B2 order transformation in a Fe - 25 at% Co - 9 at% Mo alloy
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B2 order transformation in a Fe – 25 at% Co – 9 at% Mo alloy
Christoph Turk 1, Gert Kellezi 2, Harald Leitner 2, Peter Staron 3, Weimin Gan 3, Helmut Clemens 1 and Sophie Primig 1 1
Department of Physical Metallurgy and Materials Testing, Montanuniversitaet Leoben, Roseggerstraße 12, 8700 Leoben
2
Böhler Edelstahl GmbH & Co KG, Mariazellerstraße 25, 8605 Kapfenberg, Austria
3
GEMS, Helmholtz-Zentrum Geesthacht, Max-Planck-Str.1, D-21502 Geesthacht, Germany
ABSTRACT The ternary system Fe - 25 at% Co - 9 at% Mo shows an age hardening behavior similar to aluminum alloys. After solution annealing followed by rapid quenching, the Fe-Co-matrix is hardened during subsequent aging through precipitation of the intermetallic µ-phase (Fe,Co)7Mo6. In aged condition the entire Mo content is present in coarse primary and fine µ-phase particles and, therefore, the matrix consists exclusively of 71 at% Fe and 29 at% Co. The binary system Fe-Co shows a transformation from the disordered bcc structure to the ordered B2 structure between 25 and 72 at% Co at a critical ordering temperature ranging from room temperature to 723°C. As a consequence, the remaining overaged matrix in the Fe - 25 at% Co - 9 at% Mo system should also show such a transition. However, an ordered phase is brittle and, thus, not wanted for many applications. Better mechanical properties in terms of ductility can be achieved with a partially or fully disordered phase. Such a state can be obtained by rapid quenching from temperatures above the critical ordering temperature. In this study such an approach was implemented on the ternary Fe - 25 at% Co - 9 at% Mo alloy. The effect of different cooling rates on the mechanical properties was investigated by means of hardness testing. The actual ordering transition of the Fe - 29 at% Co matrix was determined with differential scanning calorimetry and neutron diffraction. INTRODUCTION Solution annealing and subsequent aging of the ternary Fe - 25 at% Co - 9 at% Mo alloy cause the formation of nm-sized (Fe,Co)7Mo6 µ-phase precipitates, which leads to significant hardening of this alloy. However, overaging decreases the strengthening effect as the precipitates start to coarsen. In this condition, the matrix consists of 71 at% Fe and 29 at% Co only [1]. It is known that in Fe-Co alloys a bcc↔B2 order transition takes place between 25 at% and 72 at% Co. There, the ductile disordered bcc α-matrix transforms at a critical ordering temperature into a brittle B2 ordered α’-matrix showing a CsCl lattice. Moreover, rapid quenching of such alloys promotes a disordered matrix with a reduced hardness [2]. It has been
reported for a Fe - 30 at% Co (Fe30Co) alloy that rapid quenching with water leads to a completely disordered Fe30Co matrix [3]. Since the overaged matrix of the investigated alloy exhibits almost a Fe30Co composition [1], it is assumed that in our system such an order transition will also take place. To validate this order phenomenon, annealing heat treatments in the disordered bcc-region followed by different c
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