Microstructure evolution and property of annealed CoCrFeNiNbx alloys with low Nb content
The addition of small amount of Nb can strengthen FCC-type CoCrFeNi alloys because of solid solution strengthening and second phase strengthening.However,the solubility of Nb in FCC phase and the microstructure and properties of the alloys are not well understood at present.Therefore,based on thermodynamically calculated phase diagrams,the present work experimentally investigated the microstructure and microhardness of the CoCrFeNiNbx alloys containing 1%-5%(mole fraction)Nb in as-cast states before and after being vacuum annealed at 800-1200℃.Experimental results showed that,even with the addition of 1%Nb,the as-cast CoCrFeNiNbx alloy still contains about 3%of FCC+C14 eutectic structure.The Nb content in the latter solidified FCC phase can reach 2.4%-3.1%,and a needle-like nano Mg3Cd-type τ phase precipitates after 800℃annealing which lead to the microhardness of the alloy increases about 30HV.After annealing at 1000℃or 1200℃,the composition of the FCC phase is uniform,which contains 1.6%Nb and 3.1%Nb,respectively,and consists with the thermodynamic calculation.After annealing,the C14 Laves phase in the eutectic structure spheroidizes and grows,and the lamellar eutectic structure disappears,resulting in decreasing of the alloy hardness.Therefore,it is an effective method to improve the strength of the FCC-type CoCrFeNi alloy by adding less than 3.1%Nb and then applying solution and aging treatments for τ phase precipitating.
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