Mechanical Behavior of a Pt-Cr Jewelry Alloy Hardened by Nano-Sized Ordered Particles
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Mechanical Behavior of a Pt-Cr Jewelry Alloy Hardened by Nano-Sized Ordered Particles
Kamili M. Jackson, Miyelani P. Nzula, Silethelwe Nxumalo, and Candace I. Lang University of Cape Town, Mechanical Engineering Department, Centre for Materials Engineering Abstract The materials engineering of platinum jewelry is interesting because only 5wt% can be used for alloying in order to maintain hallmarking. However, pure platinum is very soft and must be alloyed in order to be used effectively as jewelry. In several binary systems an increase in hardness has been found after cold working and annealing at low temperatures. The hardening in these alloys has shown to be a result of nano-sized ordered particles. In particular, the existence of the ordered particles has previously been confirmed for a Pt-Cr alloy by TEM. Extensive work has been done on the Pt-Cr alloy to understand the crystal structure and mechanisms of the ordered phase. Hardness tests were performed to measure mechanical properties after various heat treatments. In addition, tensile tests were conducted using a smallscale tensile testing machine. An 8mm long specimen is used, which significantly reduces the cost of the specimens while providing necessary properties. Tensile tests on the Pt-Cr alloys at various post deformation heat treatments show an increase in tensile strength with no effect on ductility. They confirm results of the hardness tests while providing additional properties data. In addition, the results show a fairly good relationship between strength and hardness. Introduction Pure platinum is much too soft for the mechanical demands of jewelry, having a Vickers hardness of only 40-41 and UTS of 117-159 MPA [1,2]. While this softness (and the resulting ductility of 32% strain to failure) is ideal for mechanical forming of jewelry [2] the challenge is to harden jewelry alloys while maintaining the 95% purity hallmark required by most countries. There are various ways of hardening metals but an ideal one for jewelry would provide additional strengthening after forming by cold work. Thus, a useful hardening method for jewelry alloys might be ordering by post-deformation heat treatment, particularly where cold deformation enhances the kinetics of ordering [3]. This type of hardening is being investigated in platinum-chromium alloys at UCT. The Pt-Cr phase diagram shows evidence of ordering from 20-65 at. % chromium [4-6]; however, at the platinum rich end (greater than 95 wt. % Pt) of the phase diagram no evidence of ordering has previously been reported. Here we present the effect of ordering on hardness and tensile properties in these alloys. Experimental Methods A range of techniques was used to explore the ordering and hardening behavior of two platinum rich alloys, Pt 10 at.%Cr and Pt 11 at.% Cr (Pt 3.0 wt.% Cr and Pt 3.2 wt.% Cr), after various heat treatments. These techniques include Vickers hardness measurements, optical microscopy, transmission electron microscopy (TEM), and small sample tensile testing. Alloys were cold rolled
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