Lamellar (β Annealed) Ti-6Al-4V
The lamellar microstructure of Ti-6Al-4V is attained by the β annealed processing route. A transformation structure, Ti-6Al-4V is comprised entirely of a body centered cubic crystallographic phase at high temperatures (β phase), which upon cooling transforms nearly entirely to a hexagonal close packed crystallographic phase (α phase). Near field high-energy X-ray diffraction experiments are able to deduce the structure of the room-temperature α phase. Utilizing the knowledge of the transformation – which strictly relates the α phase orientations that may arise when transforming from a single β phase orientation – the β phase structure that existed at high temperature is deduced. Macroscopic tensile tests reveal the material has relatively low yield strength and ductility when compared to other microstructural variants of Ti-6Al-4V.
The knowledge of the phase transformation allows for a high fidelity representation of the microstructure that captures both the fine geometric structure of the material as well as the localized coupling of orientations. This representation is achieved through the use of a novel multilevel tessellation technique, in which the high-temperature β phase structure is represented by an initial tessellation, and transformation structures are represented by performing secondary and tertiary tessellations within each cell of the initial tessellation that form the α phase transformation structures within grains and the un-transformed β phase laths within transformation structures, respectively.
A parameter study is performed to gauge the influence of the geometric parameters (number of initial β phase grains, number of α phase transformation structures within each grain, thickness of un-transformed β phase laths) on the overall response of the material in the form of the macroscopic yield strength and ductility. The inclusion of remnant β phase has pronounced effects on the macroscopic response of the material, specifically possessing both a lower yield strength and ductility than comparable single phase simulations. This suggests that the inclusion of fine geometric features is important in modeling the response of the material.
Romain Quey, École des Mines de Saint-Étienne.
Paul Dawson, Cornell University
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