Recrystallization and grain boundary formation in an 18-carat gold alloy studied by mechanical spectroscopy

Heavily deformed 18-carat yellow gold samples show a recrystallization peak at 700 K during the first heating. The mechanical loss spectrum of polycrystals shows a relaxation peak at about 780 K, which is absent in single crystals made from the same alloy. Stepwise deformation of a single crystal from 2 % to 10 % causes an increase of the high temperature mechanical loss background and the appearance of a high temperature peak. At 8 % deformation the high temperature peak disappears and the peak that is normally observed in polycrystals appears. The increase of the exponential background is interpreted as due to the introduction of new dislocations whereas the high temperature peak is attributed to a relaxation mechanism in the sub grain boundaries. The peak of polycrystalline samples located at intermediate temperatures depends on the grain size: with grain growth, the peak position shifts to higher temperatures. The peak temperature can be related to the mean grain size.

Grain Boundary Relaxation in 18-carat Gold Alloys

Ann-Kathrin Audren

Grain boundaries (GBs) play an important role for the mechanical properties of metals. In addition to dislocation motion inside the grains, some metal alloys deform along the GBs at high temperatures. GBs can also be the cause of a brittle behaviour of some metallic alloys, where crack propagation along the GBs is observed. This thesis aims to understand the role of GBs on the thermo-mechanical behaviour of metals and especially to define the microscopic mechanism, which leads to a deformation at the GBs. In this thesis, polycrystals, single crystals and bi-crystals of a yellow gold alloy are studied in detail, primarily by mechanical spectroscopy. The analysis and interpretation of the experimental data identifies different anelastic relaxations (internal energy dissipation processes), that produce peaks in the mechanical loss spectrum. The mechanical loss spectrum of polycrystals shows a relaxation peak P2 at about 780K, which is absent in single crystals made from the same alloy. Stepwise deformation of a single crystal causes an increase of the high temperature mechanical loss background and the appearance of a high temperature peak (P3). Above a critical deformation, P3 disappears and the peak P2, which is normally observed in polycrystals, appears. The increase of the exponential background is interpreted as due to the introduction of new dislocations whereas the peak P3 is attributed to a dislocation relaxation mechanism in the sub-grain boundaries. The peak P2 located at intermediate temperatures depends on the grain size: with grain growth, the peak position shifts to higher temperatures. It is shown that the relaxation time is proportional to the grain size d. Such a grains size dependence is in agreement with the Zener model based on geometrical considerations of an assembly of elastic grains, which can slide against each other. The investigations on bi-crystals containing a single GB with a specific misorientation between crystal lattices and a specific boundary plane orientation show that the relaxation peak P2 is closely related to a mechanism taking place at the boundary. The peak height is proportional to the GB density in a bi-crystal, whereas a single crystalline part cut from a bi-crystal does not show a relaxation peak. Molecular dynamics simulations are performed in order to illustrate the potential microscopic mechanisms responsible for the stress relaxation peak P2 in polycrystals. A Sigma5 grain boundary is submitted to a shear deformation parallel to the boundary plane. The grain boundary shows a migration perpendicular to the boundary plane coupled to shear for temperatures below 700K. Above 1000K, only grain boundary sliding is observed. Two models are developed that provide expressions for the relaxation strength and the relaxation time that are compared to experimental measurements performed on polycrystals. The observed grain size dependence favours the sliding model over the migration model. Measurements as a function of stress allow a refinement of the sliding model. The stress amplitude dependence of the peak P2 indicates a depinning mechanism, which is interpreted as due to GB dislocations between zones where sliding of different amount occurred. Obstacles to the sliding movement like steps or extended coincidence sites in the GB plane act as pinning points. The pinning points can be overcome at high temperatures close to the melting point, which results in the onset of local microscopic creep.

