Transmission electron microscopy of deformed Laves phase NbFe2

Abstract

Laves phases are the largest class of intermetallic phases, showing very high strength up to temperatures above 1000 °C, but being very brittle at room temperature. The mechanical behavior and the deformation mechanism of these phases is very much different from that of pure metals and is still not well understood [1, 2]. The unit layer of a Laves phase AB 2 does not consist of only one plane but a slab of four planes, each of which is composed of either only A‐ or B‐atoms. This package of four atomic planes can be sub‐divided in one single B‐atom layer and a triple layer of successive A‐, B‐ and A‐atom planes [3]. The present work summarizes results obtained from transmission electron microscopy on single‐phase NbFe 2 before and after compression tests at high temperatures. The material was produced by levitation melting and has hexagonal C 14‐type structure ( hP 12, P 6 3 mmc ). The compressive stress‐strain curves are characterized by a pronounced stress peak in the stress‐strain curves at lower temperatures (up to 1200 °C) and by steady state flow at higher temperatures (above 1200 °C) [4]. Undeformed NbFe 2 is almost free of dislocations. It is assumed that the lack of dislocations in the as‐cast condition leads to the pronounced yield stress maximum observed during compression testing at lower deformation temperatures. After deformation at 1200 °C new dislocations are introduced into the material. Widely‐extended stacking faults on the basal plane dominate the microstructure. They are bounded either by partial dislocations or terminate at low angle grain boundaries (Fig. 1). Material deformed at 1300 °C shows a high density of dislocations, which are split into pairs of partial dislocations that bound stacking faults on the basal plane (Fig. 2). The observation that most dislocations are split up into Shockley partial dislocations could indicate deformation by the synchroshear process [5], as the occurrence of such partial dislocations is essential for this mechanism. Synchroshear is based on the idea of a synchronous shear motion of two adjacent planes within the triple layer. Dislocation networks (Fig. 3 and 4) indicate the activation of dislocation climb. The improvement of dislocation mobility at higher deformation temperatures leads to the absence of a pronounced stress maximum in the stress‐strain curves. The observed perfect dislocations are of (0001) 1/3<11‐20> type, the dissociated Shockley partials are (0001) 1/3<10‐10> type. The main deformation mechanism in the NbFe 2 C 14 Laves phase is basal slip.