Tensile Properties of an Age-hardenable Fe-16Ni-4Si Alloy with Duplex Microstructures

Abstract

An Fe-16%Ni-4%Si alloy having a large hardenability due to a maraging mechanism was used to study the effect of microstructures present prior to aging upon the properties of tensile strength. In an attempt to obtain microduplex structures showing a good combination of strength and toughness, the present alloy was cold rolled by 80% at a martensitic state (i.e., marforming) after conventional solution treatment in the γ field, and then annealed at the temperatures in the α+γ two phase range followed by air cooling, and finally aged.The marforming was found to increase both the tensile strength (σu) and the reduction of area (ψ) due to the formation of proper duplex structures. Large uniform elongation (eu) was observed after annealing at the temperatures around 570°C, and this was attributable to a transformation plasticity of precipitated austenite which was stabilized thermally. by partitioning of Ni. The strength, however, was rather low and σu=120kg/mm2. When annealed at a temperature range between 600°C and 650°C (still in the two-phase range), the reverted austenite was transformed on cooling into martensite which was strengthened by agingproportionally to the martensite fraction; the resultant duplex (martensite+ferrite) structure had a strength of σu=158kg/mm2, and a plasticity ofψ=27% and eu=1%, which were better than the combination, σu=132kg/mm2, ψ=25% and eu=1% in the same alloy that was age-hardened conventionally without marforming and annealing.The marforming had a dominant effect of reducing transgranular brittle fracture as well as intergranular fracture associated with preexisting coarse austenite grains.