Activation energy calculations for discontinuous yielding in Inconel 718SPF

Abstract

The kinetics of yield point return and of dynamic strain aging (DSA) were investigated for a commercial heat of Inconel 718SPF. During uniaxial tensile tests, the alloy exhibited serrated flow on the load–elongation curves at intermediate temperatures. Types A, B and C serrations were observed depending upon the test temperature and strain rate. For type A and B serrations, apparent activation energies ranging from 97 to 133 kJ mol⁻¹ were measured, which suggests that type A and B serrated flow results from the diffusion of an interstitial solute, most likely carbon. For type C serrations, apparent activation energies in the range ∼210 to ∼272 kJ mol⁻¹ were measured, which suggests that type C serrations arise from the diffusion of a substitutional solute, most likely Cr. Yield points induced during strain aging under stress experiments increased with a tⁿ relationship, where the exponent n ranged from 0.3 to 0.5 before reaching a plateau. This suggests that the yield points were a combined result of bulk lattice diffusion and dislocation pipe diffusion of solutes. The apparent activation energies, ranging from 42 to 50 kJ mol⁻¹, are consistent with prior observations, but cannot be explained by any classic models for static strain aging. It is suggested that the yield points result from the reorientation of carbon-vacancy pairs in the strain fields of dislocations.

Introduction

Strain aging is a time dependent strengthening or hardening caused by elastic interactions between mobile dislocations and diffusing solutes or point defects. This phenomenon, which is common in many metals and alloys, often manifests itself in the form of: yield stress plateaus, reduced tensile elongation, sharp upper yield points, flow stress transients upon an upward change in strain rate, negative strain rate sensitivity and serrated stress–strain curves, also known as the Portevin–LeChatelier (PLC) effect. In Ni-based superalloys, several of the aforementioned manifestations of strain aging have been reported [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. In aged Waspalloy [1], René 88DT [4] and Inconel 718 [7], [8], [9], [11], [13], for example, negative values of strain rate sensitivity and a corresponding PLC effect have been observed. In all of these studies, the flow stress serrations were preceded by a small critical onset strain, ε_c. In many alloys, ε_c increases with decreasing test temperature (normal behavior), however, in peak aged Waspalloy and Inconel 718, researchers have observed regions where ε_c decreases with increasing temperature (inverse behavior). Hayes and Hayes [1], [2] determined that the PLC effect in Waspalloy was the result of the diffusion of carbon atmospheres to mobile dislocations. They further proposed that the change from normal to inverse behavior was a result of interactions between carbon atmospheres and the strengthening precipitates, γ′-Ni₃Al in Waspalloy. In this case, the precipitates act as a sink for carbon, which diffuses down the dislocation line to the sink, while the line is arrested at the precipitates. Recently, Chen and Chaturvedi [11], noting that γ″ is the major strengthening phase in Inconel 718, observed that ε_c was proportional to interparticle spacing. In underaged specimens, where precipitates are coherent and easily cut by dislocations, they found that ε_c decreased with increasing particle spacing, L, whereas ε_c increased with L in overaged specimens where Orowan looping was the dominant mechanism. The differences were attributed to changes in the waiting time of dislocations at particles in overaged and underaged material.

In a related study, Dybiec [8] noted the occurrence of sharp upper yield points in Inconel 718 aged under stress at intermediate temperatures. Dybiec observed that the magnitude of the upper yield point increased with time according to a power law relationship, i.e. Δσ=tⁿ, where n represents the aging time exponent. He suggested that these yield points were the result of strain aging though the magnitudes of the aging time exponent and the activation energy for yield point return were inconsistent with existing strain aging data or models. Blankenship and Henry [6] observed similar yield points in prestrained and aged Inconel 718 and René 88DT. They also suggested the occurrence of strain aging, though no definitive mechanism was identified.

In the present paper, the kinetic aspects of static and dynamic strain aging were investigated in a commercial heat of Inconel 718SPF. These results are presented and discussed with respect to prior experimental observations and classical mechanisms for strain aging.