Titanium alloy with synergistic enhancement of strength and toughness based on molybdenum equivalent design: Microstructure evolution and strengthening-toughening mechanism
Authors: Yi-li Li, Hong-ze Fang, Rui-run Chen, Jia-qi Hao, Bao-hui Zhu, Jing-jie Guo
The traditional "trial and error" microstructural control method, with high cost and low efficiency, has become a key issue restricting the development of ultra-high strength and toughness titanium alloys. This study adopts the molybdenum equivalent (Mo[eq]) method to rapidly design Ti-xMo-4Al-4Zr-3Nb-2Cr-1Fe alloys (x=5-9). The as-cast alloys with different Mo[eq] exhibit a single peak of the β phase in XRD. The β grains of 5Mo alloy (the lowest Mo[eq]) exhibit elongated columnar grain characteristics. As the Mo[eq] increases, the β grains transition towards a more equiaxed form, resulting in a decrease in aspect ratio and a reduction in grain size. As the Mo[eq] increases, the α phase content gradually decreases and the α phase is almost unobservable in 9Mo alloy (the highest Mo[eq]). The α phase in 5Mo alloy exhibits short rod-shaped shapes with an average length of about 2.4 μm, while the α phase in 6Mo alloy shows an equiaxed and short rod shapes with the smallest size. The strength, plasticity, and toughness are the lowest in 5Mo alloy, with values of 867 MPa, 7.3%, and 56 MPa·m1/2, respectively. However, it reaches its maximum in 6Mo alloy, where the strength, plasticity, and toughness increase to 984 MPa, 12.8%, and 74 MPa·m1/2, respectively. The mechanical properties of Ti-xMo-4Al-4Zr-3Nb-2Cr-1Fe alloys are affected mainly by solid-solution strengthening of Mo element, refinement of β grain, and changes in α/β phase content. This study lays a certain theoretical foundation for the theoretical research and composition development of new ultra-high strength and toughness titanium alloys.
