Flow and fracture of a multiphase alloy MP35N for study of workability
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INTRODUCTION
IN the progression of metals technology there is increasing use of complex high alloys for use in high temperature and hostile environments. In contrast with the butter-like hot working qualities of more traditional alloys (e.g., carbon steels, soft aluminum alloys, brasses), the complex alloys often exhibit cracking tendencies during high temperature working processes. For this reason there is an increasing need for reliable methods of material testing and examination within a framework that is compatible with and reasonably simulative of processing at high temperatures. Hot metalworking processes are usually carried out at strain rates between 0.1 and 500 s -1 and at temperatures above 60 pct of the absolute melting point. Accordingly, it has become customary in metallurgical literature to define the hot working range by T ~> 0.6 Tm and 10 -3 < k < 102, while creep is considered to include lower strain rates.~'2 Although an extensive literature exists on creep testing and mechanisms, these results are not of use in studies of fracture during hot working. 3'4 In the complete range of temperatures and strain rates available, there is a continuous spectrum of possible metallurgical structure changes, and it is difficult on the basis of mechanisms alone to delineate between hot working and creep. The contexts in which hot working and creep information are applied provide a more meaningful basis of separation and establish guidelines for testing and examination methods. Creep of components usually involves simple tensile stresses, and dead weight tension testing within a furnace is a reasonable approach to evaluation of materials for creep deformation and rupture) Metalworking processes, on the other hand, usually involve complex, non-uniform stress G. HTZSIMONS, Assistant Research Professor, and H. A. KUHN, Professor, are with the Department of Metallurgical and Materials Engineering, University of Pittsburgh, Pittsburgh, PA 15261. Manuscript submitted August 2, 1982. METALLURGICALTRANSACTIONS A
states consisting of applied bulk compressive stresses with localized secondary tensile stresses leading to fracture. For this purpose, tension testing has limited use since instability and necking limit the amount of deformation available below that normally found in metalworking processes. Considerable work has been done previously on torsion testing for the purpose of studying the roles of dynamic recovery and dynamic recrystallization on fracture under hot working conditions. Although the application of torsion results to actual metalworking processes is difficult, at least the test is not hampered by instabilities; in addition, high strains, strain rates, and controlled temperatures can be achieved. A major conclusion drawn from torsion tests by White and Rossard, 6 Luton and Tegart, 7 Tegart, s Gittings and Sellars, 9 and many others l~ is that, for some iron alloys, stainless steels and nickel-based superalloys, increasing the deformation rate at constant temperature will lead to an increase in ducti
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