Christopher Sean Meredith
Published: 2011
Total Pages: 468
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In this study, titanium (commercially pure, grade 1) was processed by equal channel angular pressing (ECAP) for multiple passes. ECAP is a method of applying severe plastic deformation, without changing the overall dimensions. A billet material is pressed through the channel and forced to deform around the 90° turn in the channel, and this deformation converts coarse grained materials into ultrafine grained (UFG) ones, and introduces a high percentage of high angle grain boundaries. These are the main reasons for the improvement of the mechanical properties that have been observed. The uniform microstructure from the as-received condition, goes from long, coarse bands with some grains relatively undeformed after a single pass to mostly uniform UFG after four passes. The material deforms via both twinning and slip during the first pass, and almost exclusively by slip for passes beyond one. At all numbers of passes, the more coarse grains are refined by continuous dynamic recrystallization, whereby the accumulation of dislocations cause crystal rotation which subsequently form low angle grain boundaries and then fully enclosed high angle grain boundaries. These high angle grain boundaries are new ultrafine grains. Furthermore, the mechanical behavior was measured under uniaxial compression. Most of the strength increase occurs after the first pass, with incremental increases after that, but by four passes of ECAP the yield point is about three times greater than the as-received Ti. The strain rate sensitivity generally decreases as the number of passes increases, but increases with temperature. When loading in different directions, it was determined that under all conditions, the highest to lowest yield and flow stresses were in the y-, z- and x-directions, respectively. Finally, during dynamic testing in different directions, the samples failed via shear banding, which depended on the loading direction and temperature. The loading directions from the most to least total strain before the initiation of macroscopic failure by shear banding were in the x-, y- and z-directions, respectively.