Interfacial defects in Ti–Nb shape memory alloys

The structure of the martensite (α″)/martensite (α″) and parent (β)/martensite (α″) interfaces in a series of binary Ti–Nb alloys with Nb content ranging from 20 to 24 at.% was investigated. Both the α″/α″ and β/α″ interfaces comprised a series of {21¯1}β//{110}α″ terraces and steps when viewed edge-on (close to [001]α″). 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 α″/α″ and β/α″ interfaces. Using the parameters b and h  , the prediction of the α″ habit plane based on the topological model agreed well with the prediction from the phenomenological theory as well with experimental observations. The α″ habit plane in Ti–20Nb alloy is close to {75¯5¯}β and moves towards {43¯3¯}β in Ti–22Nb and Ti–24Nb alloys. The remaining transformation strain along the β/α″ interface was found to be accommodated by Type 1 twinning on (111)α″ with Burgers vector bLIS close to 〈2¯1¯3〉α″(≡〈1¯1¯2¯〉β).