The role of twinning in the cyclic stress-strain behavior of directionally solidified γ/γ′-δ

Abstract

Under cyclic straining of the γ/γ’-δ directionally solidified alloy a special type of hysteresis loop with an inflection in the compressive loading region was obtained. Observations gave direct evidence that the inflection was due to a process in which the 5 twins formed during tension untwinned under the subsequent compression. Thin foil electron microscopy established the 5 twins to be two specific variants of the four {211} variants, the selectivity being imposed by the crystallographic relation of the 5 phase with the γ/γ’ matrix. Characteristic distribution of (010) 5 faults was also noted in the foils and they were found associated with the {211} untwinning events. The compressive {011} 5 twinning mode reported in literature was not observed; the forward plastic strain in tension was reversed by {211} untwinning mode at a compressive stress level much lower than that required for the onset of the {011} twins.