On the effect of strain rate and temperature on the yield strength anomaly in L2 1 Fe 2 AlMn
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1128-U05-09
On the effect of strain rate and temperature on the yield strength anomaly in L21 Fe2AlMn Markus W. Wittmann, Janelle M. Chang, Yifeng Liao and Ian Baker Thayer School of Engineering, Dartmouth College, Hanover, NH 03755-8000, USA
ABSTRACT The effects of strain rate and temperature on the yield strength of near-stoichiometric Fe2AlMn single crystals were investigated. In the temperature range 600-800K the yield stress increased with increasing temperature, a response commonly referred to as a yield strength anomaly. No strain rate sensitivity was observed below 750K, but at higher temperatures the yield stress increased with increasing strain rate. Possible mechanisms to explaining the effects of temperature and strain rate are discussed. INTRODUCTION Interest in the microstructure and mechanical properties of Fe2AlMn arises from previous investigations on Fe-26Mn-19Al which revealed that the alloy had some intrinsic ductility and displayed a yield strength anomaly (YSA) in the range from 600-800K [1]. It was found that alloys with a composition near stoichiometric Fe2AlMn have the L21 or Heusler structure, shown in Figure 1. At room temperature the microstructure contains two sets of thermal anti-phase boundaries (APBs), viz., a/4 APBs enclosing smaller a/2 APBs. These two sets of APBs are a result of the alloy first solidifying as a disordered b.c.c. phase, subsequently ordering to a B2 phase at ~ 1300K, and finally ordering to a L21 structure at ~895K [1]. Plastic deformation was accommodated by the motion of four anti-phase boundary (APB) coupled a/4 dislocations [1].
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Figure 1: Atom positions for the L21 structure adopted by some A2BC compounds. The structure consists of eight B2 unit cells with A atoms at the corner of each unit cell and the center atom in alternate unit cells alternating between B and C atoms.
Since the L21-B2 transformation temperature (891K) occurs just above the 600-800K temperature range of the YSA, and plastic deformation involves the motion of coupled partial dislocations, numerous proposed strengthening mechanisms can account for the YSA. These include increased dislocation friction arising from the order-disorder transformation [2-4], temperature and stress-induced reconfiguration of partial dislocations [5-7], or vacancies [8]. Due to the absence of serrated yielding or yield points in the stress strain curves, dynamic strain aging was discounted as a probable strengthening mechanism. Transmission electron microscopy of polycrystalline Fe2AlMn specimens strained at the peak strength temperature (800K) contained only dislocations with no obvious evidence of pinning [1]. Thus, it was tentatively concluded that models which rely on stress and temperature aided dislocation transformations [5-7] were not the origin of the YSA Also, these models offer no physical mechanism to account for the results of quenching experiment in which the room temperature yield stress increased monotonically with increasing quench temperature [1]. Both the vac
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