Solid-State Hydrogen Storage Properties of Ti-V-Nb-Cr High-Entropy Alloys and the Associated Effects of Transitional Metals(M=Mn,Fe,Ni)

Recently,high-entropy alloys(HEAs)designed by the concepts of unique entropy-stabilized mechanisms,started to attract widespread interests for their hydrogen storage properties.HEAs with body-centered cubic(BCC)structures present a high potential for hydrogen storage due to the high hydrogen-to-metal ratio(up to H/M=2)and vastness of compositions.Although many studies reported rapid absorption kinetics,the investigation of hydrogen desorption is missing,especially in BCC HEAs.We have investigated the crystal structure,microstructure and hydrogen storage performance of a series of HEAs in the Ti-V-Nb-Cr system.Three types of TiVCrNb HEAs(Ti_(4)V_(3)NbCr_(2),Ti_(3)V_(3)Nb2Cr_(2),Ti_(2)V_(3)Nb_(3)Cr_(2))with close atomic radii and different valence electron concentrations(VECs)were designed with single BCC phase by CALPHAD method.The three alloys with fast hydrogen absorption kinetics reach the H/M ratio up to 2.Particularly,Ti_(4)V_(3)NbCr_(2)alloy shows the hydrogen storage capacity of 3.7 wt%,higher than other HEAs ever reported.The dehydrogenation activation energy of HEAs’hydride has been proved to decrease with decreasing VEC,which may be due to the weakening of alloy atom and H atom.Moreover,Ti_(4)V_(3)NbCr_(2)M(M=Mn,Fe,Ni)alloys were also synthesized to destabilize hydrides.The addition of Mn,Fe and Ni lead to precipitation of Laves phase,however,the kinetics did not improve further because of their own excellent hydrogen absorption.With increasing the content of Laves phase,there appear more pathways for hydrogen desorption so that the hydrides are more easily dissociated,which may provide new insights into how to achieve hydrogen desorption in BCC HEAs at room temperature.

Ultrashort-time liquid phase sintering of high-performance fine-grain tungsten heavy alloys by laser additive manufacturing

Liquid phase sintering(LPS)is a proven technique for preparing large-size tungsten heavy alloys(WHAs).However,for densification,this processing requires that the matrix of WHAs keeps melting for a long time,which simultaneously causes W grain coarsening that degenerates the performance.This work develops a novel ultrashort-time LPS method to form bulk high-performance fine-grain WHAs based on the principle of laser additive manufacturing(LAM).During LAM,the high-entropy alloy matrix(Al_(0.5)Cr_(0.9)FeNi_(2.5)V_(0.2))and W powders were fed simultaneously but only the matrix was melted by laser and most W particles remained solid,and the melted matrix rapidly solidified with laser moving away,producing an ultrashort-time LPS processing in the melt pool,i.e.,laser ultrashort-time liquid phase sintering(LULPS).The extreme short dwell time in liquid(-1/10,000 of conventional LPS)can effectively suppress W grain growth,obtaining a small size of 1/3 of the size in LPS WHAs.Meanwhile,strong convection in the melt pool of LULPS enables a nearly full densification in such a short sintering time.Compared with LPS WHAs,the LULPS fine-grain WHAs present a 42%higher yield strength,as well as an enhanced susceptibility to adiabatic shear banding(ASB)that is important for strong armor-piercing capability,indicating that LULPS can be a promising pathway for forming high-performance WHAs that surpass those prepared by conventional LPS.