Kinetics of discontinuous precipitation and type I discontinuous coarsening in Zn–4 at % Ag alloy

A detailed study of the kinetics of discontinuous precipitation (DP) and type I discontinuous coarsening (DCI) in Zn–4 at % Ag alloy is reported here for the first time. DCI succeeds DP during prolonged isothermal ageing. Both DP and DCI are characterized by a predominantly lamellar morphology of the precipitate phase, statistically constant interlamellar spacing and steady state reaction front (RF) velocity at a given temperature. The interlamellar spacing increases with temperature. The RF velocity shows a C-curve behavior for DP, but increases monotonically for DCI, as a function of temperature. DCI is distinguished from DP by a 3–5 times larger interlamellar spacing and 1–2 orders of magnitude lower RF velocity than those of DP under comparable conditions. DCI may be initiated from an interface between two DP colonies, a former DP-RF, or the free surface intersecting a DP colony. Kinetic analysis of DP using the models of Turnbull, Cahn, Hillert, and Petermann and Hornbogen, and of DCI using the modified Petermann and Hornbogen model (by Fournelle) have yielded grain boundary diffusivity data in the temperature range 353–573 K. Subsequent Arrhenius analysis shows that the activation energy of the DP and DCI processes lies between 50–66 kJ mol–1. The latter is comparable with the activation energy of grain-boundary self-diffusion of Zn and is nearly half that of tracer impurity diffusion (volume/bulk) of Ag in Zn. Hence, it is concluded that DP and DCI are grain-boundary diffusion-controlled processes in the present alloy.     

The grain size dependence of fracture toughness in an Al-6.0% Zn-2.5% Mg alloy

The grain size dependence of the fracture toughness (K _IC) of an aged Al-6.0% Zn-2.5% Mg alloy was studied experimentally. K _IC depended strongly upon grain size (L _G) in two ways. In the small grain size region K _IC decreased with increasing average grain size. In contrast, K _IC increased with increasing average grain size for large grain sizes. The increase in K _IC with increasing grain size arose as a result of the presence of abnormally large grains compared to the average grain size in the large-grained specimens.     

Micro-yielding in a precipitation hardening Cu 1.81 wt % Be 0.28 wt % Co alloy

The microstrain characteristics of a polycrystalline Cu 1.81 wt % Be 0.28 wt % Co precipitation hardening alloy have been determined for various precipitate conditions. The friction stress derived from measurements of closed hysteresis loops was found to remain constant ( 3 MN m^–2) for all the conditions investigated. In contrast, the microscopic yield stress (MYS) remained constant (18 to 24 MN m^–2) for most ageing conditions, but increased significantly (to 48 to 64 MN m^–2) for conditions associated with a high G.P. zone or precipitate density.     

Superplasticity of a Zn-1.1 WT.% Al alloy at high temperatures

The Zn-1.1 wt.% Al alloy exhibits superplastic properties in the temperature range from 292 K (0.42 Tm) to 635 K (0.92 Tm). The best properties were observed at the temperature of 520 K (0.75 Tm). The parameter m reaches the value m = 0.73 at the true strain-rate ϵ = 7 × 10⁻⁵s⁻¹, and the maximum ductility is A = 1020% at the initial strain-rate ϵ₀ = 1.7 × 10⁻⁴s⁻¹. This value is the highest one observed in low-alloyed ZnAl alloys and shows that materials with great volume fraction of the second phase with very good superplastic properties can be reached at comparably high homologous temperatures also in low-alloyed materials. The extreme values of m and A were observed in the material with the initial mean grain size d = 17 um.     

On the intergranular fracture mechanism due to ledge formation in an Al-6.0% Zn-2.5% Mg alloy

The ledges observed on the over-load fracture surfaces of an Al-6.0% Zn-2.5% Mg alloy variously heat treated are discussed. As pointed out by Ryum and Baardseth the ledges are formed by the reaction between slip bands and grain boundaries. The morphology of the ledges changes from a sharp saw-toothed to a sine-curved shape, and the facets of the ledges vary from a flat face to a dimpled one with the degree of ageing. Grain-boundary precipitates were frequently observed on the flat facets. The fracture stress, calculated from a modified equation of Stroh's theory, supports the possibility of fracture occurring at a stress a little beyond yielding.     

