Solid-solution softening and thermally activated flow in alloys of Fe with 3at.% Co, Ni or Si

Abstract

Iron and its binary alloys with 3 at.% Co, Ni or Si were deformed at temperatures between 77 and 295°K at a strain rate of 2.5 × 10 −5/ sec. The thermal ( σ ∗ ) and athermal ( σ i ) components of the flow stress and the activation parameters for yielding ( V ∗ and ΔH ∗ ) were determined from analyses of double strain-rate cycling and premacroyield tests. Solid-solution softening occurs in Fe-3Si and Fe-3Ni at temperatures between 77 and 200°K. Nickel and Si decrease σ ∗ and ΔH ∗ in this temperature range. Solid-solution softening was not observed in Fe-3Co; σ ∗ and ΔH ∗ for this alloy between 77 and 295°K are essentially those for Fe. For all these materials, at temperatures of 200°K and below, the relations of σ ∗ with T, ΔH ∗ and V ∗ are consistent with the predictions of the lattice-hardening theory of Dorn and Rajnak. The component of stress attributable to solid-solution hardening appears to remain constant between 77 and 295°K. This component is about ten times greater in Fe-3Ni and Fe-3Si (15–20 ksi, 103–138 MN/m 2) than in Fe-3Co (1–2 ksi, 6.9–13.8 MN/m 2). Solid-solution hardening in these alloys can be related to solute atom size misfit, as indicated by lattice parameter measurements. The results suggest that solid-solution softening is greater in alloys which show pronounced solidsolution hardening at temperatures near 295°K; therefore, solid-solution softening also appears to be determined by solute atom size misfit. Solid-solution softening by Ni and Si appears to be most readily explained in terms of promotion of kink nucleation on screw dislocations as a result of relatively strong solute atom-dislocation interactions. The absence of solid-solution softening in Fe-3Co is attributed to the small atomic misfit of Co which results in negligible interactions between dislocations and solute atoms.