The Effect of Cooling Conditions on the Microstructure of Rapidly Solidified Ti-6Al-4V

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The effect of cooling conditions, giving estimated cooling rates in the range 104 °C per second to 107 °C per second, on the microstructure of Ti-6Al-4V has been evaluated. The microstructures of as-solidified particulates were martensitic, with the martensite lath length decreasing with beta grain size, L, which in turn decreased with increasing cooling rate. For material alpha + beta heat-treated or vacuum hot pressed, the alpha morphology was dependent on the prior cooling rate. For materials cooled at <5 × 105 °C per second martensite transformed to lenticular alpha, while material cooled at >5 × 105 °C per second developed an equiaxed alpha morphology. This change in morphology was explained in terms of high dislocation density or grain size refinement, both of which result from the high cooling rate. When the beta grain size (L) was plottedvs section thickness (z), and estimated cooling rate (T), power law relationships analogous to those reported for secondary dendrite arm spacing were found: L = 1.3 ± 0.4z089±006 (thin, chill-substrate quenched), L = 0.17 ± 0.05z0.86±0.01(thick, convection-cooled material), andL = 3.1 × 106 T−0.93±0.12 (all material), where Land z are in μm and T is in K/s. The last relationship is in agreement with the 0.9 exponent predicted using a model developed for the effect of grain size on cooling rate assuming classical homogeneous nucleation and isotropic linear growth during solidification. The first two relationships were rationalized by assuming that the two materials cooled under near-Newtonian conditions.