Interfacial defects in Ti-Nb shape memory alloys

The structure of the martensite (alpha '')/marten site (alpha '') and parent (beta)/martensite (alpha '') interfaces in a series of binary Ti-Nb alloys with Nb content ranging from 20 to 24 at.% was investigated. Both the alpha ''/alpha '' and beta/alpha '' interfaces comprised a series of {2 (1) over bar 11}(beta)//{110}(alpha '')terraces and steps when viewed edge-on (close to [001](alpha '')). Interfacial defects, particularly the transformation disconnections (b,h) superimposed along the terrace-step interface structure, have been identified. They were responsible for accommodating most of the transformation strain along the alpha ''/alpha '' and beta/alpha '' interfaces. Using the parameters b and h, the prediction of the alpha '' habit plane based on the topological model agreed well with the prediction from the phenomenological theory as well with experimental observations. The alpha '' habit plane in Ti-20Nb alloy is close to {7 (5) over bar(5) over bar}(beta) and moves towards {4 (3) over bar(3) over bar}(beta) in Ti-22Nb and Ti-24Nb alloys. The remaining transformation strain along the beta/alpha '' interface was found to be accommodated by Type 1 twinning on (111)(alpha '') with Burgers vector b(LIS) close to <(2) over bar(1) over bar3 >alpha '' (equivalent to <(1) over bar(1) over bar(2) over bar >beta). (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.