Study On The Effect Of Thermal Exposure On The Micstructure And Properties Of Laves Phase NbCr₂Alloys

Effect of thermal exposure on the room temperature mechanical properties of fourkinds of Laves phase NbCr₂alloys （Cr-9Nb、Cr-12Nb、Cr-33.3Nb and Cr-77.5Nb） invacuum were investigated as to study their thermal stabilities.The microstructralcharacteristics and fractograph during thermal exposure were also analyzed.It is found that the four NbCr₂alloys’ relative densities increase gradually under amuch small range with increasing the thermal exposure time,because they have a rearlyfull density before thermal exposure.When using Indentation Test Method measuredthe four alloys’s fracture toughness,there are no cracks amoung Cr-12Nb and Cr-9Nballoy before and after thermal exposure,so they have a good fracture thoughness,withincreasing thermal exposure time,Cr-33.3Nb alloy had lower fracture toughness,whenexposured at800℃、1000℃and1200℃for50hours,Cr-33.3Nb’s fracture toughnessdescreas as5.5MPa·m^1/2、5.2MPa·m^1/2and4.4MPa·m^1/2，Cr-77.5Nb’s fracturetoughness changed little during the thermal exposure,generally keepde in about6MPa·m^1/2;With increasing the thermal exposure time,Cr-33.3Nb alloy has lowerVickers hardness,but Cr-12Nb、Cr-9Nb and Cr-77.5Nb alloys’ Vickers increased at theearly time then decreased later.Cr-33.3Nb alloy had no strain before and after thermal exposure,its yield strengthdescreased gradually with increasing the exposure time, when exposured at800℃、1000℃and1200℃for50hours,it’s yield strength decreased by22.5%、26.1%and42.1%respectively.All of the Cr-9Nb、Cr-12Nb and Cr-77.5Nb alloys have yieldphenomenon and strain during exposure,thermal exposure had little effect on the roomtemperature compression yield strength of Cr-12Nb、Cr-9Nb and Cr-77.5Nb alloys.Cr-12Nb and Cr-9Nb alloys’ compression strength increased at the early exposuretime,and then descreased latter,they had a rising trend totally; Cr-77.5Nb alloy’scompressive strength increased with the exposure time increased.When exposured at800℃、1000℃and1200℃for50hours,Cr-12Nb alloy’s strain descrease by32%、1.6%and5.7%respectively,Cr-9Nb alloy’s strain descrease by10.9%、1.6%and15.8%respectively while Cr-77.5Nb alloy’s strain increase by146%、170%and188%respectively.Before and after exposure,the fracture analysis shows that Cr-33.3Nb alloy is allfor brittle fracture mode, Cr-12Nb alloy is mainly ductile fracture,with increasing exposure time,it changes gradually into brittle fracture,at the same time,Cr-77.5Nballoy’s fracture mode before exposure is all for brittle fracture, with increasingexposure time,there are more and more dimples in fracture surface structure,and it’sfracture mode changes into ductile fracture.Amoung the four components,Cr-77.5Nb alloy had the best thermal stability,Cr-9Nb alloy was better than Cr-12Nb alloy, and Cr-33.3Nb was the worst. Afterexposured at800℃、1000℃and1200℃for50hours,Cr-77.5Nb alloy’s compressioncompressive strengthes were3284MPa、3150MPa and3504MPa respectively,compression yield strengthes were2468MPa、2026MPa and2342MPa respectively,andthe strains were13.5%、20%and16.8%respectively.