EPFL2014

High temperature mechanical spectroscopy of fine-grained zirconia and alumina containing nano-sized reinforcements

Claudia Ionascu

Polycrystalline zirconia (3Y-TZP grade) unreinforced and reinforced with multiwalled carbon nanotubes (CNTs) or silicon carbide whiskers (SiCw) or silicon carbide particles (SiCp) have been processed and studied by mechanical spectroscopy with complementary observations of electron microscopy and creep tests. Moreover, for comparison with zirconia, polycrystalline alumina specimens, unreinforced and reinforced with silicon carbide nanoparticles, have also been studied. The high-temperature mechanical loss spectrum of pure zirconia presents an exponential background (exponential increase with temperature) accompanied by a decrease of the dynamic shear modulus above 1200 K. A dissipation peak, which transforms into an exponential increase, appears in the spectra obtained as a function of frequency (isothermal spectrum). The peak does not depend on the amplitude of the applied stress, whereas the exponential increase is stress dependent above a certain threshold. This result has been linked to the interface reaction, which plays an important role in deformation accommodation during creep. The high-temperature mechanical loss is grain size dependent. Smaller the grain size, higher the mechanical loss. The presence of the impurities in the starting powder of zirconia leads to a shift of the entire mechanical- loss spectrum towards lower temperatures. Doping zirconia with different amounts of CNTs results in a decrease of the isothermal-mechanical loss level with respect to pure zirconia. A dissipation peak that transforms into an exponential increase is present for a higher amount of CNTs in the mechanical-loss spectrum as a function of temperature. Zirconia doped with CNTs exhibits a lower creep strain than pure zirconia when submitted to a compressive stress at high temperature. Also, addition of different amounts of SiCw to zirconia leads to a decrease of the mechanical-loss level. A high-temperature dissipation peak is presents in the isochronal spectrum. The peak is better resolved for a higher amount of SiCw. The creep-strain curve is lower in SiCw-doped zirconia than in pure zirconia at high temperature. Addition of SiCp to zirconia may shift or not the mechanical-loss spectrum towards lower temperatures. Doping zirconia with different amounts of SiCp results in the appearance of a peak or peaks in the high temperature isochronal and isothermal spectrum. The height of the peaks depends on the amount of SiCp to zirconia. Similar to zirconia, the high-temperature mechanical loss spectrum of alumina consists of an exponential increase of the mechanical loss and a decrease in the dynamic shear modulus above 1200 K. Additions of SiCp lower the high-temperature mechanical loss with respect to the one in pure alumina. High-temperature plastic deformation of pure zirconia and alumina is interpreted in terms of a theoretical model of GB sliding. When GB sliding is not limited by obstacles like multiple junctions, GB asperities or other defects, creep occur and an exponential background is observed in the mechanical-loss spectrum. Addition of nano-sized reinforcements on the GBs results in a decrease of the exponential background. This means that the GB sliding is more difficult and as a consequence a better creep resistance is observed. The activation parameters obtained from the Arrhenius plots were apparent. Two methods are used to correct the apparent values. However, even if the corrected values seem more reasonable than the values obtained from the Arrhenius plots, they do not bring useful information. It is proposed that apparent too high values of the activation enthalpy account for the evolution of the GB microstructure with temperature. Thermal evolution of the GB microstructure is believed to offer short circuit paths for GB diffusion, which allows the pinning nano-sized reinforcements to be overcome. Then pinning effect is less efficient.

EPFL2008

Strain-amplitude-dependent mechanical loss at intermediate temperatures in a Ni-3(Al, Ta) single crystal

Nadine Baluc, Joël Bonneville, Robert Schaller

Dislocation dynamics in a Ni-3(Al, Ta) single-crystal alloy have been studied by means of mechanical spectroscopy at intermediate temperatures, that is in the temperature range of the flow stress anomaly. The mechanical loss of specimens predeformed at 300 K was found to be strongly dependent on temperature, pre-deformation level and heating rate. Thermal cycling (heating-cooling) and ageing experiments were also performed, which provided additional information about the annealing time dependence of the mechanical loss, elastic modulus of the material and plastic deformation of the specimens. The obtained results can be explained by a combination of two phenomena simultaneously occurring as the temperature is increased from 300 to about 500 K exhaustion of the mobile dislocation parts (superkinks) and pinning of the screw dislocation segments via cross-slip from the (111) onto the (010) planes.