Equiaxed grained structure: A structure in titanium alloys with higher compressive mechanical properties

We report an equiaxed grained (EGed) structure in Ti66Nb13Cu8Ni6.8Al6.2 bulk alloys with higher compressive mechanical properties, which was fabricated by the spark plasma sintering of milled alloy powders. Microstructural analysis indicates that all of the alloys consolidated from the alloy powders with different milling times consist of the same phases, β-Ti and (Cu, Ni)-Ti2, but exhibit different phase contents, morphologies, and distributions. The alloys consolidated from alloy powders that were milled for less than 30 h exhibit a typical bimodal microstructure with EGed (Cu, Ni)-Ti2 regions surrounded by a Widmanstätten structure-like matrix that encompasses bcc β-Ti regions and the nanosized interleaving of plate-like fcc (Cu, Ni)-Ti2 lamellae. In contrast, the alloys consolidated from alloy powders that were milled for more than 30 h have predominant EGed structures, i.e., EGed (Cu, Ni)-Ti2 regions embedded in an EGed β-Ti matrix. With an increasing volume fraction of the EGed structure or a decreasing volume fraction of the Widmanstätten structure, the alloys exhibit an invariant compressive yield strength and increasing compressive fracture strength and strain. Fracture mechanism analysis based on TEM observations indicates that under compression stress, cracking occurs preferentially at the grain boundaries of soft EGed (Cu, Ni)-Ti2 regions and at the interfaces between soft EGed (Cu, Ni)-Ti2 and hard EGed β-Ti regions, while the penetration of the hard β-Ti regions due to cracking in the middle of the interleaving of the plate-like (Cu, Ni)-Ti2 lamellae deteriorates the compression fracture strength and strain of the alloys with a Widmanstätten-structure matrix. The obtained results further confirm a promising method for fabricating fine EGed bulk titanium alloys with excellent mechanical properties by powder metallurgy.