Microstructure and Mechanical Behaviour of Ordered Au-Cu-Pt Alloys

This thesis investigates nine new compositions of the ternary Au-Cu-Pt system, containing from 75 to 78 wt% Au with 0 to 15 wt% Pt, by means of in-situ synchrotron radiation X-ray diffraction, differential scanning calorimetry, transmission electron microscopy and mechanical tests, including hardness, tensile and repeated stress-relaxation testing. X-ray diffraction data show that the alloys can be separated into three groups according to their stable low-temperature ordered phase(s), namely L10, L12, or the two combined. Platinum increases transformation temperatures in comparison with binary AuCu, two-phase ordered alloys showing the highest transformation temperatures. Details of the evolution of the peak structures upon heating and cooling point to significant differences between mechanisms of disordering and ordering: whereas ordering visibly proceeds at significant undercooling by nucleation and growth, disordering appears, from the visible shifts in peak position, to progress in more homogeneous fashion within the alloy. Alloys containing 2.5 wt% Pt show different microstructural evolutions when ordered at 250 or 400 °C after annealing at 650 °C and a water quench. With 75 and 76.5 wt% Au, the microstructural evolution depends on the ordering temperature. At 400 °C, a classical polytwin structure with two L10 crystal orientation variants is formed; this evolves into a two-phase, L10 + A1, checkerboard-like microstructure after 106 s. At 250 °C, ordering develops a stable (up to 105 s) structure containing all three L10 crystal variants and a network of perpendicular {110} twins roughly 30 nm wide. The 78 wt %Au alloy develops at both ordering temperatures a similar three-variant nanotwinned structure that also remains stable up to 105 s. With all three alloys containing 2.5 wt% Pt, superior hardness and tensile strength, coupled with lower ductility, are obtained with the three-variant nanotwinned structure formed at 250 °C compared with the more classical polytwin structure that develops at 400 °C. Activation volumes characteristic of room-temperature compressive plastic deformation in Au- 21 wt% Cu- 2.5 at% Pt, are measured using the repeated stress-relaxation method on the classical polytwin and the three-variant structures. All ordered nanotwinned microstructures obtained have an initial effective activation volume Veff of 100 to 120 b3; this is values typical of cross-slip or twin boundary bypass in FCC metals. The two structures differ strongly as they work-harden: the two-variant polytwin structure yields a straight line on a Haasen plot, suggesting that work hardening is associated with an increase in slip obstacle density, most likely dislocation debris at intervariant boundaries. The three-variant structure on the other hand shows a constant activation volume. This suggests the presence of a mechanism by which obstacles that accumulate during deformation of the two-variant structure can be removed when three variants are present; a possible mechanism is debris elimination by intraboundary glide coupled with triple line leakage or loop annihilation. Alloys containing 7.5 or 15 wt% Pt have microstructures in which the L12 phase is found, either alone or in combination with L10. Whereas fully L12 alloys (of higher Pt content) are relatively soft, two-phase alloys are nearly as hard and ductile as fully L10 alloys, and feature relatively stable structures, which TEM observation suggests consist in several variants of the L10 phase embedded in a matrix of L12.


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