Solidification Microstructure Refinement And Mechanism Of NiCoCr Multi-Principal-Element Alloy

In recent years,multi-principal-element alloys have become an important research direction in the field of metal materials,and a variety of alloy systems with excellent properties have been developed.Among them,Ni Co Cr multi-element alloy has excellent room temperature and low temperature toughness,which is considered to be an alloy system with application potential.However,the solidification structure of Ni Co Cr multi-principal-element alloy is composed of coarse columnar grains,which leads to low strength and anisotropy of the alloy,and casting is an important way to produce complex structural parts,so the refinement of solidification structure of Ni Co Cr multi-principal-element alloy is very important for application.However,there is still a lack of systematic research on the solidification microstructure refinement of the alloys.Especially,many refining methods are difficult to implement for vacuum casting,so it is urgent to develop grain refinement methods suitable for vacuum casting.In this study,firstly,the solidification structure of Ni Co Cr multi-principal-element alloy is refined by solute effect.The addition of solute is suitable for vacuum casting,and can achieve fine grains and solid solution strengthening.By adding B element,the enrichment of solute at the front of the solid-liquid interface is promoted,and the constitutional undercooling in the melt is improved to promote the nucleation process,so as to refine the solidification structure of Ni Co Cr multi-principal-element alloy.With the increase of B element,the grains transform from columnar grains to equiaxed grains in the casting.When the B element content reaches 1.5at.%,the solidification structure of Ni Co Cr multi-principal-element alloy achieves complete equiaxed,and the average grain size is about 50 μm.Cr element and B element segregated at grain boundary to form borides.The yield strength and ultimate tensile strength of the alloy increase with the increase of B element content,while the alloy remains high elongation.In view of the accuracy requirements of some castings for composition,it is necessary to use a clean and pollution-free grain refinement method.In this paper,the mechanical vibration is further used to refine the solidification structure of Ni Co Cr multi-principal-element alloy.After the horizontal vibration is applied to the Ni Co Cr multi-principal-element alloy,under the condition of mechanical vibration,with the increase of vibration amplitude and vibration frequency,the area of columnar grains on the surface was reduced,the columnar grain size is reduced and the columnar grains gradually transform into equiaxed grains,and the shrinkage cavity of the ingot is gradually reduced at the same time.When the vibration parameter is 50 Hz and the amplitude is 2 mm,the casting completely transforms into equiaxed grain structure,and the average grain size is about 150 μm.In this study,the solidification structure of Ni Co Cr multi-principal-element alloy is refined by B alloying and mechanical vibration,which is of great significance to improve the quality of multi-principal-element alloy vacuum castings.