The Mode of Deformation in a Cold-Swaged Multifunctional Ti-Nb-Ta-Zr-O Alloy
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INTRODUCTION
A certain class of body centered cubic (BCC) titanium alloy, termed b-phase Ti alloy, is an attractive engineering material for the aerospace, automobile, and medical industries due to its high specific strength, good hardenability, excellent fatigue performance, and crack-propagation resistance.[1,2] Multifunctional Ti alloys (MFTAs), termed ‘‘Gum Metal,’’ are a relatively new class of b-phase Ti alloy exhibiting many interesting properties such as superplasticity at room temperature without extensive work hardening. In the cold-deformed states, they also possess an ultra-low elastic modulus, ultra-high strength, large elasticity, and Elinvar and Invar properties.[3] MFTAs were originally prepared through a complex sinter-forging-solution treatment–swaging process using pure elemental powder,[3] but other routes have been devised to generate comparable alloys and properties.[4] To generate MFTAs exhibiting this suite of desirable properties, Saito et al.[3] proposed that the following criteria must be satisfied: W. GUO, Ph.D. Researcher, is with the School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia. M.Z. QUADIR, Materials Manager, is with the Electron Microscope Unit, University of New South Wales, Sydney, NSW 2052, Australia and also Lecturer with the School of Materials Science and Engineering, University of New South Wales, and also with the Australian Research Council Centre of Excellence for Design in Light Metals, Sydney, Australia. M. FERRY, Professor, is with the School of Materials Science and Engineering, University of New South Wales, and also with the Australian Research Council Centre of Excellence for Design in Light Metals. Contact e-mail: [email protected] Manuscript submitted September 24, 2012. Article published online January 17, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
(1) Composition range (mol. pct): Ti-24(Ta+Nb+V)(Zr,Hf)-(0.7 to 3)O (2) Simultaneous satisfaction of the following ‘‘magic numbers’’: (i) A compositional average valence electron/ atom ratio e/a ~4.24; (ii) A bond order (Bo-value) ~2.87 based on the DV-Xa cluster method; and (iii) d-electron orbital energy level (Md-value) ~2.45eV. (3) Substantial cold working of the billet. It is well known that deformation of conventional b-phase Ti alloys occurs by three possible mechanisms, depending on the stability of the b phase: (i) slip; (ii) twinning, and (iii) stress-induced a¢¢ martensite transformation.[5–7] However, the discovery of MFTAs indicated that a new plastic deformation mechanism is operative in these unique alloys during severe plastic deformation such as cold swaging. In the current study, the deformation microstructure was characterized by a marble-like structure at low resolution and a distorted lattice containing nanodisturbances (‘‘planar nanoscopic areas of local shear’’) at high resolution.[4] It was reported that ‘‘virtually no dislocations’’ were present in the alloys, but unusual nanometer-sized strain contours were evident. Since dislocati
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