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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