Microstructure and Tensile Properties of the Fe-32%Ni-4%Co Alloy During Cryorolling and Subsequent Annealing

The Fe-32%Ni-4%Co alloy was cryorolled to produce thickness reductions of 10, 40, 60 and 90%; then, the strips with a final thickness of 0.52 mm were annealed at 893, 923 and 943 K for different soaking times. The x-ray diffraction measurements indicated that both thermally induced α ′-martensite ( α ′-M) and deformation-induced α ′-M were present in the tested alloy. The thermally induced α ′-M was mainly lath-shaped, while the morphology of the deformation-induced α ′-M gradually changed from lath-shaped to irregular-shaped according to transmission electron microscopy observations. After the 90% reduction due to cryorolling, the total volume fraction of α ′-M reached 75.6%, and the average ultimate tensile strength (UTS av. ) was significantly improved from 430.3 to 1306.8 MPa. Upon subsequent annealing at 923 K for 5 min, the tested alloy mainly consisted of a large number of fine grains and a few coarse grains (CGs). In addition, several deformed structures were also observed. As the annealing time increased to 10 and 30 min, the fraction of CGs increased, and a bimodal grain size distribution was obtained, while the deformed structures were still present. The temperature of the reverse phase transformation of α ′-M → γ was lower than the recrystallization temperature, which may be the main reason for the bimodal grain distribution. With this bimodal grain structure, the alloy exhibited a combination tensile strength of 543.3 MPa and an average elongation of 34.1%.