1987 Volume 51 Issue 8 Pages 715-722
Effects of austenitic phase (γ) stability on the low temperature tensile and Charpy impact properties of γ′-strengthened iron-based alloys have been investigated by using transmission electron microscopy and magnetic techniques. The mechanical tests at 4, 77 and 300 K were conducted on a series of Fe-(16-28.5)Ni-(14, 16)Cr-2.4Ti-(1.5, 2)Mo (mass%) alloys after aging at 973 K for 172.8 ks which yieled the same hardness level of 320-330 Hv.
The (18.5-21)%Ni alloys showed an enhancement in ductility at 4 and 77 K due to transformation-induced plasticity (TRIP), while the yield and tensile strengths and Charpy absorbed energy were almost independent of the γ-phase stability of the alloys. The tensile deformation characteristics at 4 K of the metastable alloys have been discussed on the basis of the morphology of the α′ phase as follows:
(1) Fine lath-like α′ formed at the intersections of microscopic {111}γ shear bands. The formation of the α′ is accompanied by a decrease in work-hardening rate at an eary stage of deformation and continues to near necking point for the alloys that cause the TRIP.
(2) Blocky or large lath-like α′ which grow from the fine α′ at a later stage of deformation where the amount of α′ phase exceeds about 4 mass%. This phase is hardener for the suppression of necking but acts as a site for stress concentration in subsequent straining.
