Acta Metallurgica最新文献

The site preference of ternary additions in an A₃B intermetallic compound with the L1₂ structure is modeled using the tetrahedron approximation of the Cluster Variation Method. At T = 0 K, there are three fundamental site preference behaviors for the ternary additions, depending on the relative magnitude of the effective pair interactions. It is found that the site preference of the ternary element is generally not related to the direction of the L1₂ solubility lobe. Our model also indicates that, under some conditions, the site preference can change with alloy composition and with temperature.

Observations are reported on the precipitation kinetics of a range of Al-Ag alloys under different conditions of aging temperature and supersaturation. For most of each precipitation reaction a large and constant aspect ratio is seen together with the thickening and lengthening rates expected for diffusion-controlled growth. The aspect ratios increase with mean precipitate spacing. The interpretation of the results is that at the start of the reaction thickening is inhibited due to a low density of growth ledges. The thickening inhibition causes an increase in the aspect ratio, continuing until the precipitates, growing on nonparallel {111} matrix habit planes, intersect each other. After intersection, growth ledges are generated and appear to have become sufficiently numerous to allow diffusion limited thickening. However observations of coarsening at both 406°C and at 225°C suggests that the thickening reaction is again inhibited, despite residual precipitate intersections, under the very low driving forces of the coarsening reaction.

The microstructure and mechanical properties of Al₆₇Ni₈Ti₂₅ have been investigated. After homogenization, the microstructure consists primarily of large grains of the π phase with the L1₂ crystal structure, but second phase particles and pores are also present. The dislocation density in the π phase is low after homogenization. but increases substantially during compressive plastic deformation. In compression. Al₆₇Ni₈Ti₂₅ has a yield strength of 355 MPa at room temperature and can exhibit substantial plasticity. The mechanism of compressive plastic deformation is a₀〈110〉{111} slip. The a₀〈110〉{111} dislocations are not dissociated by more than 2 nm, indicative of high {111} APB and SISF energies. Under the influence of a tensile stress, Al₆₇Ni₈Ti₂₅is brittle and fails by transgranular cleavage. The brittle fracture behavior is consistent with ideas originally proposed by Rice and Thomson concerning an activation barrier for emission of dislocations from crack tips.

The compressive flow stress of Pt₃ Al single crystals was measured as a function of temperature and orientation of the compression axis to check the predictions of the theory of Tichy et al. [Phil. Mag.A53, 485 (1986)] regarding the operative slip systems. It was found that samples oriented near [001] slip predominately on (111) planes while those oriented on the opposite side of the triangle slip on (001) planes. The temperature dependences of the CRSS for (111) and (001) slip are similar, viz. the CRSS increases dramatically with decreasing temperature from an almost constant value at elevated temperatures. The CRSS for cube slip is always less than that for octahedral slip. The transition from octahedral to cube slip occurs over a small range of orientations, as predicted by the Tichy et al. model, but slip on only one system was observed in samples of a given orientation.

The reciprocal lattice of a single R-phase variant formed in a Ti₄₀NI₅₁ shape memory alloy was investigated and it was found that $\text{1}\text{3}$$\text{1}\text{3}$$\text{1}\text{3}$ reflections arc situated on the elongated 〈111〉_B2^∗ direction corresponding to the rhombohedral expansion axis and that 0 $\text{1}\text{3}$$\text{1}\text{3}$ reflections lie on three 〈011〉_B2^∗ directions which are all perpendicular to the elongated 〈111〉_B2^∗ direction. There are four distinct R-phase reciprocal lattices corresponding to four distinct R-phasc variants. The four R-phase reciprocal lattices are related by twinning. Based on the R-phase reciprocal lattice, atomic shuffles in the R-phase transition are proposed. The high density of Ti₁₁ Ni₁₄ precipitates in the aged Ti₄₉Ni₅₁ alloy restrict the growth of R-phase variants, and therefore, superimposed reciprocal lattices of the distinct R-phase variants are often observed.

