The First Principles Study On The Alloying Effect Of Mo And Re In Tungsten And The Precipitation Mechanism Of M(W,Hf)-X(C,O,H) Compounds

The materials used in the engine of the roket and missles endure extremely poor environments such as high temperature and corrosion,and required to possess the high thermal stability and good mechanical properties at high temperature.The refractory elements tungsten(W)was selected as the base materials due to its high melting point,high modulus and low thermal expansion coefficient.However,the application of tungsten materials is limited by its low temperature brittleness,recrystallization brittleness and poor high temperature strength,which may be improved by adding Molybdenum(Mo),Rhenium(Re)and HfC particles.To date,the influencing mechanism of Mo and Re in W is still remainning unclear,and the formation of HfC in W is hard to control.Further research on these two aspects is needed.First-principles calculations can not only accurately predict and design the structures and properties of materials,but also reveal the influencing details in the atomic scale which is hard to be observed in the experiment.Therefore,the present work investigated the alloying effect of Mo and Re in W and the formation mechanism of HfC by using the first-principles methods.The results show that,The addition of Mo can facilitate the phase stability of W,decrease bulk modulus B,and increase the thermal expansion coefficient ?V and heat capacity CP.W50Mo50 possesses most stable phase stability of the W-Mo binary alloys;Mo can slightly reduce the hardness and strength of the W,and improve the ductility due to the overlap and hybridization of metallic bonding of Mo and W.The addition of Re may deteriorate the phase stability,and cause the formation energy changing from negative to positive when Re addition is about 4.8 at.%.Re can increase B,?V and CP.Both the increase of the strength and the ductility of Re for W is due to the enhancement of W-Re metallic bonding and the directional Re-W-Re bonding in Re rich regions.However,the ductility improvement is not obvious when Re addition is fairly small(<2.5 at.%).Out of the range,the effect of Re on the ductility of W is much more obvious than that of Mo.The formation energies of M(Hf,W)-X(C,O,H)compounds shows that,HfO2 has the lowest formation energy,and the reaction of Hf+O2?HfO2 has the lowest ?rG? at 298K?2600 K.This result indicates that HfO2 will be preferentially generated and HfO2 possess the most stability.While for HfC,the ?rG? of reaction Hf+C?HfC,is little,and therefore may not be the main formation way of HfC.For the HfO2 related reaction HfO2+3C->HfC+3W+2CO,HfO2+3WC?HfC+3W+2CO,HfO2+3W2C?HfC+6W+2CO,the corresponding ?rG? turns from positive to negative in the 1923?2580 K,and decreased with the increase of temperature.This result shows that,when temperature is exceeding 1923 K,the formation of HfC is dominant.While for reaction of HfO2+3Mo2C?HfC+6Mo+2CO,the ?rG?is positive,shows that this process is reversed and HfC is easy to discompose.