Thermomechanical response of a powder metallurgy Ti-6Al-4V alloy modified with 2.9 pct boron
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I. INTRODUCTION
THE drive toward achieving higher structural efficiency in aircraft technology has motivated research efforts in the last decade aimed at improving the specific properties of Ti alloys. From a performance viewpoint, Ti alloys reinforced with continuous fibers, notably SiC (popularly known as titanium matrix composites (TMCs)), have been very promising.[1] Although TMCs are now in commercial production for aerospace applications,[2] widespread application of these materials has been hindered by high fiber costs, design limitations imposed by the anisotropy of properties, and relatively weak matrix/fiber interfaces.[3] Conventional titanium alloys modified with small additions of boron have been attracting considerable attention in recent years as potential candidates for replacing structural components requiring high specific properties at room and moderately elevated temperatures.[4] Boron additions to Ti alloys produce the intermetallic TiB by an in-situ reaction at high temperatures. TiB is chemically and thermomechanically stable and B is essentially insoluble in or Ti at all temperatures of interest. These materials can be considered modified Ti alloys at low-to-modest boron levels, while at boron additions beyond 6 pct, they are best classified as discontinuously reinforced titanium metal-matrix composites.[5] Other advantages of Ti-B alloys include reaction-free interfaces, minimal residual stresses (due to similarity of thermal expansion coefficients of matrix and TiB), good creep and fatigue resistance, and damage tolerance.[6] Moreover, the TiB forms in RADHAKRISHNA B. BHAT, Research Scientist, is with UES, Inc., Dayton, OH 45432. Contact e-mail: [email protected] SESHACHARYULU TAMIRISAKANDALA, Assistant Research Professor, is with the Department of Mechanical Engineering, Ohio University, Athens, OH 45701. DANIEL B. MIRACLE, Senior Scientist, is with the Metallic Composites Research Group, Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLLMD, Wright-Patterson Air Force Base, OH 45433. VILUPANUR A. RAVI, Associate Professor, is with the Chemical and Materials Engineering Department, Cal Poly Pomona, Pomona, CA 91768. Manuscript submitted March 1, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
situ in the form of whiskers that are intrinsically stiff and strong, leading to effective stiffening and strengthening. In addition, these alloys can be subjected to conventional thermomechanical processing (TMP) treatments, unlike TMCs.[7] Ti-B alloys can be produced by a variety of synthesis techniques such as liquid metallurgy (LM),[8] powder metallurgy (PM),[9] combustion-assisted synthesis,[10] mechanical alloying,[11] and laser-engineered net shaping (LENS*).[12] *LENS is a trademark of Sandia National Laboratories, Albuquerque, NM.
Approaches based on the LM route limit the maximum B concentration to below the eutectic limit for avoiding formation of coarse primary TiB, whereas B levels in excess of eutectic composition can be easily introduced u
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