The effect of hydrogen on the mechanical properties of high purity iron II. Effect of quenched-in hydrogen below room temperature

Abstract

High purity iron specimens prestrained to 1–2% were hydrogen charged at room temperature and quenched to testing temperatures below room temperature. The yield stress of specimens with a residual resistivity ratio (RRR H) of 4600 or larger was decreased at and above 200 K and increased below 190 K. At 200 K a sharp yield point was observed in specimens with RRR H = 5500. After about 0.5% yield strain a rapid work hardening took place and the flow stress soon exceeded that of uncharged specimens. In low purity specimens (RRR H ≈ 3500) the yield stress was always increased by hydrogen. The effect of hydrogen disappeared on aging for 10–20 min at room temperature after charging. The above observations for the specimens with RRR H = 4600 or larger are satisfactorily explained by two hypotheses: (1) hydrogen atoms trapped at the screw dislocation core increase the mobility of screw dislocations; (2) edge dislocations introduced by prestraining are pinned or heavily dragged by hydrogen atoms in the core below 190 K.