Effect of Thermal Cycling on Microstructures and Mechanical Properties of Lath and Lenticular Martensites in Fe-Ni Alloys.

Abstract

Effects of thermal cycling on the microstructures and mechanical properties of Fe-15%Ni and Fe-31 %Ni alloys having lath and lenticular martensite, respectively, have been studied. The average width of laths in lath martensite of Fe-15%Ni alloy decreased with the increase in number of thermal cycles, and the width of internal twins in lenticular martensite of Fe-31 %Ni alloy also decreased with thermal cycling. The hardness of martensite increased up to 3 thermal cycles in Fe-31 %Ni alloy, over which it remained constant. On the other hand, the hardness of martensite in Fe-15%Ni alloy increased up to 1 cycle, and remained nearly constant on further cycles. The tensile strength of martensite in Fe-31 %Ni alloy increased up to 3 cycles, showing a similar tendency of the hardness variation. However, the tensile strength of martensite in Fe-15%Ni alloy decreased with increasing the number of thermal cycles in spite of the increase in hardness with thermal cycling. The reason is that no strain hardening occurred in the thermal-cycled specimens owing to the brittle fracture due to the segregation of sulfur at prior austenite grain boundaries.