PSEUDOTWINNING AND PSEUDOELASTICITY IN BETA FE-BE ALLOYS

Pseudoelasticity in β Fe-Be has been observed and rationalized in terms of pseudotwinning. a type of stress-assisted martensitic transformation which arises from the interaction of long-range order and twinning in this alloy. The ordered structure, which gives rise to a chemical contribution for the reversion of the pseudotwin upon release of the stress, is not homogeneous DO3 as was previously reported, but a decomposed α + B2 microstructure. A crystallographic model predicts that the pseudotwinning process is describable by the stress-assisted martensitic phase transformation Pm3m (B2) → Cmmm. The electron diffraction evidence presented here to confirm this prediction is believed to be the first experimental proof of long-standing theories regarding the interaction of order and twinning. The macroscopic mechanical behavior of β Fe-Be has been investigated in compression at room temperature. Maximum pseudoelasticity is observed for specimens water-cooled and aged at 325-425°C. which gives rise to a modulated α + B2 microstructure. Aging at higher temperatures causes a sharp drop-off in pseudoelasticity and the disappearance of the modulated microstructure. In addition, pseudotwins in specimens exhibiting little pseudoelasticity always have accommodation dislocations associated with them. It is concluded that the modulated microstructure promotes pseudoelasticity by strengthening the parent matrix, thereby enabling it to accommodate the pseudotwins in an elastic manner. © 1979.