Preparation,microstructure And Properties Of Powder Metallurgy NbMoTaWVCr_x Refractory High Entropy Alloys

High entropy alloys?HEAs?possess excellent properties,such as mechanical properties,corrosion resistance,electric and magnetic properties,radiation resistance and so on.It has a wide application prospect and important research value.HEAs were not found until 2004,there are still many theories need to be investigated.The refractory high entropy alloys show excellent properties such as high strength,high hardness and good thermal stability.At present,vacuum arc melting is the main technique used to prepare refractory high entropy alloys.It is inevitable to produce various casting defects in the HEAs.What's more,the vacuum arc melting requires high processing temperature,the prepared alloys consist of coarse grains,and the as-cast refractory HEAs usually show poor plasticity at room temperature,thus limits the development of refractory HEAs.In the present paper,NbMoTaWVCr_x?x=0,0.2,0.6,0.8,1?high entropy alloys were synthesized by mechanical alloying and spark plasma sintering.The main research results are as follows:The alloying sequence of NbMoTaWVCr_x refractory HEAs in the process of mechanical alloying was studied.It was found that the alloying sequence was related to the melting points of the elements,the lower the melting point,the faster the alloying speed was.After 40 h ball milling the alloying was basically completed,and homogeneous and fine HEA powders were prepared.The phase structure of the alloy powders was BCC single-phase.The effect of Cr content on the alloying speed was studied.It was found that the increase of Cr content would slow the process of alloying,mainly due to the sluggish diffusion effect of high entropy alloy hindering the interdiffusion of atoms.The phase composition,microstructure and mechanical properties of Nb MoTaWVCr_0.6refractory high entropy alloys were studied.It was found that after SPS,two kinds of precipitates formed,one was tetragonal structure?Ta,V?O₂ phase that often precipitates inside the grains and at grain boundaries and grain boundary,and the other was Laves phase Cr₂Ta,which was rich in Cr,Ta,Nb and V.The shape of the Laves phase was irregular,it often precipitated at the triple junctions.A phase transition from C15 to C14 occured in Cr₂Ta at a certain temperature.When the sintering temperature was 1450?and 1500?,the C15 Laves phase with cubic structure was formed.When the sintering temperature was 1550?and1600?,the C14 Laves phase with hexagonal structure was formed.With the increase of sintering temperature,the volume fraction of Lavess phase increased first and then decreased.When the sintering temperature was 1500?,the compressive yield strength,plastic strain and microhardnessof NbMoTaWVCr_0.6 at room temperature was 2902 MPa,8.2%,and 859Hv,respectively.NbMoTaWVCr_x?x=0,0.2,0.6,0.8,1?refractory HEAs were fabricated.The effect of Cr content on the phase composition,microstructure and mechanical properties of the sintered alloys was investigated.It was found that Cr played an important role in the formation of Laves phase in the alloys.When the content of Cr increased,the volume fraction of Laves phase in the alloys increased and percentage of the?Ta,V?O₂ phase decreases.When the content of Cr increased,the compressive yield strength of NbMoTaWVCr_x refractory HEAs was increased from 2756 MPa?x=0?to 3409 MPa?x=1?at room temperature,and the Vickers hardness increases from 795 Hv to 919 Hv.This was mainly due to the increasing effect of the solid solution strengthening and precipitation strengthening in the alloy with the increase of Cr content.The fracture mechanism of NbMoTa WVCr_x refractory high entropy alloy at room temperature was related to the sintering temperature.The fracture mechanism of alloy sintered at 1450?and 1500?is mainly intergranular fracture,with a small amount of shear fracture.When the sintering temperature rose to more than 1550?,the fracture mechanism of the alloy changed to the transgranular fracture,accompanied by a small amount of shear fracture.