Microstructure Evolution And Mechanical Properties Of Directionally Solidified ZrCr Alloy

Zr alloy has the characteristics of high melting point,low neutron absorption cross section,good radiation resistance and oxidation resistance,and excellent corrosion resistance.It has broad application prospects in the nuclear industry,aerospace,biomedicine and other fields.However,Zr alloys still have problems such as high room temperature brittleness and insufficient high temperature strength,which limit their further wide application.In this paper,a Zr-Cr binary system alloy was taken as the research object,and ZrCr alloy with directional columnar grain structure was prepared by directional solidification technology,and the comprehensive mechanical properties of strong and plastic were greatly improved.The morphology of the solid-liquid interface and the evolution law of solidification structure were investigated,and the mechanism of the precipitation eutectoid sheets in the oriented eutectic structure of the alloy was explored.The main results achieved are as follows:1.During the directional solidification process of ZrCr alloy,the primary phase of cellularβ-Zr is precipitated by liquid-solid phase transformation,and then the remaining liquid phase undergoes eutectic transformation L→β-Zr+α-ZrCr₂,resulting in a primary worm-likeα-ZrCr₂andβ-Zr With the decrease of temperature,the eutecticβ-Zr undergoes solid-state phase transformation,and the secondaryα-ZrCr₂ phase is gradually precipitated;finally,the primary cellularβ-Zr occursβ-Zr→α-Zr+α-ZrCr₂ Analytical transformation,resulting in three lamellar structures ofα-ZrCr₂ andα-Zr.Therefore,the ZrCr alloy at room temperature is composed ofα-Zr phase andα-ZrCr₂ phase.2.Under the experimental conditions of this paper,with the increase of the directional solidification pulling rate,the alloy solid-liquid interface gradually undergoes the transformation of cellular dendrite→dendrite;the growth direction ofβdendrite gradually approaches the<100>preferred orientation,and the primary The size of the primary phase of cellularβ-Zr keeps getting smaller,and the size of the primary worm-likeα-ZrCr₂ phase precipitated in the eutectic structure keeps getting smaller,and theα-ZrCr₂ andα-Zr precipitated inβ-Zr are co-precipitated.Thinning of the lamella.3.With the increase of the directional solidification pulling rate,the tensile strength of the alloy along the directional growth direction increases gradually,and the elongation first decreases and then increases.The reason for the increase in strength is the continuous refinement ofβdendrites and the gradual consistency of the growth orientation.Therefore,when the pulling rate increases to V=100μm/s,the tensile strength of the alloy is the largest;the reason why the elongation first decreases and then increases It may be that:βdendrites gradually change from equiaxed growth to preferential growth under low rate conditions,but the growth orientation is inconsistent with the directional growth direction,and the growth direction ofβdendrites gradually coincides with the directional growth direction,resulting in a gradual increase in plasticity.4.The strength and plasticity along the growth direction of the alloy after orientation are significantly improved:the room temperature compressive strength is increased from 1005 MPa as-cast to 1351 MPa,an increase of 34.4%;the room temperature tensile strength is increased from as-cast 481 MPa to 576 MPa,an increase of 19.7%;Elongation increased from as-cast0.2%to 1%.