Zhang, Kun (2010)
Ph.D. thesis, University of Birmingham.
In the present study, the effect of hot isostatic pressing (HIPping) variables such as HIPping temperature, HIPping cycle and powder particle size, on the microstructure and mechanical properties of HIPped samples of two Ti alloys have been assessed. Powders of the most commonly used (α + β) alloy, Ti-6Al-4V and one specific beta alloy, Ti-25V-15Cr-2Al-0.2C wt% (burn resistant titanium alloy, BuRTi) were studied. The Ti-6Al-4V powder was made by the plasma rotating electrode process (PREP). BuRTi powders, which were made both by gas atomisation and by PREP were HIPped to investigate the influence of the initial structure of the powder on the microstructure and associated mechanical properties of the HIPped alloy. The PREP Ti-6Al-4V powder was shown to be fully martensitic in the as-atomised condition. The gas-atomised and PREP powders of BuRTi showed very different as-atomised structures, but in both cases the structure was, as expected single phase beta, with the carbon retained insolution. The individual particles of gas-atomised BuRTi powder were always polycrystalline, although there was a significant scatter in grain sizes within different particles. In contrast the individual particles in the PREP powder were either coarse grained polycrystals or single crystals. These differences led to significant differences in the microstructures and properties of HIPped samples. It was found that HIPping of Ti-6Al-4V samples resulted in the formation of equiaxed regions and lath-like microstructure. The small equiaxed regions are formed by recrystallisation which occurs at original particle boundaries where most of the deformation occurs during HIPping; the lath-like microstructure is formed by simply tempering the (less deformed) original alpha prime martensite within the central part of original particles. Among the three HIPping temperatures used, samples machined from powder HIPped at 930°C exhibited a better balance of properties than those HIPped at 880°C or 1020°C. The fatigue properties of samples HIPped at 930°C, made using different HIPping procedures were compared. It was found that samples which contain the as-HIPped surface, which were made using a new HIPping procedure, have better fatigue properties than samples with as-HIPped, machined or electro-polished surfaces which were produced by conventional HIPping . The properties of optimally HIPped Ti-6Al-4V samples are as good as or better than ingot-route samples. In the case of BuRTi the original single crystals or coarse grained polycrystals in the PREP powder are retained after HIPping and limited grain growth occurs in the gas-atomised samples. The tensile strength is comparable for the gas-atomised and PREP samples, but samples tested to failure showed a significant scatter in ductility (a larger scatter in the PREP powder samples) and all fracture surfaces contained large circular fracture initiation sites, with larger sites associated with lower ductility. Initiation occurs in the centre of these circular regions in large grains or in adjacent grains which have similar orientations and the failed region expands symmetrically in powder samples where no texture is expected. The fatigue properties of the PREP samples are much lower whereas the fatigue properties of the gas atomised samples are better than those of samples from ingot route. This behaviour is associated with obvious facetted failure sites in the PREP powder samples where it is suggested that the coarser microstructure has allowed persistent slip to occur leading to localised deformation and to premature failure. These observations are discussed in terms of the potential of net shape HIPping for the production of engineering components and in this context the fact that a new HIPping schedule has been developed during this study, where the fatigue properties of samples containing an as-HIPped surface are excellent, is very significant.
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