Ageing of a Pd-35 at % Ag-25 at % Cu solid solution alloy

A palladium-35 at % silver-25 at % copper alloy has been prepared to simulate a succesful, commercial alloy. The commercial alloy is widely used because of its good corrosion resistance and formability and the fact that it develops good wear resistance and low electrical resistivity after ageing. However, little is known about the transformations which result from heat treatment of this alloy. Thus, the laboratory prepared material was used for determination of ageing transformations by means of resistance measurements, metallography, transmission microscopy, and X-ray diffraction. The following sequence of reaction was found to occur: $$\begin{gathered} Supersaturated \to Spinodal \to Ordering and discontinuous \hfill \\ solid solution decomposition precipitation \hfill \\ \end{gathered} $$ The transformations are quite similar to those occurring in several other solid solution systems whose initial spinodal decompositions are followed by discontinuous transformations with one or more ordered phases.     

Effect of heat treatments on discontinuous precipitation kinetics in Al-30 wt.% Zn alloy

The discontinuous precipitation kinetics in Al-30% wt. Zn alloy have been investigated at temperatures ranging from 348 to 503 K (75 to 230 °C) by using an optical microscopy, X-ray diffraction, differential dilatometer, differential scanning calorimetry and microhardness measurement. We have found that at all temperatures less than 180 °C the supersaturated solid solution of quenched alloy was observed to decompose completely by a cellular precipitation reaction. Quantitative metallography methods have been applied to measure the corresponding transformed volume fractions at different temperatures and times of precipitation. The variation of the heating rate and the application of different methods have allowed us to calculate two kinetic parameters of precipitation: the activation energy of the process and the Avrami exponent.     

The effect of direct electric current on precipitation in a bulk Al-4 wt % Cu alloy

The effect of the stress of direct electric current on precipitation in a quenched Al-4.15 wt % Cu alloy is investigated by measurement of the change in electrical resistance of a bulk specimen as a function of current density up to about 3000 A cm^–2 at a given annealing time. A similar effect of d.c. stress is observed at 50, 75 and 100 C. For a lower current density, a constant or slightly increasing resistance is observed, while for a current density higher than a critical value, the resistance decreases with an increase in density. On the assumption that the temperature increase of a specimen due to the Joule heating is proportional to the square of the current density, it is concluded that there exists some effect of d.c. stress per se to prevent the precipitation reaction, aside from the effect of the temperature increase due to the Joule heating, and it is suggested that this retarding effect is related to the sweeping out of quenched-in excess vacancies into grain boundaries by electromigration.     

Misorientation dependence of discontinuous precipitation in Cu-Be alloy bicrystals

The boundary-dependent discontinuous precipitation (DP) for various [001] symmetric tilt boundaries in Cu–5.2 at% Be alloy bicrystals has been examined in the temperature range 523–698 K. The precipitate phase maintains a constant interlamellar spacing under isothermal growth conditions. The interlamellar spacing increases with an increase in temperature. The incubation period to initiate DP and cell growth rate for DP against misorientation angle diagrams show local peaks and cusps at the same misorientation angles. The positions of the peaks and cusps agree with those of cusps in the boundary energy against misorientation diagram. The formation and growth of DP occur more easily at higher-energy boundaries. A detailed kinetic analysis of the experimental data using the models by Turnbull and Petermann–Hornbogen has enabled the determination of the grain-boundary diffusivity of Be along each boundary in the temperature range studied. A close correlation is found between the diffusivity and the energy of boundaries. A higher-energy boundary has a higher diffusivity with a smaller activation energy and a smaller pre-exponential factor.     

