The determination of the energy stored in a metal during plastic deformation

Abstract

A new apparatus for measurement of the stored energy is described and the results obtained are presented. The method involves the heating, in vacuo , of an annealed and a deformed specimen, of equal weight, by small heating elements located inside each specimen. During heating the specimens are kept at the same temperature as each other, and they are surrounded by a metal shield continuously adjusted to this temperature. The stored energy is calculated from measurements of the difference in the powers supplied to the specimens made by means of a differential wattmeter constructed for this purpose. The apparatus has proved to be capable of measuring, with an accuracy of ± 5 %, a difference of 0.1 cal/g in the energy supplied to the specimens in heating them through a temperature range of 240° C in a period of 40 min. Results are given for specimens of electrolytic copper deformed in torsion, and they show that the stored energy increases with deformation in a similar way to the torque equired to produce the deformation. Small amounts of some impurities appear to be responsible for large differences both in the recrystallization temperatures and in the shapes of the curves showing the release of the stored energy as a function of temperature, but the total stored energy is not changed markedly. However, in other cases impurities may produce a significant alteration in the stored energy. It is suggested that differences in composition are the cause of discrepancies between the present results and those of previous workers.