A New Ordered Transformation-Induced Plasticity Enables Exceptional Strength and Ductility in a B2 Medium-Entropy Alloy.

Ordered aluminide intermetallics are promising lightweight structural materials but suffer from limited room-temperature tensile ductility due to the constrained dislocation activity inherent to their ordered lattices. Here, we demonstrate that controlled reduction of phase stability can overcome this limitation in B2-ordered Ti-Zr-V-Al medium-entropy intermetallics. By tailoring the Al content to lower the stability of the B2 matrix while preserving long-range order, the metastable alloy activates an ordered transformation-induced plasticity (ordered TRIP) effect under tensile loading. This involves the cooperative operation of {110}<111> slip and an unprecedented stress-induced B2 to D019-α2 martensitic transformation. The transformation follows an orientation relationship of {0001}α2//{110}B2 and <11-20>α2//<100>B2, representing an atypical pathway distinct from conventional B2→B19' or B2→B33 transformations. The designed alloy achieves a tensile elongation of about 7%, an ultimate tensile strength of about 1.1 GPa, and a specific yield strength of about 197 MPa·g-1·cm3, a combination of strength, ductility, and density that surpasses most reported lightweight B2 intermetallics. By establishing a controlled metastability strategy that couples dislocation plasticity with a novel ordered martensitic transformation, this work provides a new paradigm for designing ductile, high-strength ordered alloys and may inspire similar approaches in other brittle ordered alloy systems.