Processing and Characterization of Porous Ti-6Al-4V and NiTi

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A series of investigations was undertaken to explore the processing and properties of porous titanium with 6 wt% Al and 4 wt% vanadium (Ti-6Al-4V) and an equiatomic nickel-titanium alloy (NiTi). In this study, porous materials were created in the solid state by entrapping argon in a powder compact, and subsequently expanding the entrapped gas through creep of the matrix. The gas quantity entrapped was varied by altering the pressure of argon in the powder prior to compaction, which led to initial porosities between 0.2% and 5%. Isothermal creep expansion of both the Ti-6Al-4V and the NiTi was examined in relation to the initial gas quantity. It was found that the higher the initial porosity the faster the porosity increase. In Ti-6Al-4V, the effect of transformation superplasticity (induced by thermal cycling) on pore expansion kinetics was explored, and found to significantly increase the pore growth rate. Thermal cycling was also used in NiTi to produce graded porosity using the non-linear response of creep to temperature. The porous metals created through the above method were characterized in a number of ways. The pore size distribution of the Ti-6Al-4V porous material was measured using imaging methods. The elastic response of all the samples was determined using ultrasonic testing. Transformation properties of porous superelastic NiTi under stress were examined using synchrotron x-ray analysis. The growth of porosity was explored through simulation using numerical as well as analytical methods. An existing analytical method was modified to include corrections to the gas law, initial porosity changes due to argon density, and a surface tension pressure.

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  • 08/27/2018
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