Physical and Mechanical Properties of Two-Phase Zr-Cr-Mn Alloys
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*Present address: Cercon, Howmet-Cercast Group, Dallas, TX. ABSTRACT The Zr-Cr-Mn system is used to explore the effect of a lowered SFE on the room temperature mechanical properties of a Laves phase using elements of similar atomic size. The ternary Zr-Cr-Mn
diagram in the region from 0 to 12 at. % Mn is first determined and it is shown that Mn substitutes only for Cr in the Laves phase. TEM analysis of the density of stacking fault energy related defects
such as annealing twins indicates that Mn substitution for Cr in ZrCr 2 lowers the SFE of the cubic
Laves phase. Mechanical testing of the two phase alloys is used to explore the effects of Mn content and the volume fraction of each phase on the ductility and fracture behavior in compression. It is found that the mechanical properties are well-described by a model incorporating solid solution strengthening in a ductile-brittle two phase alloy.
INTRODUCTION Recent reports of some room-temperature ductility via twinning in two phase alloys containing
the cubic Laves phase compound HfV2+Nb suggest that the Laves phases deserve more scientific attention [1-3]. It has been proposed that the ease of twinning may be controlled either by a lowered stacking fault energy (SFE) or by the atomic size of Nb, which is intermediate between Hf and V. Unfortunately it is impossible to isolate either effect in the Hf-V-Nb system since a lowered SFE has been shown to occur with increased concentration of the intermediate-sized element [4]. In the present work, the Zr-Cr-Mn system is chosen to explore the effect of a lowered SFE on the mechanical properties of the Laves phase using elements of similar atomic size. Within the literature, the pseudo-binary ZrCr 2 - ZrMn 2 phase diagram has been determined [5]. A transition from the cubic C15 Laves phase to the hexagonal C14 Laves phase with increasing Mn content was reported. This information is used as a basis with which to choose alloy compositions. In the present work, the ternary Zr-Cr-Mn diagram inthe region from 0 to 12 at. % Mn is first determined. It is important to determine on which sites in the cubic Laves phase the Mn substitutes as well as the alloy composition at which the C15 to C14 transition is found. This latter information is required since two-phase alloys containing the Cr-Mn BCC solid solution and the Laves phase are studied in order to facilitate mechanical testing. Characterization of the density of planar defects in the undeformed material is used as a qualitative indication of the effect of Mn alloying on the SFE of the cubic phase. The mechanical properties and fracture behavior of the alloys are then characterized to determine whether Mn alloying enables some deformation of the cubic Laves phase at room temperature. EXPERIMENTAL PROCEDURE Alloy buttons were prepared by arc melting using a nonconsumable tungsten electrode on a water-cooled copper hearth in an argon atmosphere. The purity of the elemental Cr, Zr and Mn used to produce the alloys was 99.995 at. %,99.8 at. % and 99.99 at. %,respectively. Bo
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