W.C. Hayes’s research while affiliated with CA Technologies and other places

In the present study on AISI 1020 and 2 1 4 Cr-1 Mo steel, serrated flow is found to disappear at higher temperatures and lower strain rates by (1) a progressive delay to the onset of serrated flow and by (2) a disappearance of serrations off the end of the flow curve. The activation energy for the disappearance by the delay process was found to be 1.15 eV for the 1020 steel and 1.35-1.6 eV for the 2 1 4 Cr-1 Mo. The delay to the onset of serrations in both materials is proposed to be due to the presence of either Fe3C or complex metal carbides which act as a second phase sink depleting the arrested dislocations of their atmosphere species. In both materials the strain delay is found to increase with decreasing particle spacing. In temperature ranges where the strain delay exists, serrations appear on the flow curve only after the delay sinks approach saturation. Serrations then progress until disappearance off the end of the flow curve occurs. The disappearance of serrations off the end of the flow curve in both materials is proposed to occur by a reaction between the carbon atmospheres on arrested dislocations and the surrounding Fe atoms. This reaction occurs nearly simultaneously at all reaction sites with no long-range diffusion of either reactant required.

The Portevin-Le Chatelier effect was investigated for a nickel-based γ′ age-hardened alloy. The activation energy for strain aging was found to be 0.57 to 0.68 eV which is consistent with a carbon atmosphere aging mechanism. Stress decrement (σD) as a criterion for strain aging was found to be effective in determining the activation energy for strain aging at temperatures where critical strain (ε{lunate}c) values were giving anomalous results. Lower temperatures gave normal ε{lunate}c behavior while higher temperatures gave an anomalous delay to the onset of serrated yielding. It is proposed that this delay is the result of the Ni3(Al, Ti) precipitate acting as a sink for carbon which diffuses down the dislocation line to the sink while the line is arrested at the precipitates.