Technical Notes - Observations on Elevated-Temperature Tensile Deformation

DURING the last several years the authors have been conducting tests on a tensile machine that autographically records load-elongation data. In certain tests at elevated temperatures on face-centered cubic, hexagonal close-packed, and body-centered cubic materials (binary alloys of nickel, zirconium, and iron), the load-elongation curve as shown in Fig. 1 reaches a maximum load (Pmax,) at relatively small amounts of strain and a large amount of elongation occurs subsequently with a continuously decreasing load, until finally localized necking and fracture take place. This early maximum load behavior has been found without the accompanying evidence of localized necking as has been assumed by some, or of evidence of cracking as observed by others.' This behavior has also been observed by Nadai and Manjoinez whose results are in good agreement with ours. The prior strain, strain rate, test temperature, and crystal structure affect the observations. The influence of prior strain was observed in tests on unalloyed zirconium at 300°C in which specimens containing large amounts of prior strain (cold work) exhibited the type of curve shown in Fig. 1. No localized necking was observed after strain as much as that of point B. Annealed and recrystallized specimens of the same material showed a load-elongation curve with maximum load occurring after considerable strain hardening and at the beginning of localized necking. Interrupted tests (Fig. 2) show no loss in strength because of recovery or recrys-tallization during an interval at temperature without load. Recovery under stress such as found by Wood and Suiter3 in aluminum is now being investigated as one possible explanation. The strain rate is a contributing variable as shown in Fig. 3, a test on high purity nickel. Here the strain rate was decreased by a factor of ten during a test in which the maximum load was reached at a small strain. The slope of the true stress-true strain curve was positive with the more rapid strain rate and negative with the slower strain rate. In many binary ferrites an added effect of temperature has been observed. Fig. 4 shows the load-elongation plots for a binary ferritic alloy containing 2 pct Mo. The slope of the small portion of the stress-strain curve obtained at a strain rate of 0.01 mhr1 changes sign from negative to positive on going