The Research On Solidification Behavior Of Zr-Based Bulk Amorphous Alloy And Composites

The solidification behavior of the alloy directly affects the properties of the material.The cooling rate possesses a more obvious influence on the solidification structures of the alloy.Only when the cooling rate is beyond the critical cooling rate Rc,amorphous structure can be formed.This research work follows our previous work,wedge-shaped samples and rod samples of Zr₇₂Cu₂₈,(Zr_0.72Cu_0.28)₈₈Al₁₂,(Zr_0.72Cu_0.276Ag_0.004)₈₈Al₁₂,and(Zr_0.72Cu_0.165Ni_0.115)₈₈Al₁₂2 alloys were prepared.X-ray diffraction,room temperature compression test,microhardness test and scanning electron microscopy were used to test and analyze the solidification structure and mechanical properties.The results are as follows:?1?Four kinds of alloy wedge-shaped samples were prepared and their corresponding amorphous composites were obtained.Firstly,the optimal etching process of four kinds of amorphous composites was explored,and the solidification structure of the wedge-shaped samples was studied.The Zr₇₂Cu₂₈8 wedge-shaped samples have two areas:the composite area and the crystal area.The cross-sectional width of the wedge-shaped sample b<2.68mm means composite region,organized as columnar crystal and amorphous network structure.The crystal generated in the cross-sectional width b>2.68mm,the organization is equiaxed crystal and columnar crystal.The other three types of alloy wedge-shaped samples were divided into three regions:amorphous regions,composite regions,and crystal regions.For the(Zr_0.72Cu_0.28)₈₈Al₁₂2 wedge-shaped sample,width b<2.06mm means amorphous region and the microstructure presents network.2.06mm<b<4.18mm means composite region,and the microstructure presents network and columnar crystals,b>4.18mm means the crystal area,the microstructure presents columnar crystal and equiaxed crystal.For the(Zr_0.72Cu_0.276Ag_0.004)₈₈Al₁₂2 wedge-shaped samples,width b<3.22mm means amorphous region and a network structure.3.22mm<b<4.72mm means composite region and the microstructure presents network and equiaxed crystals.b>4.72mm means crystal region and the microstructure presents equiaxed crystal.For the(Zr_0.72Cu_0.165Ni_0.115)₈₈Al₁₂2 Wedge-shaped samples,width b<4.52mm means amorphous and the structure is a network structure;4.52mm<b<7.36mm means composite region with a network structure and equiaxed grains.b>7.36mm means the crystal region,the microstructure presents equiaxed crystal.?2?The surface hardness distribution of(Zr_0.72Cu_0.28)₈₈Al₁₂2 wedge sample was measured and plotted.It was found that the hardness of amorphous region,recombination region and crystal region increased in turn.The transition region is irregular,and they are not a theoretical parabolic shape.The hardness values of the amorphous and composite regions are relatively stable,and the hardness values are in the range of 450-495 and 495-560,respectively,which is due to the difference in the cooling rates of the points in the two regions,but the difference is not significant.Thus the performance is relatively stable.The hardness of the crystal area fluctuates relatively large,in the range of 530-630.The crystals of the same component of the melt formed at different cooling rates may cause little difference in their microstructure under the microscope,but their hardness values may differ greatly.Under the light microscope,the crystal area is basically columnar crystals and equiaxed crystals,but it can be seen from the hardness distribution chart that the columnar crystals formed near the edge where the cooling rate is relatively high are relatively high in hardness.The equiaxed crystals formed with a lower cooling rate in the heart are less rigid.The surface hardness of the surface layer formed on the copper mold wall reached a maximum of 700.?3?Four kinds of alloy rods with different diameters were prepared.The influence of cooling rate on the microstructure and mechanical properties of Zr-based amorphous alloys were studied.The results show that increasing the cooling rate can improve the plasticity and strength of amorphous alloy samples.Among them,the(Zr_0.72Cu_0.276Ag_0.004)₈₈Al₁₂2 alloy 2mm sample possess the best plasticity,reaching31.29%,the highest strength,and the yield strength and compressive strength reach1916MPa and 2934MPa,respectively.Comparing the mechanical properties of samples of the same diameter with different compositions,it was found that microalloying can significantly promote the strength and plasticity of the alloy.(Zr_0.72Cu_0.28)₈₈Al₁₂2 is even more plastic and stronger than(Zr_0.72Cu_0.165Ni_0.115)₈₈Al₁₂with adding of small amount of Ag.?4?Since the cooling rate is very fast,there is no macroscopic segregation in the whole wedge-shaped sample,and the distribution of components in the amorphous region,the composite region,and the crystal region is also uniform.The transitional zone microstructure is more complex and there is segregation of components.