Acta Metallurgica et Materialia最新文献

Stoichiometric Mo₃₃Si₆₆ and Nb₃₃Si₆₆ have been alloyed by mechanical alloying (MA) from the elemental powders and the process has been monitored by X-ray diffraction (XRD) technique. Nanocrystalline intermetallic compounds were obtained for either compositions: the NbSi system gave the expected NbSi₂ compound, while in the MoSi system both αMoSi₂ (low temperature phase) and βMoSi₂ (high temperature phase) with the latter being the dominant phase, were promoted by the milling action. The metastable equilibrium αMoSi₂↔βMoSi₂, as well as the tendency to reaction of prealloyed MoSi samples, have been examined by means of differential thermal analysis (DTA), followed by X-ray identification of the formed phases. Finally, the oxygen influence on phase transformation induced by both milling action and thermal treatment has been examined. It has been found that oxygen pick-up not only strongly inhibited the solid state reactions in the process of mechanical alloying but also caused the destabilization of the already formed intermetallic compounds.

Experiments were performed on orthotropic columnar fresh-water ice of ≈ 6mm column diamter proportionally loaded across the columns under biaxial compression at −10°C under various combinations of strain rate and stress ratio, R = σ₂₂/σ₁₁. The ductile-to-brittle transition strain rate increased from about 8 × 10⁻⁵ s⁻¹ under no confinement to about 3 × 10⁻⁴s⁻¹ under intermediate confinement ($\text{R ⋍ 0.3}$), and then decreased to about 5 × 10⁻⁵s⁻¹ under full confinement (R = 1). The ductile failure envelopes are semi-elliptical in shape and conform fairly well to those created with the use of Hill's [9] criterion for the yielding of plastically orthotropic materials. The transition strain rate is rationalized in terms of the effects of the confining stress on the ductile and the brittle failure stresses, and is modelled in terms of the suppression of creep deformation at the tips of wing cracks.

This study aims to determine the different deformation mechanisms of grade 702 zirconium under uniaxial tension and at room temperature. The grade 702 zirconium tested had undergone an α → β → α cycle at a slow cooling rate (∼ 15° s⁻¹). Three deformation mechanisms were identified: prismatic slip, (1012) 〈101〉 twinning and (1121) 〈1126〉 twinning. The critical resolved shear stress for prismatic slip, (1012) 〈1011〉 twinning and (1121) 〈1126〉 twinning was also determined. The effect of the non-uniform redistribution of the hardening elements on location through the grain of the various mechanisms and on the tendency for localized deformation to develop is also discussed.

In split cube geometries the specific fracture energies of sandstone-mortar (14 Nm⁻¹) and limestone-mortar (6 Nm⁻¹) interfaces were determined. They are much lower than for bulk mortar (80 Nm⁻¹). A damage type law describes well the development of a process zone around the interfacial cracks. In finite element calculations the damage law descrobes the behaviour of flat interfaces, but needs improvement for curved interfaces.

The main part of the line energy of dislocations in solids can be described by elastic continuum theory (linear, nonlinear and nonlocal). There remains, however, a core part which must be treated atomistically. Based on dimensional arguments it is shown that independent of the interatomic potential and independent of the core configuration there exists a unique correlation between the elastic energy and the atomic misfit energy. This results from the fact that the displacements are coupled to the asymptotic field of a singular dislocation.

For prismatic slip in α titanium, the temperature dependence of the activation area peaks at around 350 K in specimens of moderate purity. TEM in situ deformation experiments have been performed between 150 and 473 K in order to determine the origin of this peak. In the entire temperature range, rectilinear screw dislocations are found to move by jumps between locking positions. The jump length decreases as the temperature is increased. This last experimental result isat the origin of the present model. This jump length variation leads indeed to several transitions between the microscopic mechanisms controlling the dislocation propagation. For each mechanism, the activation area is calculated as a function of stress. A global evolution of the activation area is obtained and compared with the data reported in the literature.