Bimodal α Phase Transformation and Strengthening‐Toughening Mechanisms in Ti–7Mo–4Al–3Nb–2Cr–2Zr Alloy via Synergistic Control of Cooling Rate and Thermal Cycling Frequency

Abstract

To systematically investigate the influence of thermal history on microstructure evolution along the deposition direction in Ti–7Mo–4Al–3Nb–2Cr–2Zr alloy during laser melting deposition, the morphological changes in the α phase have been mainly researched. Results indicate that the region from the substrate up to 20 mm in height consists of columnar grains, with the equivalent circle diameter decreasing from 344 to 308 μm. Between 20 and 40 mm, equiaxed grains are observed, with diameters decreasing from 260 to 246 μm. Concurrently, the α‐phase morphology evolves from basket‐weave to a bimodal structure, consisting of forked primary α p and dispersed secondary α s , and finally to acicular α. The grain boundary α phase transitions from a continuous to a discontinuous state, eventually forming a continuous thin film.The observed microstructural evolution is attributed to the shift in heat dissipation behavior with increasing height: from a predominantly unidirectional thermal gradient to multidirectional heat flow, which induces the columnar‐to‐equiaxed transition of β grains. Meanwhile, the decreased cooling rate promotes an increase in the initial size of the α phase, whereas the subsequent reduction in thermal cycling suppresses its coarsening. The ultimate tensile strength increases from 1038.0 MPa in region D to 1182.3 MPa in region B and subsequently decreases slightly to 1169.6 MPa in region A. Meanwhile, fracture toughness increases from 62.3 MPam 1/2 in region D to 71.5 MPa m 1/2 in region B, followed by a decrease to 59.0 MPa m 1/2 in region A. The mid‐upper region (B) exhibits an optimal strength‐toughness combination, mainly due to the refined α phase within the bimodal structure: lamellar α hinders dislocation motion, promoting accumulation at α/β interfaces and enhancing strength; meanwhile, nanoscale α accommodates localized plastic deformation during fracture, delaying crack initiation and improving toughness.