Strengthening Mechanisms in NiAl Bronze: Hot Deformation by Rolling and Friction-Stir Processing
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INTRODUCTION
GRADIENTS in strain, strain rate, and temperature during deformation processing may lead to gradients in microstructure that, in turn, affect material response during subsequent mechanical testing of as-processed material. Assessing the role of such gradients in mechanical response can be facilitated by microstructure-based models of strength, especially when gradients are steep or when such gradients are an inherent feature of the deformation technique. The latter is the case for friction-stir processing (FSP), which is an allied technique of friction stir welding (FSW), a solid-state joining method developed at The Welding Institute.[1–3] In FSP, a rotating, nonconsumable cylindrical tool with a concentric projecting pin is pressed into a work piece surface. A combination of frictional and adiabatic heating leads to formation of a plasticized column of material around the pin as it penetrates the surface. When the tool shoulder contacts the work piece surface the tool may be traversed across the surface to process the volume of material that is swept by the pin profile. SARATH K. MENON, Research Professor, and TERRY R. McNELLEY, Distinguished Professor, are with the Department of Mechanical and Aerospace Engineering, Naval Postgraduate School, 700 Dyer Road, Monterey, CA 93943-5146. Contact e-mail: tmcnelley@ nps.edu FRANK A. PIERCE, formerly Student, with the Department of Mechanical and Aerospace Engineering, Naval Postgraduate School, is now Engineer Officer, with United States Coastguard, CGS STRATTON, Alameda, CA. BRIAN P. ROSEMARK, formerly Student, with the Department of Mechanical and Aerospace Engineering, Naval Postgraduate School, is now with the NUWCDIVNPT, Code 3423, Newport, RI. KEIICHIRO OH-ISHI, Associate Professor, is with the Nagaoka University of Technology, 1603-01 Kamitomioka, Nagaoka 940-2188, Japan. SRINIVASAN SWAMINATHAN, Research Scientist, is with General Electric Global Research, Bangalore 560066, India. Manuscript submitted August 17, 2011. Article published online May 17, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
This volume of material is termed the stir zone (SZ). The FSP thermomechanical cycle includes rapid transients as well as steep gradients in strain, strain rate, and temperature. Accordingly, there are gradients in microstructures and mechanical properties within the SZ and, especially, outward into the surrounding base material.[3–5] The application of FSP to as-cast NiAl bronze materials results in simultaneous increases in both tensile strength and ductility in the SZ material.[6,7] However, reduced ductility has been observed when the deforming gage sections of tensile samples include regions of inhomogeneous microstructure. Such reduced ductility may reflect strain localization but could also be caused by other processing-related defects, such as inclusions or inadequate bonding of stir zone to base material.[6,7] The latter may occur on the advancing side of the tool, i.e., on the side where tool traversing and rotation speeds are additive and where
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