快速凝固Ni-Ge合金的组织演化与力学性能研究

Microstructural evolution and mechanical properties of rapidly solidified Ni-Ge alloys

采用铜型喷铸和自由落体两种方法实现了Ni95Ge5单相合金和Ni33Ge67共晶合金的快速凝固,对其组织演化规律、硬化机理和多尺度力学性能进行了系统研究.实验发现,随着合金熔体冷却速率的提升,Ni95Ge5单相合金的微观组织由粗大枝晶转变为等轴晶,Ni33Ge67共晶合金则由规则共晶逐渐细化并转为含少量初生Ge相的过共晶组织.两种合金均发生显著硬化,其中单相合金为晶界主导的硬化行为,共晶合金则为多相协同硬化.纳米力学分析表明,共晶合金的表面应力较大,卸载时出现更高的弹性回复,而单相合金更易发生塑性变形,其独特的组织形态与较小的应变硬化指数会显著增强合金硬化能力,冷速提升引起的硬度增幅高达56.6%,远高于共晶合金的6.3%.另一方面,两者呈现不同的硬度与强度关系,单相合金显微硬度与屈服强度的平均比值为2.75,共晶合金则为9.09,由此可知,高冷速引起的组织演变同样会提升合金材料的屈服强度.由于落管中微小液滴的冷速更高,快速凝固后其硬度与强度均显著优于喷铸合金.

Structural evolution and mechanical performances of rapidly solidified Ni95Ge5 and Ni33Ge67 alloys were investigated under two conditions involving drop-tube containerless processing and copper-mold injection casting.As the cooling rate increased,the coarse dendritic structure of the primary(Ni)phase for Ni95Ge5 alloy changed to equiaxed grains structure,and the microstructure of Ni33Ge67 alloy showed a transition from coarse(Ni Ge+Ge)eutectic to refined structure plus some primary Ge phase.Containerless processing can produce finer microstructures than those of injection casting owing to higher cooling rate.Nanoindentation measurements indicated that both alloys showed a remarkable enhancement of Vickers hardness along with the microstructural evolution,and the hardening mechanism for a single-phase alloy was found to be grain-boundary strengthening,while that of the eutectic alloy was attributed to the multiphase coordinated strengthening.The strain hardening exponents of Ni95Ge5 and Ni33Ge67 alloys were calculated as 0.28 and 0.77,respectively.The special morphology evolution and lower strain hardening exponent of Ni95Ge5 alloy remarkably improved the hardening ability.Its hardness increased up to 56.6%that was much higher than the eutectic alloy.Moreover,these alloys displayed different relations between hardness and strength.The average ratio of hardness to strength for Ni95Ge5 alloy was 2.75,while that for Ni33Ge67 alloy was 9.09.Both the experimental evidence and theoretical analysis revealed that obvious enhancements of hardness and strength were achieved using a high cooling rate;thus,the drop-tube processed alloy particles showed better mechanical properties than those of the as-cast alloys owing to their higher cooling rates.It is obvious that rapid solidification conditions can improve the mechanical properties of both the alloys to various degrees.