Mechanism and kinetics of type II discontinuous coarsening in a Zn-4 at% Ag alloy

Discontinuous coarsening (DC) may succeed discontinuous precipitation (DP) either at the same (DCI) or another temperature (DCII). The present study concerns mechanism and kinetics of DCII in a Zn-4 at% Ag alloy in the range 353–513 K following DP at 393 K for 60 h. DCII colonies prefer to initiate either from one or both sides of the interfaces between the former DP colonies. A suitable comparison of the kinetic data reveals that interlamellar spacing (,) and steady-state growth velocity (v) values in DCII are significantly different than those in DP. On the other hand, the kinetics of DCIvis-à-vis DCII in terms of andv are comparable to each other, though the calculated values of the driving forces between them differ marginally. A detailed kinetic analysis of DCII through the Livingston-Cahn model leads to an underestimation of the activation energy (Q _b) of grain boundary chemical diffusion of Ag in Zn-Ag (=30.7 kJ mol^–1), whereas the same obtained from the modified Petermann-Hornbogen model (=61.0 kJ mol^–1) compares well with that for DP/DCI (reported elsewhere by us), and grain boundary self diffusion of Zn. Considering thatQ _b in DCII is nearly 50% of the activation energy for volume/matrix diffusion of Ag in Zn, it appears that DCII in the present alloy is a boundary diffusion controlled process.     

Lamellar precipitation in Cu‐4,5 at. % In alloy

The purpose of this study is to study the lamellar precipitate morphology in the Cu‐4, 5 at. % In alloy by optical, scanning electron and transmission microscopy. It was observed that the lamellar precipitates develop starting from a grain boundary by a discontinuous reaction during an ageing at 673 K, and different lamellar orientations in the same cell were observed. The discontinuous reaction can also develop in the interphase boundaries. In the same way, the reaction front form of the precipitate during ageing was analyzed in this work.     

Low-stress creep behaviour of superplastic Zn-22% Al alloy

Low-stress creep behaviour of microduplex Zn-22% Al alloy was studied using spring specimen geometry. The average phase size in the specimens investigated was 0.87, 1.48 and 1.98 m. Experiments were conducted in the temperature range 393–473 K at stresses below about 1.0 MN m^–2. The present study has established that the stress exponent of the creep rate is unity and, therefore, a viscous creep process dominates the flow in Region I superplasticity. The activation energy corresponds to that for boundary diffusion. However, the phase-size exponent was found to be –2 instead of –3, as predicted by the Coble creep theory. Further, the measured creep rates are three to four orders of magnitude slower than those predicted by the Coble theory. Transmission electron microscopy revealed precipitation, along / grain interfaces, whose inhibiting action on plastic flow should at least be partly responsible for the lower values of measured creep rates. There also exist two other interfaces, namely / and /, whose comprehensive role in diffusion creep is not yet fully understood. Therefore, it seems illogical to describe the creep behaviour of Zn-22% Al by the classical Coble theory, originally developed for single-phase polycrystals.     

Cu clusters evolvement in Fe-1.18% Cu binary alloy

Microstructure evolution of Fe-1.18%Cu binary alloy during solution and aging at 550°C was investigated under a high-resolution electron microscopy (HREM). In addition, the aging strengthening mechanism was investigated based on the precipitation strengthening theory. Results show that there were lots of Cu atom clusters in the ferrite matrix during solid solution and initial aging stage, and Cu-rich metastable Fe-Cu particles precipitate subsequently at the aging hardness peak. It is found that there were high-density dislocations and stacking fault substructure in the Cu clusters that forms the obstacle of the dislocation motion, which should be the dominant reason of strengthening in the Fe-Cu alloy.     

Characterization of deformation processes in a Zn-22% Al alloy using atomic force microscopy

The Zn-22% Al eutectoid alloy was subjected to equal-channel angular pressing at a temperature of 473 K to give an as-pressed grain size of 1.3 m. Subsequent tensile testing of the as-pressed alloy at room temperature revealed a transition from deformation by a dislocation mechanism at the higher strain rates to superplastic flow at strain rates below 5 × 10^–3 s^–1: this corresponds to the transition from region III to region II in conventional superplasticity. Samples were pulled to relatively low total strains, of the order of 0.2–0.5, and the surface topography was then examined using an atomic force microscope (AFM). The AFM observations confirm the transition in deformation mechanisms with decreasing strain rate and they provide direct evidence for the occurrence of grain boundary sliding within the superplastic regime.