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Particulate reinforced material

This thesis is an exploration of deformation and fracture properties of a novel two-phase α2 + γ Ti46Al8Ta (at.%) alloy featuring a "convoluted" microstructure, produced by partners within a European Union project named IMPRESS. Properties of this alloy are compared with those of other two-phase TiAl alloys, to assess the performance of the novel convoluted microstructure in critical, load-bearing high-temperature applications. Uniaxial compression tests featuring repeated relaxation cycles are conducted from room-temperature to 1073K on three different α2 + γ TiAl alloys, namely Ti-46Al with 8Ta, 8Nb or 7Nb (at.%). Of these, the first two are novel "convoluted" cast titanium aluminides developed within the project, while the last is a more conventional alloy displaying a duplex microstructure. The data include measurements of the flow stress and activation volume of these alloys at 293, 673, 873, 923, 1023 and 1073 K. It is found that, despite differences in composition, processing and microstructure, the values and evolution with temperature or with stress of these parameters are relatively similar for the three alloys. This observation, coupled with the agreement of present results with data in the literature for similar alloys, leads to conclude that the strain-rate dependence of plastic flow in alloys of this class is governed by features of the γ-phase that are relatively insensitive to the α2 phase morphology. The convoluted alloys furthermore feature a yield stress anomaly previously documented for this class of intermetallic alloy. Fracture toughness measurements are conducted from RT to 923K on the Ti46Al8Ta (at.%) alloy, and compared with similar data gathered on two alloys featuring a lamellar microstructure, namely the same Ti46Al8Ta alloy heat-treated to feature a coarse lamellar structure, and a boron-refined industrial alloy of the XD class, of composition Ti45Al2Mn2Nb (at.%) + 0.2 (vol. %) TiB2, also heat-treated to display a lamellar structure. The convoluted alloy displays an initiation fracture toughness ranging from 12MPa√m at RT to 20MPa√gm at 923 K. Comparatively, at room temperature it slightly outperforms the fine lamellar XD alloy, while the coarse lamellar alloy of similar composition is significantly tougher. At 923 K, the convoluted alloy outperforms the fine lamellar XD alloy, the fracture toughness of the coarse lamellar is still superior but to a lesser extent. All alloys, including the convoluted alloy, show pronounced R-curve behaviour at both room temperature and 923 K. Examination of the crack path by cross-sectioning of samples having undergone interrupted toughness tests and slightly reloaded in the scanning electron microscope show roughly similar fracture characteristics for all three alloy. These observations, coupled with the fracture toughness data of this work, show that the convoluted alloy is toughened by the same mechanisms, of crack deflection, microcracking, and shear ligament toughening, as two-phase TiAl alloys having the lamellar structure, known to be optimal from the perspective of fracture toughness. The effects of temperature and atmosphere (vacuum, dry air and atmospheric air) are assessed on the Ti46Al8Ta alloy featuring a convoluted microstructure. Fracture toughness data suggest a marginal difference between dry or atmospheric air; however, the difference is within experimental error. During the tests, however, clear evidence was found to document that ambient humidity causes subcritical crack growth, presumably due to hydrogen embrittlement.

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.