Metastable hexagonal modifications of the NbCr 2 Laves phase as function of cooling rate
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1128-U08-07
Metastable hexagonal modifications of the NbCr2 Laves phase as function of cooling rate Jochen Aufrecht, Andreas Leineweber and Eric Jan Mittemeijer Max Planck Institute for Metals Research, Heisenbergstrasse 3, D-70569 Stuttgart, Germany
ABSTRACT In as-cast ingots produced by arc-melting, several metastable polytypic modifications of NbCr2 were found additional to the cubic C15 phase stable at room temperature: C14, C36 and 6H-type structures, often highly faulted and/or intergrown. Strikingly, these phases had formed at locations of the specimen which had experienced a relatively low cooling rate, whereas the C15 phase was formed preferentially in regions which had experienced the highest cooling rates. INTRODUCTION In the binary Nb-Cr system, the Laves phase NbCr2 exhibits the cubic C15-type structure (prototype: MgCu2) at room temperature [1]. Nevertheless, in as-cast ingots, other Laves-phase polytypes of NbCr2 have been found: Guseva found a “hexagonal structure” [2]. Thoma et. al. reported hexagonal C14 and C36 modifications (prototypes: MgZn2 and MgNi2) in as-cast ingots prepared by arc-melting, together with the stable C15 phase [3, 4]. The amount of the metastable phases was reported to depend on the location within the ingot: more metastable phase at locations adjacent to the water-cooled copper hearth (i.e. the ground plate) of the arc-melting furnace, i.e. at locations where relatively high cooling rates occurred. Splat-quenched NbCr2 samples contained an even larger fraction of C14, but always together with the C15 modification [3]. Kazantzis et al. [5] reported various Laves-phase polytypes in solidified arc-melted alloys together with the C15 modification: Besides C14 and C36 also more complex polytypes (Ramsdell symbols 6H, 8H and 9R) were suggested to be present. The occurrence of a C14 modification seems to be in agreement with the commonly accepted phase diagram [1], which incooperates a C14-type high-temperature modification of NbCr2 stable between about 1600°C and the congruent melting point at 1770°C. It has thus been supposed that NbCr2 solidifies into this crystal structure, in the first stage of cooling upon solidification, which structure is only partially retained upon continued cooling to room temperature. The high cooling rates occurring after arc-melting or during splat-quenching necessitate very fast kinetics to complete the C14 → C15 transformation, while this type of transformation has been reported to be very sluggish in Laves phases like TaCr2, HfCr2 and TiCr2 [6]. The transformation is assumed to proceed by the “synchro-shear” process, involving mediation by a certain type of complex partial dislocations, changing the stacking sequence of the Laves-phase structures [6, 7]. On this basis the occurrence of stacking sequences of longer period than corresponding with C14 and C15, as 4H ( = C36), 6H, 8H and 9R, which could represent intermediate stages, might be understood. Contamination with O and N has also been suggested as possible reason for the formation of these l
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