The structure of the high-temperature phase MnAl(h) and the displacive transformation from MnAl(h) into Mn 5 Al 8
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I.
INTRODUCTION
THE phase
diagram Mn-A1 investigated by G6decke and K6ster tu shows in the interval of mole fractions from x~ = 0.5 to0.7 and at temperatures T = 1465 and 1321 K, respectively, two peritectic reactions which yield the hightemperature phases MnAl(h) (= y) and 71 (Figure 1). The structures of these compounds have been unknown up to the present. By peritectoid reaction at 1260 K, MnAl(h) and 71 form the room-temperature phase MnsA18 (= 72), according to Schubert et al. 121and Meil3ner and Schubert, ~31 isotypic to CrsA18. A new description of the phase diagram Mn-A1 by Murray et a1.[41 and by McAlister and Murray 151 accepts the phase equilibria among MnAl(h), ")/1, and MnsA18 investigated by G6decke and K6ster. t~1 It was shown by K6ster and Wachtel I61 that the phase transformation MnAl(h) --> MnsA18 exhibits a significant change of resistivity: the rhombohedrally distorted gamma brass structure of MnsA18 shows a higher resistivity than the high-temperature phase MnAl(h). According to a report of Schonover and Mohanty, 171 MnAl(h) cannot be retained by quenching. In this paper, high-temperature X-ray powder diffraction was used for the structural investigation on MnAl(h). For the lowtemperature phase MnsA18, the dependence of lattice parameters on mole fraction was measured over the whole range of homogeneity.
II.
EXPERIMENTAL
Alloys consisting of manganese (Ventron Puratronic) and aluminum (Ventron 99.999 pct) were melted under argon (Messer-Griesheim 5,0) in a high-frequency furnace. Because no significant weight loss of melted alloys was observed, no chemical analysis was carried out. The melted alloys were homogenized in corundum crucibles introduced in the evacuated silica tubes at 1150 K for 1 day. For splat cooling, a shock wave tube 181 and a rotating wing apparatus I91 were used. M. ELLNER, Research Associate, is with the Max-Planck-Institut fiir Metallforschung, Institut fiir Werkstoffwissenschaft, D-7000 Stuttgart, Federal Republic of Germany. Manuscript submitted April 19, 1989. METALLURGICAL TRANSACTIONS A
Powder diffraction patterns were recorded in an EnrafNonius FR 552 Guinier camera using C u K ~ , and Fe K~, radiation. Silicon was used as an internal calibration standard. The lattice parameters were refined by least-squares calculations using all observed reflexions. Metallographic analysis was carried out by use of optical and scanning electron microscopy (SEM). For the hightemperature X-ray powder diffraction measurements, an Enraf-Nonius FR 553 Guinier-Simon camera with Cu K~, and Fe K~, radiation was used.
III.
EXPERIMENTAL RESULTS
Guinier photographs with Fe K~, radiation of the as cast alloys from the composition range of MnsoAls0 to Mn35A165 at room temperature showed the diffraction lines of MnsA18 (CrsA18 type). No change of the type of structure was observed in the Guinier photographs of these alloys, splatted either in the shock wave tube or in the rotating wing apparatus. Powdered alloys MnsoA150 9 9 9 Mn37A163 were heat treated at 1325 K for 10 minutes and wat
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