Effects of Heat Treatment on Microstructure and Mechanical Properties of a Near-a Titanium Alloy

This thesis is concerned with the effect of heat treatment on the microstructure of the near-a titanium alloy IMI 834. Furthermore, it investigates the crack growth rate behavior in two microstructures of this alloy differing in their primary alpha ( ap) volume fractions. IMI 834 is an aerospace high temperature material that possesses the highest creep resistance of duplex near-alpha alloys at temperatures up to 650°C. A series of heat treatment experiments are carried out to examine the influence of recrystalization temperature, cooling rate, aging time and over-aging time, on the volume fractions of the primary alpha as well as the corresponding hardness characteristics. It is shown that for ap volume fractions up to 20%, colony size decreases with increasing the volume fraction. For higher ap volume fractions, the colony size remains relatively constant. Results show that between approximately 10% and 30% ap volume fractions, the material has a constant hardness, defined by the cooling rate. On the other hand, the hardness decreases significantly for volume fractions lower than 10% and for volume fractions higher than 30%. It is also found that element partitioning effects are most dominant in volume fractions above 30%. Over-aging specimens with 5%, 23% and 39% ap volume fractions at 650°C shows that only S2 type silicides precipitated in IMI 834. Furthermore, results show that the critical silicide size is achieved at about the same over-aging period, 2,000 minutes, for ap volume fractions greater than 23%. Over-aging the material with 5% ap volume fraction results, however, in precipitation of S2 type silicides after a shorter time, 80 minutes, and a noticeable increase in hardness is observed when compared with that of the other two microstructures. The influence of two selected CX.p volume fractions, 20% and 30%, on the fatigue crack growth rate (FCGR) is examined at 650°C for three different loading frequencies, 10 Hz, 0.05 Hz, and 0.003 Hz. It is shown that for the 10 Hz and 0.05 Hz tests, the FCGR is similar in the two microstructures. The fracture mechanisms under these two test conditions involves transgranular cracking of both the a!P colony and the CX.p particles. For the loading frequency of 0.003 Hz, the fatigue crack growth is higher in the 30% ap volume fraction than in the 20%. For this loading condition, the fracture mechanism involves transgranular cracking in the ex.IP colony accompanied by interboundary cracking in the CX.p particles. These fracture