Plastic deformation in a multifunctional Ti-Nb-Ta-Zr-O alloy
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NTRODUCTION
THE phase stability of titanium alloys can be changed by adding alloying elements such as aluminum, vanadium, molybdenum, and iron, most of which are known to stabilize the (bcc) phase around room temperature, excluding a few (hcp) stabilizing elements such as aluminum and oxygen. The stabilizing elements are often added to practical titanium alloys because the phase has a good combination of deformation capability and strength. Plastic deformation in or near- titanium alloys is known to be complicated because it does not proceed with a simple dislocation motion in the phase. It has been reported that deformation twinning or stressinduced martensitic transformation has an important role along with dislocation motion during the plastic deformation; which mechanism dominates the deformation process depends on the alloy composition or fabrication processes.[1,2] We have recently reported a series of new titanium alloys, GUM METAL*[3,4] which show “super” properties such *GUM METAL is a trademark of Toyota Central R&D Labs, Inc., Aichi, Japan.
as ultra-low Young’s modulus with nonlinear elastic behavior, ultra-high strength, extended elastic limit, superplasticlike deformability, Invar-like thermal expansion, and Elinvar-like thermal dependence of the elastic modulus. These super properties are only achieved after a certain amount of cold working, which implies that the deformation process in the alloy is dominated by somewhat different mechanisms from those reported in conventional titanium alloys, and result in a peculiar microstructure characterized by a “marble-like” appearance.[3,4] Recently, several new mechanisms for plastic deformation in nanostructured metallic materials or in thin foil specimens of pure metals have been reported. In a recent review article by Ovid’ko, the importance of rotational deformation, grain boundS. KURAMOTO, T. FURUTA, and J.H. HWANG, Researchers, K. NISHINO, Principal Researcher, and T. SAITO, Director, are with Toyota Central R&D Laboratories Inc., Nagakute, Aichi 480-1192, Japan. Contact e-mail: [email protected] Manuscript submitted March 15, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
ary sliding, and stress-induced mass transfer in respect to underlying mechanisms in the plastic deformation in nanocrystalline materials was pointed out.[5] Kiritani and co-workers have reported a dislocation-free mechanism in thin foil specimens of fcc metals[6,7,8] and they also have found that the similar mechanism operates in thin foil of bcc metals.[9] In the case of Gum Metal, the alloys show the possibility of plastic deformation without aid from dislocation glide, even though they have a coarse-grained microstructure and are processed with conventional methods.[3] The value of ideal shear stress for the alloy is considered to be so small as to be comparable to the actual strength. In this report, we study the characteristics of plastic deformation in Gum Metal through transmission electron microscopy (TEM) and the electron backscattering pattern
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