Acta Metallurgica最新文献

Ultrasonic measurements have been done of the elastic constants of a number of binary intermetallic compounds that melt above 1500°C. The data give directly the stiffness and specific stiffness of these materials and are expected to be indicators of their strength and specific strength.

Crystals of MnZn ferrite are deformed by indentation with a loaded sphere. On all planes studied, (100), (111) and (110) the deformation consists of slip lines, above a certain threshold of stress. At higher loads new lines appear, apparently coupled to the first ones. At still higher loads cracks are found as well. The slip patterns are discussed for {100}, {111} and {110} slip systems with a Burgers vector along <110>. The analysis is supported by calculations of the resolved shear stress (RSS) on the basis of the elastic equations for a halfspace loaded by a sphere. The source of dislocations is located where the RSS is largest. From there slip propagates into the sample and to the surface. The predicted position and orientation of the lines on the surface agree with experiment. Depending on the plane of indentation, slip starts on {100} or {111}, then continues on the other systems. The observed cross-slip requires an extension of the RSS model used here.

Friction stress σ₀ was measured on specially prepared, uniaxial. carbon yarn and carbon-carbon composite specimens up to a temperature of 2310°C. Relations between the friction stress, applied stress and other parameters are derived. The amount of work needed to activate the apparent activation volume V₀ is V₀(σ − σ₀) = 4 kTn'. The friction stress and the applied stress relation is $\text{σ}{}_0\text{= [}\text{(n − n ')}\text{n}\text{]σ}$, and the friction stress and the test temperature relations can be expressed as σ₀ ∝ exp ($\text{l}\text{T}$). These relations are consistent with test results.

A grain-bridging model of crack-resistance or toughness (R-curve, or T-curve) properties of nontransforming ceramics is developed. A key new feature of the fracture mechanics treatment is the inclusion of internal residual (thermal expansion mismatch) stresses in the constitutive stress-separation relation for pullout of interlocking grains from an embedding matrix. These internal stresses play a controlling role in the toughness properties by determining the scale of frictional tractions at the sliding grain-matrix interface. By providing a physical account of the underlying micromechanics of the bridging process the analysis allows for predetermination of the material factors in the constitutive relation, thereby reducing parametric adjustments necessary in fitting the theoretical toughness curve to experimental data. The applicability of the model is illustrated in a case study on indentation-strength data for a “reference” polycrystalline alumina with particularly strong T-curve characteristics. From theoretical fits to these data the constitutive relation, and thence the entire T-curve, can be deconvolved. This “parametric calibration”, apart from demonstrating the plausibility of the model, allows for quantitative predictions as to how the toughness and strength characteristics of ceramics depend on such microstructural variables as grain size and shape, grain boundary energy, level of internal stress and sliding friction coefficient. An indication of this predictive capacity is provided by a preliminary calculation of the grain-size dependence of strength, using some existing data for other aluminas as a basis for comparison.

Stress rate change experiments have been used to assess the contribution of mechanical activation of dislocation glide to the inelastic deformation of 304 stainless steel. These experiments were conducted in a servohydraulic testing machine by loading at stress rates from 5 to 40 MPa/s to a constant stress between 250 and 450 MPa. An extrapolation method was developed to determine the inelastic strain rates just prior to and just after the transition in stress rate. The results of these experiments demonstrate that the inelastic strain rate is continuous in this material through a transition in stress rate. These findings, in contrast to previously published results, indicate that mechanical activation does not occur under these testing conditions.

Polycrystals of aluminum, initially cubic in shape, were deformed in various sequences of free or plane-strain compression increments to three different final shapes: one corresponding to compression, one to tension, and one to no net shape change. The orientation distributions were determined both initially and after the various strain paths. All of the samples exhibited significant changes in texture, in accordance with the total symmetry of each test. The final textures were in quantitative agreement with the results of computer simulations of polycrystal plasticity. The Taylor factors determined by this calculation were used to convert the measured stress-strain curves to crystallographic shear stress vs shear curves; this improved the comparison with experimental stress-strain curves, but the strain hardening in some tests involving path changes remained below that of unidirectional compression. The difference may be attributed to instabilities in the dislocation structure upon a path change.

Creep crack growth of a 12% CrMoV steel has been investigated at 650°C in SENT specimens with and without side-grooves approaching plane strain and plane stress conditions respectively. Experiments have been conducted under different loading conditions including constant $\text{C}{}^{\mathrm{\ast }}$, the J-integral rate parameter. Transitions in crack growth behaviour are observed early in the crack growth and later when the crack becomes long with respect to the specimen width. The early transition is dominated by the initial development of cavitation damage ahead of the crack tip. Once this transition is complete, the correlation between $\text{C}{}^{\mathrm{\ast }}$ and the crack growth rate becomes valid. This correlation is independent of loading conditions, it coincides with theoretical expectations and it is valid until the second transition where the extent of damage has a dominant influence on the stress field ahead of the crack tip. A trend has been observed that crack growth is slower in smooth-sided specimens as compared to the crack growth in side-grooved specimens for the same value of $\text{C}{}^{\mathrm{\ast }}$. Metallographic investigations indicate that crack growth occurs by the formation and growth of cavities and microcracks ahead of the crack tip. A comparison of the damage in smooth-sided and side-grooved specimens shows that creep damage is much more extensive under plane stress conditions than under plane strain conditions for the same value of $\text{C}{}^{\mathrm{\ast }}$. This difference is due to the multiaxial stress field, produced by the side-grooves, which influences cavitation and microcrack formation in the material. It also accounts for the smaller crack growth rates in smooth-sided specimens.

The ductility of a preoxidized Ni₃Al (Ni-23Al-0.5Hf-0.2B. at.%) alloy with various grain sizes (17–193 μm) was evaluated by means of tensile tests at 600 and 760°C in vacuum. The preoxidation does not affect the ductility of the finest-grained material at either temperature, whereas it causes severe embrittiement in the largest-grained material, especially at 760°C. Auger studies revealed very little oxygen penetration along grain boundaries in the finest-grained material but substantial oxygen penetration in the largest-grained one. A continuous, thin Al-rich oxide layer which forms on the fine-grained samples protects the underlying alloy from oxygen penetration, preventing any loss of ductility, whereas the nickel-rich oxide which forms on the large-grained samples allows oxygen to penetrate along grain boundaries, causing severe embrittiement. The grain boundaries act as short-circuit paths for rapid diffusion of aluminum atoms from the bulk to the surfaces, and this is responsible for the change in oxidation product from Ni-rich to Al-rich oxide with decreasing grain size.