Influence of crystallinity on thermo-process ability and mechanical properties in a Au-based bulk metallic glass

cited By 2International audienceThe present investigation addresses the impact of crystallinity on the mechanical properties: hardness (HV) and toughness (KIC) in an Au49Cu26.9Ag5.5Pd2.3Si16.3 BMG which appears especially attractive for applications in jewelry and watch making industries. Thermal stability is first determined using differential scanning calorimetry (DSC), X-ray diffraction and thermo-mechanical analysis (TMA). Then the conditions (time, temperature) in which crystallization is observed (during annealing above the glass transition temperature) and the kinetics are determined. Results show an increase of hardness proportional to the volume fraction of the crystalline phase (φ) (from about 350 HV up to about 480 HV), and a drastic reduction in fracture toughness from about 20 MPa√m down to 1.5 MPa√m for fully crystalline structure. Finally the conditions required achieving a good compromise between hardness and toughness are established. © 2016 Elsevier B.V

Main and secondary relaxations in an Au-based bulk metallic glass investigated by mechanical spectroscopy

cited By 2International audienceThe dynamic mechanical relaxation behavior of an Au49Cu26.9Si16.3Ag5.5Pd2.3 (at.%) bulk metallic glass was investigated by mechanical spectroscopy as a function of temperature or driving frequency. The mechanical spectra show the evidence of a secondary relaxation (or β) process at low temperature or high driving frequency. The main relaxation (α) is clearly evident at higher temperature or lower frequency. Crystallization induces an increase in elastic modulus, in two steps, which correspond to the formation of two different crystalline phases, as determined by in-situ X-ray diffraction experiments. Master curves could be obtained based on the time-temperature superposition principle. From these curves the apparent activation energies for the two relaxation phenomena were determined: Eβ = 1.10 eV and Eα = 3.65 eV, respectively. These values indicate that the secondary relaxation may be attributed to local atomic movements, while the α relaxation corresponds to correlated movements. © 2016 Elsevier B.V

Investigation of Ta-MX/Z-Phase and Laves Phase as Precipitation Hardening Particles in a 12 Pct Cr Heat-Resistant Steel

A 12%Cr martensitic/ferritic steel was designed and produced to study Laves and Z-phase as precipitation hardening particles under creep conditions at 650ºC. To ensure the precipitation of Laves after tempering, additions of W and Cu were selected according to thermodynamic calculations. It is known that Z-phase formation does not follows the classical nucleation theory. Indeed, MX particles are transformed into Z-phase by Cr diffusion from the matrix to the precipitate. Therefore, to promote fast Z-phase formation, Ta, Co and N additions were used to produce Ta-MX which will be transformed into Z-phase. As main results, Laves precipitation was successfully achieved after tempering with a particle size of 196nm. Concerning Z-phase, the transformation of Ta-MX into Z-phase after tempering was confirmed by the formation of hybrid nanoparticles of 30nm. Although, W and Ta have a low diffusion in martensitic/ferritic matrix, characterization of the precipitates after isothermal aging revealed that Laves and Z-phase have a fast growth kinetic, reaching 400nm and 143nm respectively at 8760h. As consequence, creep test at 650ºC showed prematurely fails after few thousand hours. Therefore, investigations focused on the growth and coarsening behavior of Laves and Z-phase, seem to be the next researcher field of martensitic/ferritic steels