It is investigated by Monte Carlo simulations of short-range order reactions in an f.c.c. alloy A₇₅B₂₅ how the different short-range order parameters are correlated with each other. Thereby three alloys with an increasing number of energy parameters for the different coordination shells are studied at an annealing temperature, always 10% above the critical long-range ordering temperature. The values of the energy parameters are such that the long-range order below this temperature is always of Ll₂-type (Cu₃Au). The initial state of the crystal is always totally disordered. The correlations have been found to depend on the shell energies, on the atom occupations of those lattice sites, which directly influence the performance probability of the reaction step and on the crystallographic features of the operating diffusion mechanism. The latter point is investigated by employing besides the actual mechanism of vacancy interchanges with first neighbourhood atoms also e.g. the fictitious vacancy interchanges with atoms at random distances. The correlations, found by the Monte Carlo simulations, are quantitatively explained for the beginning of the reaction, where the atoms were randomly distributed onto the lattice sites.

The diffusion of silver in nanocrystalline copper was measured at various temperatures between 303 and 373 K. The specimens used were disc shaped pellets of nanocrystalline copper (average crystal size 8 nm). The silver was diffused in from one of the flat surfaces. The resulting silver concentration profile was determined by electron beam microanalysis on taper sectioned specimens. The values of the silver diffusion coefficients range from 0.3 · 10⁻¹⁸m²/s at 303 K to 12 · 10⁻¹⁸m²/sat 373 K. The activation enthalpies deduced were 0.63 eV (for T > 353 K) and 0.39 eV (for T < 343 K).

Dislocations are observed in many shape memory alloys after thermal or stress cycling. The amount of dislocations (with Burgers vector $\text{b}{}_{\mathrm{\beta }}\text{= a}{}_0\text{〈010〉}$ and 〈1̄11〉 line direction in the β phase) increases with the number of cycles. The dislocations are accumulated in the sample and are incorporated in the corresponding growing phase. The relative energy of the dislocations when embedded in the parent phase or in one or another variant of martensite is evaluated in this work. The crystallographic changes of the dislocations provide a primary selection rule for those martensite variants in which the dislocations have the lowest energy. In order to proceed more quantitatively a full calculation of the dislocation energies has to be performed using the anisotropic theory. In this work these calculations have been made on the basis of measured elastic constants of the β and 2H phases of a Cu-Al-Ni alloy. It is found that a given dislocation could indeed have different energies depending whether it is embedded in the β phase or one or another oriented variant of martensite. In particular, those dislocations with Burgers vector lying on the basal plane of the 2H martensite show the lowest energy as compared to their self energy when embedded in other oriented variants (Burgers vector out of the basal plane) or in the β phase.

The increasing use of diffusion bonding as a commercial process has, over the last 15 years, been paralleled by a series of progressively more complex attempts to model the mechanisms and processes occurring during bonding. This paper describes a theoretical model for diffusion bonding which proposes a new and simplified void geometry. The mechanisms operating during diffusion bonding are based on those derived from pressure sintering studies, although the driving forces and rate terms for these mechanisms have been altered to allow for the quite different geometries of the two processes. Also included in this work is an analysis of the effect of grain size which can lead to an enhancement in the contribution to bonding from additional grain boundary diffusion and so may be particularly relevant when joining materials of fine grain size. The extent to which this new model offers significant developments over existing models is discussed; an initial comparison between experimental results and predictions from this new model shows that there is good agreement between practice and theory.

The effect of interfacial friction on the use of the J-integral for calculating the toughness of a brittle-fibre reinforced composite has been examined. A simple micro-mechanical model that includes friction as the only dissipative mechanism yields a lower bound on the toughness. When the same constitutive equations are used with a J-integral formulation, an upper bound is obtained which incorporates other energy losses associated with the mechanics of fibre failure. Finally, a method is provided by which the J-integral can be correctly combined with statistical models of fibre failure to incorporate the effects of fibre pull-out on the toughness of a composite.