Room Temperature Fracture of FeCo
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ROOM TEMPERATURE FRACTURE OF FeCo L. ZHAO*, I. BAKER* AND E. P. GEORGE**
*Thayer School of Engineering, Dartmouth College, Hanover, NH 03755 **Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831
ABSTRACT FeCo is a B2 intermetallic compound which undergoes an order-disorder transformation. In this paper, the effects of changes in both constitutional and thermal disorder on the room temperature fracture of FeCo are presented. Tensile tests were performed on three compositions of FeCo, Fe 3 0 Co 7 0 , Fe 50 Co 50 and Fe 70 CO3 0 . The resulting fracture surfaces were examined by SEM and the grain boundary chemistry was investigated by Auger electron spectroscopy, AES. Ordered Fe5 0Co 50 and Fe 70 Co 30 were very brittle, fracture occurring before yielding, with intergranular fracture occurring in most grains. In contrast, ordered Fe 30 CO7 0 showed about 18% elongation and exhibited a dimple-type fracture. It was also found that disordering improved the ductility of each composition but had little influence on the fracture mode. AES showed that a low level of sulfur segregation was present at the grain boundaries, suggesting that sulfur segregation alone was not responsible for the brittle behavior of the ordered alloys. INTRODUCTION For an ordered polycrystal it has been suggested [1] that five independent slip systems are not a sufficient criterion for ductility. In addition, a (partially) disordered grain boundary region may be needed. This suggestion implies that both thermal and constitutional disorder could improve ductility for an ordered alloy. This paper presents a study designed to test this hypothesis. FeCo is an ideal B2 intermetallic compound with which to study the effects of constitutional and thermal disorder. It undergoes an order-disorder transformation over a considerable range, from 23 at.% Co to 74 at.% Co. It is well known that stoichiometric FeCo has less ductility in the ordered state than in the disordered state. Accompanying this decrease in ductility is a change from transgranular cleavage to intergranular fracture [2,3]. Since grain boundary fracture in bcc metals is usually caused by grain boundary segregation, the entirely intergranular fracture of the ordered Fe50 Co5 0 raises the possibility that segregation of impurities to the grain boundaries occurs during the relatively slow cooling required to induce long range order, in turn weakening the boundaries and causing intergranular fracture [4,5]. However, Glezer and Maleyeva [3] argued that impurities are not responsible for grain boundary fracture and that the order of grain boundary itself is important feature. EXPERIMENTAL Dumbbell-shaped specimens for tensile tests were made from hot-extruded ingots of Fe3 0 Co 70 , Fe 5oCo 50 and Fe7 0 Co 30 annealed at 860"C for 5 hrs in order to produce fullyrecrystallized microstructures with a grain size of about 25-35 A.rm. Details of the extrusions are given elsewhere [6]. The specimens were further heat-treated to obtain either the ordered or the disordered sta
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