High Temperature Mechanical Behavior And Ablative Properties Of W-3Re-xHfC Alloys

Tungsten alloys is favored in the field of high temperature application because of its high melling point,high strength and good thermal conductivity.It is widely used in high temperature resistant and ablation resistant components such as rocket engine nozzle,satellite propeller nozzle and kinetic energy penetrator.With the rapid development of modern aerospace industry,more stringent requirements are put forward for the performance of tungsten alloys in extreme service environment,especially to ensure excellent high-temperature strength and good ablation resistance.Therefore,the research and development of tungsten alloys with high recrystallization temperature,excellent.high temperature strength and good ablation resistance has important theoretical significance and engineering application value.In order to develop tungsten alloy materials with excellent high-temperature mechanical properties and ablation resistance,W-3Re-xHfC(x=0,0.5 wt.%,1 wt.%,5 wt.%,10 wt.%)alloys with different HfC contents were prepared by powder metallurgy technology by adding Re and doping HfC particles as the reinforcing phase.The microstructure and high temperature properties of alloys were systematically analysed and tested.Coupled with the theory of solid solution strengthening,interface synergy and second phase strengthening,the high temperature deformation behavior of alloys was clarified.The effects of W-3Re alloy with different HfC contents were revealed on the friction and wear properties and ablation behavior and the friction and ablation mechanism of W-3Re-xHfC alloy in high temperature environment clarified.Finally,in order to explore the hot working ability of W-3Re-xHfC alloys and obtain appropriate process parameters,the thermal deformation behavior of W-3Re-5HfC alloy with optimal high temperature mechanical properties and ablation resistance is studied.The constitutive equation and hot working diagram are established,and the thermal deformation mechanism of the alloy is revealed.The addition of HfC improves the sintering activity of tungsten alloy powder,and the higher HfC content(0～10 wt.%),the smaller the average grain size of tungsten alloy,the more obvious the effect of increasing the recrystallization temperature and hindering grain growth of W-3Re alloy matrix.When the HfC content is less than 5%,HfC particles are evenly distributed in the grain boundary and grain interior of W-3Re matrix,which plays the role of pinning dislocation and grain boundary,so as to refine the grain,improve the matrix strength and uniform fracture strain ability.However,10%HfC has obvious agglomeration in W-3Re alloy matrix,which results in the reduction of densification and uniform strain of W-3Re alloy matrix.The friction coefficient of W-3Re-5HfC alloy increases(0.14～0.24)and the friction performance improves in the test temperature(RT-800℃).The microstructure of wear mark surface shows that the friction properties and wear mark surface morphology are mainly related to the source of wear debris,oxidation and tribo-chemical reaction.The wear mechanism of W3Re-5HfC alloy belongs to the interaction mechanism of adhesive wear and oxidation wear at room temperature.The wear mechanism changes to the interaction mechanism of adhesive wear,particle wear and oxidation wear with the gradual increase of friction temperature.The crosssectional morphology of the wear mark shows that there is an intermittently dispersed oxide layer at the wear mark,and a certain degree of plastic deformation has occurred under the oxide layer(800℃).The addition of HfC particles significantly improves the ablation properties of W-3Re alloy.The order of ablation rate is W-3Re-5HfC<W-3Re-10HfC<W-3Re-0.5HfC<W-3Re under the same ablation conditions(30 s,10 mm).Compared with other W-3Re alloys with HfC content,the W-3Re-5HfC alloy not only has high temperature compressive strength(1500℃,605 MPa),hardness(447.1 HV)and low thermal conductivity,but also has a high density(98.8%),so its high temperature ablation resistance is relatively excellent.The addition of HfC particles reduces the thermal diffusion coefficient and thermal conductivity of the alloy and effectively prevents external heat flow from diffusing into the interior of the material,leading to hightemperature heat only existing in the surface area of the material.At the same time,HfC and HfO2 particles with high melting point nail the matrix grains in the ablation process,which is conducive to the formation of the protective layer and improves the ablation resistance.The chemical composition of the ablated surface shows that the thermochemical oxidation of W-3Re matrix and HfC particles is the main ablation mechanism.Combined with thermal simulation test,the thermal deformation behavior of W-3Re-5HfC alloy was studied at different temperatures(1200 0C～1500 0C)and different strain rates(1 s1～0.001 s-1).The results demonstrate that the stress decreases with the increase of temperature or the decrease of strain rate.The constitutive equation between flow stress and temperature,strain rate and strain was constructed based on the Arrhenius model.At the same time,the hot working diagram was established based on energy dissipation rate and instability criterion,and the optimum temperature and strain rate parameter range during hot working were given.