction product with a small size.Such regular spherical particles were formed by the solidification of molten liquid droplets,which confirms that a very high temperature was generated during the reaction process.According to the EDS results,the particle is mainly composed of three phases: the dark phase is Al2O3;the bright phase has a W/O element ratio of 2:3,so we think it is an insufficient combustion product of W;the light gray phase adjacent to Al2O3was also detected to be an oxide of tungsten,but the W/O element ratio of the bright phase is close to 1:3,which can be presumed to be the tungsten combustion product WO3(Fig.9(d)).Similar results can also be obtained in Fig.9(e)and Fig.9(f).Fig.9(e)shows an irregular large particle formed by the combustion and solidification of multiple small particles.A widely distributed WOXnetwork was found around the Al2O3phases.According to the EDS results,this phase may also be an insufficient combustion product of W.The above reaction product analyses prove that the W(Al60)super-saturated solid solution powder modified by MA has excellent reactivity.Driven by the impact kinetic energy and the combustion heat of the Al matrix,the W(Al60) particles successfully break the barrier of the high thermal inertia of W and deflagrate at the moment of contact air.In addition,the droplet-like reaction products indicate that the flame temperature is very high,which is conducive to the self-sustaining combustion of modified W(Al60)super-saturated solid solution powder.

In traditional ESMs with W addition,the role of W is mainly reflected in the improvement of density and penetration ability,resulting in poor energy release characteristics of ESMs[50].For the W(Al60)-Al composite,the energy release rate is significantly improved due to the combustion of W.According to the study of Ames [51],the reaction products venting from the hole can be ignored since the time of reaction is short and the diameter of the hole on front plate is small.Thus,the chemical energy released in the chamber can be expressed as follows:

where Δp is the peak value of quasi-static pressure,V is the chamber volume and is equal to 0.027 m3,and γ is the gas adiabatic index in the chamber,which is assumed to be 1.4.

The energy release rate per unit mass iswheremis the mass of the projectile.Data on the energy release rate are tabulated in Table 1.The energy release rate corresponding to the impact velocity of 1362 m/s reaches 2.71 kJ/g,which is 65%of the energy released by TNT explosion (4.19 kJ/g).Besides,Fig.10 summarizes the reaction rate of some typical Al-base composites[52-55].It is worth mentioning that the density of the W(Al60)-Al composite is 6.44 g/cm3,which is higher than that of other Al-basedESMs shown in Fig.10.Generally,high density materials have better penetration characteristics.On this basis,the energy release rate of the W(Al60)-Al composite is still at a high level among all the Albased ESMs,even slightly better than that of the Al-Ni composites,which makes the W(Al60)-Al composite has greater competitive advantages in ESMs.

Table 1 Energy release rate of W(Al60)-Al composite under different impact velocities.

Fig.10.Energy release rate of typical Al-base composites.

4.Conclusions

This paper reports a W(Al60)-Al composite based on W(Al60)super-saturated solid solution powder.The density of the material has reached 6.44 g/cm3,which is higher than most Al-based ESMs.The impact initiation experiments show that the W(Al60)-Al composite is a low threshold ESMs and has excellent energy release characteristics.The energy release rate reaches 2.71 kJ/g at an impact velocity of 1362 m/s,which is even better than that of Al/Ni ESMs.In the reaction product analysis,a variety of tungsten oxides were detected,proving that the modified W based on supersaturated solid solution is combustible under impact load.The high activity and combustion heat of Al help W overcome the barrier of high thermal inertia,and effectively make up for the low adiabatic flame temperature of W.The droplet-like reaction products imply that the combustion process of W(Al60)alloy powder is accompanied by a high temperature field,which is of great significance for the self-sustaining combustion of W.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China,[Award number: 11972372] and [Award number: U20A20231].