Date of Award

12-2011

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Materials Science and Engineering

Advisor

Rack, Henry J

Committee Member

Skaar , Eric C

Committee Member

Luo , Jian

Abstract

Martensitic phase transformations were investigated for Ti -50.6 at % Ni processed via severe plastic deformation (SPD) and subsequent aging for temperatures between 400 and 550C and times ranging from 1hr to 100hrs. Phase transformations of the SPD material were compared to material processed via solution treated at 800C and 850C for 1hr. Transformations were investigated with differential scanning calorimetry (DSC) and confirmed by X-ray diffraction (XRD) with optical microscopy being used to determine grain size. Solution treated samples displayed a single (1-1) B2↔B19' transformation prior to and upon initial aging, i.e. 400C/1h, 450C/1/10h. Intermediate aging conditions, 450C/100h; 500C/1/10/100h; and 550C/1h, resulted in multiple B2↔B19' transformations (2-2). Aging at 400C/10/100h, induced the R phase and 3-2 transformation behavior. Intensive aging caused the solution treated material and the SPD material to revert to a single (1-1) B19'↔B2 transformation. A 2-1 transformation was exhibited by the bulk of the SPD samples, i.e. unaged SPD, 400C and 450C at 1/10/100h, 500C 1/10h, 550C 100h. XRD of SPD material indicated the presence of metastable Ni4Ti3 precipitates prior to aging, and equilibrium Ni3Ti precipitates after aging for 500C/100h. Multiple peaks in the solution treated samples result from composition heterogeneities. 3-2 behavior is the result critical microstrains inducing the R phase. 2-1 behavior of the SPD material results from a homogenous microstructure and macrostrains which induce the R phase. Aging the SPD material causes recovery, suppressing the R phase transformation and increasing the Ms temperature, eventually resulting in a 1-1 transformation.

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