A novel multiphase Zr–30Ti–7Nb–4Sn alloy consisting of β, α′ and α″ phases was designed and fabricated, with an aim to elucidate the mechanical mechanisms responsible for the deformation behavior in the multiphase Zr-based alloys involving two different martensites. Based on the synchrotron X-ray diffraction (SXRD) and electron backscatter diffraction (EBSD) analyses, these two different martensites can be unambiguously discriminated: one type is hexagonal closed-packed (hcp) α′ martensite exhibiting an orthorhombicity value of 1.00 and a typical acicular morphology, the other is orthorhombic α″ martensite possessing an orthorhombicity value of 0.97 and a lath-like morphology with (110)α″-type internal twinning structures. In situ SXRD reveals that during loading, no stress-induced martensitic (SIM) transformation occurs between β phase and α′ martensite, and the notable “stress-plateau” is attributable to the extensive β→α″ SIM transformation and the reorientation of pre-existing α″ martensitic variants. During unloading, the reverse α″→β SIM transformation occurs in an incomplete manner, eventually leading to the residual macroscopic strain (∼4.3%) that persists in the Zr–30Ti–7Nb–4Sn specimen after unloading.

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