Materials Science Forum Vols. 747-748

Abstract: A newly near-β titanium alloy Ti-7Mo-3Cr-3Nb-3Al (Ti-7333) was subjected to β phase solution treatment and ageing in the present work. The characteristics of α phase transformation in ageing treatment were studied. Results show that isothermal aging at a low temperature (350) will result in lots of ω particles with small size homogeneously distributing in the parent phase. These ω particles can act as nucleation sites for α phase and lead to the uniform precipitation of fine α phase within the β grain after further ageing treatment. However, when ageing at a higher temperature, the α phase tends to precipitate direct from the β matrix and the morphology of α phase is determined by the temperature and period of ageing treatment. After aging at 550 for 5min, acicular α phase precipitates in the β grains as well as along β grain boundaries and the size and quantity of α phase increase with the holding time. Note that Ti-7333 alloy has a quick ageing response. When aging at 700 for 1h, coarser α laths precipitate both on the grain boundary and within the grain. Increase the ageing temperature to 800, α phase precipitates within the β grain as short rod-like morphology. It is suggested that the driving force for α phase nucleation and the amount of defects in the intragranular decrease with the increasing of temperature, leading to the grain boundaries become the prior nucleation sites. Substantial α phase precipitate-free regions adjacent to β grain boundaries remained after ageing at 700 for 1h due to the rejection of β-stabilizer from coarse α lath on β grain boundaries. Aging at 800 for 1h resulted in pronounced continuous α-films along β grain boundaries.

Abstract: After two-stage annealing heat treatment process, the near β-Titanium alloys reveal a mixed microstructure containing lath-like α phase and finer acicular α phase in β matrix, leading to the improvement of strength-ductility balance. In this paper, the microstructural evolution and the behaviour of α precipitate during high temperature ageing process were investigated by SEM in a near β-Titanium alloy called Ti-55531.The relationship between α precipitates and the β grain orientation in high temperature was investigated by EBSD. The results show that the α-phase precipitated only at some places of the β grain boundaries at higher ageing temperature (~780 °C); the amount of grain boundary α increased with the decreased of the ageing temperature; after ageing at 720 °C for 45 min, we found that the α-phase precipitated not only at grain boundaries but also within the grains. It seems that the precipitation of grain boundary α is strongly influenced by β grain boundary energy which means that grain boundary α tends to form preferentially at high energy grain boundaries (high-angle grain boundaries); The α-phase precipitates more easily at the grain boundaries where the {110} plane of adjacent β grains have the same orientation.

Abstract: The effect of common solution treatment, two-step solution treatment and aging, solution treatment and aging (STA) on the microstructure and mechanical properties of a new low cost titanium alloy used in aviation field were investigated by optical microscope (OM), scanning electron microscopy (SEM) and tensile test. The results show that a typical equiaxed structure can be obtained by common solution treatment leading to a good combination of strength and plasticity. Besides, solution heat treating in the β region and subsequently ageing at a low temperature results in a significant increase in mechanical strength and a little decrease in plasticity. When the solution temperature is at α+β two-phase region (895), the low cost titanium alloy acquires the best combination of strength and ductility.

Abstract: In this work, 2.5vol. % (TiB+TiC)/Ti composite was prepared by in situ casting route then 1-D forging. The microstructure and tensile properties were presented and discussed. The results indicate that the as cast microstructure can be significantly modified by 1-D forging. After forging, TiB and TiC segregated at the prior β grain boundaries within the as-cast composite tend to fracture and align perpendicular to forging direction. Reduction in aspect ratio of reinforcements and α lath is also observed. 1-D forging can enhance the strength and elongation of as cast composite significantly. However, the increment in strength is quite limited as strain temperature increases to 700 °C. Additionally, room temperature and high temperature fracture mechanisms are also discussed.

Abstract: The evolution regularities of microstructure and property of a new metastable β titanium alloy with different solution treatment and aging treatment were studied using optical microscope (OM), scanning electron microscopy (SEM) and tensile test. The results show that, the volume fraction of primary α phase decreases and globularization of α phase occurs with the increasing aging temperature from 540 to 580 and solution treated temperature from 800 to 820. When the solution treated temperature is 820, the acicular secondary α phase precipitates along β grain boundary. The strength of the investigated alloy increases and the ductility decreases with the solution treated temperature increasing. While the strength of the investigated alloy increases and the ductility decreases with the aging temperature decreasing.

Abstract: In this paper, a series of relevant theoretical analyses and experimental researches have been carried out for precision forming of a typical aerospace revolver. By use of the material stress relaxation principle, the finite element numerical simulation and the preforming sizing process technology of stress relaxation, it have been revealed that the new process can be used for precision forming of titanium alloy sheet and its forming precision can meet the manufacturing requirements of aerospace sheet-metal parts. The precision forming technology of titanium alloy can also be used to improve the digital manufacturing technologies of aerocraft sheet-metal parts, and thus has potential application prospects.

Abstract: Metastable β type TiNb based alloys of Ti-35Nb, Ti-35Nb-4Sn and Ti-42Nb-4Sn (wt. %) with different stability were prepared and thermo-mechanical treatment was carried out to investigate their microstructural evolution and mechanical properties. It was found that although the TiNb based alloy with lower stability performs lower strength in its solution state due to the stress induced martensitic transformation, they could be strengthened significantly by severe cold rolling followed by aging, remaining a relative low elastic modulus. X-ray diffraction, transmission electron microscopy and mechanical test were conducted to characterize the microstructural evolution and mechanical behavior of the metastable β type TiNb based alloys with different β stability. The strengthening mechanism was discussed on the basis of the cold deformation and martensitic transformation.