Tailoring the Bain strain of martensitic transformations in TiNb alloys by controlling the Nb content

•Bain strain of β → α′/α″ in TiNb is studied emphasizing its hydrostatic part.•Middle principal strain λ2 becomes zero for ≈21.0 at.% Nb (≈34.0 wt.%).•Volume dilatation is highest (about +1%) close to compositional α′/α″ boundary.•For all compositions, martensite α′ or α″ with the lowest atomic volume forms.•Strain energy calculations match the observed volume dependent transformation path.

The lattice (Bain) strain and corresponding volume change of a martensitic phase transformation are key parameters that determine the formation of a self-accommodated martensitic microstructure and related functional properties. Based on experimentally determined lattice parameters of the cubic austenite β and of the hexagonal and orthorhombic martensites α′ and α″, the lattice strain and the volume change related to the β → α′/α″ martensitic transformations in TiNb alloys are analyzed in dependence of composition. The lattice strains of the transformation β → α″ increase with decreasing Nb content but those of β → α′ are not affected by the amount of Nb. Depending on the composition, both a volume dilatation and contraction is encountered. It is proposed that the volume change plays a decisive role in determining the lattice structure of the martensitic phase that evolves when the transformation is thermally induced.

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