Effect Of Long-term Exposure And Alloying On Thermal Sability Of Laves Phase NbCr₂Alloy

The thermal stability of Laves phase NbCr₂alloy （including Cr-33.3Nb,Cr-12Nb and Cr-77.5Nb） when exposed at1200℃for100hours in vacuumconditions were studied respectively in this paper. On the basis of that, the effect ofNi and Si on the thermal stability of Cr-12Nb and Cr-77.5Nb was investigated.The results show that after thermal exposure at1200℃for100h,themicrostructure of Cr-33.3Nb, Cr-12Nb and Cr-77.5Nb alloys has a certain degree ofcoarsening. Vickers hardness, fracture toughness and compressive strength ofCr-33.3Nb alloy decrease to6.68GPa,3.11MPa·m1/2and717MPa respectively.Vickers hardness, yield strength and plastic strain of Cr-12Nb alloy reduce to4.24GPa,1659MPa and5.9%respectively. Vickers hardness, fracture toughness andcompressive strength of Cr-77.5Nb alloy change to6.8GPa,5.96MPa·m1/2and2293MPa respectively. The thermal stability of those alloys decrease with thevariation of microstructure and mechanical properties during the thermal exposureprocess. In terms of reduction of thermal stability, Cr-33.3Nb alloy is the mostobvious, followed by Cr-12Nb alloy, while Cr-77.5Nb alloy is not obvious.During the entire vacuum thermal exposure process at1200℃for100hours,the relative density of （Cr-77.5Nb）-5at.%Ni and （Cr-77.5Nb）-5at.%Si alloys keephigher, the microstructure of those alloys is fine and homogeneous because of noobvious coarsening. At the same time, the room-temperature mechanical propertiesof those alloy are stable, especially fracture toughness keeps higher value. After100h thermal exposure, the value of yield strength, compressive strength and plasticstrain of （Cr-77.5Nb）-5at.%Ni alloy becomes to2058MPa,2915MPa and14.8%respectively. At the same time, those values of （Cr-77.5Nb）-5at.%Si alloy change to2098MPa,3045MPa and15.4%respectively. The results indicate that thermalstability of Cr-77.5Nb alloy can be improved by adding alloying elements Ni and Si.During the whole thermal exposure process, relative density of （Cr-12Nb）-5at.%Ni alloy maintains at around98.8%. The microstructure of（Cr-12Nb）-5at.%Nialloy is fine and uniform, no obvious coarsening is found. Meanwhile, fracturetoughness maintains well and Vickers hardness, which decrease to5.15GPa after100hthermal exposure, changes slightly after100h thermal exposure at1200℃,（Cr-12Nb）-5at.%Ni has yield strength of1406MPa, compressive strength of2170MPa,and plastic strain of9.5%. The results show that using Ni as alloy element can improve thermal stability of Cr-12Nb alloy.