Acta Metallurgica最新文献

An estimate is made of the geometrical rate constant G₃ in the parabolic grain growth law d〈r〉²/dt = kG₃ in three dimensions, where 〈r〉 is the average volume-equivalent grain radius and where k = -u_n/K is a positive constant in which u_n is the local boundary velocity along an outward pointing normal and K is the local mean curvature (positive for a sphere). The parabolic law follows from the above velocity rule and a hypothesis of statistical self-similarity of the structure. The estimate is based on (l) a theorem deduced from the preceeding assumptions, (2) an approximate formula for the rate of change of the volume of a given grain, (3) a model of polyhedral geometry and (4) the experimental data of Hull on separated β-brass grains. We estimate G₃ = 0.5 ± 0.l. Comparison with previous estimates is made.

The toughening of Al₂O₃ by Al has been investigated, using electron microscopy to provide in sit measurements of the microstructural parameters that govern ductile ligament toughening. The results are used to compare the measured toughness with values calculated for ligaments that exhibit limited debonding during ductile rupture. Good agreement confirms that toughening is dominated by the plastic work expended in the ductile rupture of ligaments stretched between the crack surfaces. Furthermore, it is demonstrated that in situ measurements are needed to assign the appropriate flow stress, because of the occurrence of coupled precipitation/solution hardening, and to ascertain the extent of the plastic stretch.

A cylindrical test specimen for evaluating the toughening of brittle intermetallics by ductile reinforcements has been evaluated. A processing procedure capable of producing specimens has been devised, using HIPing, and a method for the tensile testing of the specimen has been established. Results are presented for γ-TiAl reinforced with Nb and a Ti-33 at.% Nb alloy. The toughening imparted by these materials is interpreted in terms of their strength, ductility and interface reactions, as well as loss of constraint mediated by the extent of debonding along the reaction product layers. Finally, the results are compared with those previously obtained on actual composites.

Auger electron spectroscopy has been used to study indium grain boundary segregation in symmetric «110å tilt boundaries and randomly oriented boundaries in bicrystals as well as polycrystals of NiIn containing 0.l–1.4 at. % In. Quench-induced non-equilibrium segregation of In, caused by fluxes of In vacancy complexes to the interfaces, influenced the results for quenching temperatures above 1000–1100 K. A careful analysis of sputtering profiles supplied additional informations for the discussion with respect to current models of QINS. From the equilibrium segregation behaviour for the symmetric boundaries both the segregation enthalpies (ΔH_se = 38 ± 3 kJ/mol and ΔH_seg = 39 ± 3 kJ/mol) and the mean segregation entropy [ΔS_seg = (0 ± 0.5)R] have been determined for the first time. The segregation free energy for 970 K is ΔG_seg = 50 ± 5 kJ/mol for the polycrystals. The symmetric atomic arrangement along the tilt boundaries is assumed to be responsible for the low segregation entropy.

A grain size dependent rheological model is presented for high temperature creep that is capable of predicting both primary and tertiary as well as secondary creep. Primary and tertiary creep rate are shown to be strongly influenced by grain size, whereas secondary or minimum creep rate is rather insensitive to it. During the primary creep stage creep rate increases with decreasing grain size, but the reverse is true in the tertiary or accelerating range. An increase in grain size also dictates decrease in the time to reach minimum creep rate concomitant with a decrease in strain. The model is based on intragranular dislocation creep enhanced by grain-facet size cracks produced during deformation by the embrittlement process that is caused by an intergranular sliding mechanism. Incorporation of the kinetics of microcracking activity is the foundation of the theory.

