Study On Phase Transformation Characteristics And Mechanical Behaviors Of CrCoNi Alloys

CrCoNi medium entropy alloy(also known as multi principal component alloy)has become a research hotspot in recent years because of its good strength and plasticity.The physical sources of its excellent properties mainly include high entropy effect,lattice distortion effect,hysteretic diffusion effect and cocktail effect.Recent studies on the mechanical behavior of CrCoNi alloy show that the plastic deformation process of CrCoNi alloy is often accompanied by martensitic transformation,and the interaction between martensite and dislocation contributes to the excellent work hardening ability and crack propagation resistance of the alloy.Therefore,the behavior and law of martensitic transformation in CrCoNi alloy deserve attention.The martensitic transformation in CrCoNi alloy is mainly closely related to Co and Ni elements,and the transformation is generally difficult to occur when the Cr content is high.Therefore,based on the Co Ni binary alloy,a series of CrCoNi alloys were prepared by means of melting,solution treatment,cold(hot)rolling and heat treatment by adjusting the content of Co and Ni elements and adding a small amount of Cr elements The phase transformation characteristics were studied by means of resistance and scanning electron microscope.The mechanical behavior was preliminarily explored by means of uniaxial tension and nano indentation,and the influence law of element content on the phase transformation and mechanical behavior of the alloy was tried to be mastered.The main results are as follows:It is found that the transformation point of Co Ni binary alloy decreases below room temperature with the increase of Ni content.The addition of Cr does not directly affect the phase stability of the alloy,but indirectly affects the phase stability by affecting the ratio of Ni,Co and(Co + Ni)-the phase transformation point of CrCoNi alloy increases with the decrease of Ni content.It is roughly estimated that when the Ni/(Co + Ni)value of CrCoNi alloy is greater than 0.3275 at room temperature,the alloy is composed of a single fcc phase;Less than this value,hcp phase is produced.Based on the existing data of 26 CrCoNi alloys in our research group,it is found that Ni content plays a major role in the phase stability of the alloy.When Ni(at.%)> 32%,the alloy is composed of fcc phase at room temperature regardless of the proportion of Cr and Co elements.The tensile test results show that CrCoNi alloy exhibits the best strength plastic combination at room temperature after annealing near the recrystallization temperature.The mechanical behavior of CrCoNi alloy is related to the volume fraction of fcc and hcp phase,and the lower the value of Ni /(Co + Ni),the weaker the stability of fcc phase,and martensitic transformation is easy to occur in the tensile process.The phase transformation leads to an increase in the work hardening rate,relieves stress concentration,and has a plasticizing effect.The results of nano indentation tests show that the value of Ni /(Co + Ni)in CrCoNi alloy decreases and the hardness of CrCoNi alloy increases.The substitution of Cr for Co(Ni)changes the value of Ni /(Co + Ni)and affects the stability of fcc phase.With the decrease of Ni /(Co + Ni),the stability of fcc phase decreases and martensitic transformation is more likely to occur.The hardness of hcp phase produced by transformation is high,so the hardness of the alloy increases.