Year

2012

Degree Name

Master of Engineering - Research

Department

School of Mechanical, Materials and Mechatronic Engineering

Abstract

This study investigates the recrystallisation behaviour of cold drawn low cost beta (LCB) Ti-1.5Al-6.8Mo-4.5Fe-0.2O (wt.%) by tracking the evolution of its microstructure and microtexture during rapid resistance annealing. The studied material was solution treated at 850oC for 1800 s, followed by water quenching, then cold-drawn at room temperature to 80% (CD80) and 90% (CD90) reduction in diameter and consequently rapid resistance heating to select peak temperatures at rates of 10 and 20oC.s-1. All aspects of the present study have been characterised using advanced Electron Back-Scattering Diffraction (EBSD) and X-ray diffraction techniques. The EBSD maps were deconstructed into deformed, recovered, newly nucleated and growing grain subsets based on average internal misorientation, morphology and size of the microstructural features. XRD was used to obtain the bulk texture after accounting for background, irradiated volume and defocusing errors. The microhardness of LCB-Ti alloys after each annealing condition was measured by Vickers hardness testing.

Recovery was found to play an important role during rapid annealing. The partially recrystallised microstructures of CD80 and CD90 LCB-Ti were found to be heterogeneous such that the unrecrystallised grains had high aspect ratios with some deformed grains containing in-grain shear bands. A fewer number of shear bands were noted in the CD80 samples as compared to the CD90 samples. Discontinuous recrystallisation was manifested as newly nucleated (sub)grains formed mostly at the grain boundaries of deformed grains; with fewer nucleation events occurring at the in-grain shear bands. The density of the newly nucleated (sub)grains in the CD90 samples is approximately double that of the newly nucleated (sub)grains in the CD80 samples. In the nearly full recrystallised condition, the average equivalent grain size of the CD80 sample is nearly 1.5 times larger than that of the CD90 sample. An increase of heating rate from 10 to 20oC.s-1 resulted in 1.7 times smaller final recrystallised grain size of the CD90 samples.

Bulk and micro-textures textures of the cold drawn and partially recrystallised conditions in CD80 and CD90 LCB-Ti were chacracterised by thepartial fiber parallel to the drawing direction. During the rapid annealing of CD80 and CD90 samples, the texture of the newly nucleated subset comprised a combination of orientation components that inherently possess high ((110)[110] and (111)[110] ) and low ( {001}and (110)[001]) stored energy carried over from the cold drawing stage. At the end of the rapid annealing, texture of the approximately fully recrystallised conditions for CD80 and CD90 showed the presence of weak {001}and (110)[001] orientation components.

The values of yield strength estimated from the Hall-Petch relationship is in general agreement with the values determined experimentally from Vickers hardness. The contribution from LAGBs to the true stress was found such to decrease with the progress of recrystallisation.

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