Specimens of amorphous Fe₄₀Ni₄₀P₂₀ exposed to neutron-irradiation and subsequently annealed at different temperatures are shown to undergo a similar phase separation into amorphous P-enriched and P-depleted regions as occurs in specimens annealed without prior irradiation. Whilst the radius (∼ 3.2 nm) of the P-rich regions is independent of whether the specimen has been irradiated or not, the onset of phase separation occurs for irradiated samples at lower temperatures; under identical annealing conditions the volume fraction of P-rich clusters is much larger in irradiated FeNiP than in un-irradiated material. The faster phase separation kinetics are a consequence of the irradiation-induced excess volume which allows for an increased mobility of individual atoms.

The driving forces for various possible reactions within the bainitic transformation temperature range of Cu 40 at.% Zn alloy have been calculated. The results show that because of the occurrence of β→β′ ordering transition within the temperature range, the driving forces ΔG^{β′→α′} and ΔG^{β′→β′+α} increase inversely with decreasing temperature, with ΔG^{β′→α′} > 0 and ΔG^{β′→β′+α} < 0. Moreover, the equilibrium temperature T₀ of the two phases β′ and α′ is far below the experimental B_s for alloys of various compositions. Therefore, the bainitic transformation in CuZn alloys can only proceed as that of diffusional reaction β′→β₁′+α.

Ductile fracture studies have been conducted on four high purity AlCuMgZr (2134 type) alloys containing 0, 0.31. 0.61 and 1.02 wt% Mn in the under and overaged conditions having similar yield strengths. The second phase particle content, i.e, Mn rich dispersoids and Mn containing large particles, increased with increasing Mn content. In both aging conditions maximum ductility and toughness were observed for the 0.31% Mn alloy and minimum values were observed for the 1.02% Mn alloy, The largest void content or damage accumulation due to void nucleation and growth at any strain level occurred in the 1.02% Mn alloy, consistent with ductility values. The 0.31% Mn alloy showed the highest ductility in both aging conditions, Although the void volume fractions for the 0.31% Mn alloy were similar to those of the 1.02% Mn alloy, accumulation occurred at higher strains. The void nucleation and growth data and microstructural analysis suggest that the 0.31% Mn additions provide sufficient submicrometer Mn-dispersoids to homogenize slip without producing large Mn-rich primary particles which decrease ductility. Ductility was observed to decrease with increasing triaxial constraint which increased void volume fraction and void growth rates. However, the degree of triaxiality had little or no effect on the nucleation rate of voids.

Slip behavior in cyclic-deformation has been studied using the α-β brass two-phase bicrystals, the phase-interface plane of which is parallel to the stress axis. Tension-compression tests were performed at room temperature under constant total strain amplitude of ±0.4%. Different slip patterns were developed in the vicinity of the interface at opposite surfaces of the α phase, while the patterns in the β phase were almost similar. It was thought that the behavior in β reflects its intrinsic monocrystalline property in response to the external stress. On the other hand, different slip behavior in α was interpreted in terms of the effect due to elastic strain incompatibility at the interface as given by Hook and Hirth [Acta metall.15, 535 (1967)]. The effect due to elastic strain incompatibility was shown to be more significant in cyclic straining.

For both the solid solutions α and β, composed of Cu and Zn, approximated by the regular solutions, the interaction parameters E^α and E^β of components in the a and β phases are calculated with the experimental activities. By the application of lattice stability parameters ΔG_Cu^β→α and Δ_Zn^β→α of Cu and Zn, which are obtained from the phase diagram of CuZn system in this paper, a general formula for ΔG^β→α is derived. On the basis of Inden formula for the change of free energy ΔG^β→β′ in the β→β′ ordering transition, it is proposed, through discussing the critical temperature T_c of ordering and ordering degree as a function of temperature, that the theoretical maximum ordering degree can not be obtained for an alloy with given composition. The maximum ordering degree attained is approximately independent of composition for alloys with X_Zn = 0.35–0.65, so an approximate equation for ordering degree as a function of temperature is suggested. This equation is used to calculate the $\text{α}\text{(α + β′)}$ and $\text{β′}\text{(α + β′)}$ phase boundaries of CuZn system, and the results are in good agreement with the phase diagram.