Decomposition in Al-Zn alloys: Part I. Isothermal decomposition in an Al-22 at. pct Zn-0.1 at. pct Mg alloy

A small-angle X-ray scattering (SAS) study has been made on solution treated and isothermally annealed specimens of an Al-22 at. pct Zn-0.1 at. pct Mg alloy. The changes in peak position and integrated area of the SAS spectra with time and temperature indicate that de composition is nearly complete immediately after quenching, in agreement with the earlier interpretation that Gerold and Merz placed on the results of Rundman and Hilliard in the binary Al-22 at. pct Zn alloy. Furthermore, structural changes occurring during annealing are consistent with a coarsening or maturation of the fluctuations in the solution. The domi nant wavelength varies ast ^1/3 over a large time span and the temperature dependence of the coarsening process yields an activation energy of 94.2 kJ/mole. The effect of Mg is to re tard the formation of the equilibrium phases while having a small effect on the growth of the composition fluctuations during the coarsening process. Now on sabbatical leave to Caterpillar Tractor Co., Mapleton Plant, Mapleton, Ill. 61554.     

Decomposition in Al-Zn alloys: Part II. Decomposition during continuous cooling

A small angle X-ray scattering study (SAS) has been made of decomposition during contin uous cooling in four binary Al-Zn alloys with compositions spanning the miscibility gap and in two ternary alloys, each containing 22 at. pct Zn plus small amounts of Sn and Mg. Plots of logλ _m (wavelength receiving maximum amplification during the quench)vs logQ (quench rate) yield slopes of approximately -1/3 for all alloys, indicating that coarsening plays an important role during the quench. In addition, measurements of integrated area under the SAS spectra indicate that decomposition is essentially complete in the quenched condition for all of the alloys studied.     

Decomposition in Al-Zn alloys: Part II. Decomposition during continuous cooling

A small angle X-ray scattering study (SAS) has been made of decomposition during contin uous cooling in four binary Al-Zn alloys with compositions spanning the miscibility gap and in two ternary alloys, each containing 22 at. pct Zn plus small amounts of Sn and Mg. Plots of logλ _m (wavelength receiving maximum amplification during the quench)vs logQ (quench rate) yield slopes of approximately -1/3 for all alloys, indicating that coarsening plays an important role during the quench. In addition, measurements of integrated area under the SAS spectra indicate that decomposition is essentially complete in the quenched condition for all of the alloys studied. DENNIS T. LEWANDOWSKI, formerly a graduate student at Michigan Technological University. Now on sabbatical leave to Caterpillar Tractor Co., Mapleton Plant, Mapleton, Ill. 61554.     

Influence of the precipitate-free zone width on the tensile properties of an Al-6 Wt pct Zn-1.2 Wt pct Mg alloy

The mechanical properties of an Al-6 wt pct Zn-1.2 wt pct Mg alloy with various width of precipitate-free zones have been investigated. The width of the precipitate-free zone (PFZ) has been changed by the quench interruption technique without any appreciable change in the size and distribution of precipitates. An important relationship has been observed between the width of the PFZ and the quench-interruption period;i.e., the width of the PFZ increases in proportion to the square root of the holding time at 200°C. From the analysis of stress-strain curves as well as the observation of dislocation arrangements in slightly deformed specimens, the plastic deformation has been found to occur preferentially in the PFZ. The initial stage of deformation is much affected by the change in the width of the PFZ, but in the later stage, the work-hardening rate seems to be almost independent of the PFZ width. Tensile tests show that the ultimate tensile strength and the 0.2 pct proof stress decrease very little with increasing width of the PFZ, while the uniform elongation is practically constant regardless of the reduction in the nonuniform elongation. The work-hardening rate at the initial stage of deformation is found to decrease in proportion to the reciprocal of the PFZ width. This relationship can be explained from the dislocation model for work hardening.     

The effect of reversion treatments on precipitation mechanisms in an Al-1.35 at. pct Mg2Si alloy

The effect of reversion treatments on an Al-1.35 at. pct Mg₂Si alloy fully age hardened for 24 h at 160°C was studied by electron microscopy and tension tests. This alloy aged to full strength at 160°C did not show true reversion when heated 15 min at 200 to 300°C. The G.P. zones did not dissolve rapidly at a particular temperature but instead were replaced by the more stable phase, β′ (the intermediate partially coherent form of Mg₂Si). After reheating the fully age hardened alloy 15 min at 250°C, a slight increase in strength was obtained, but the ductility was slightly lowered. Reversion treatments at higher temperatures (275 to 300°C) gradually decreased the strength of the alloy. Two simultaneous reactions are believed to occur during the reversion treatments: 1) the growth of some of the G.P. zones and the dissolution of others and 2) the formation of needles of β′.     

Isothermal transformations in an Fe-9 pct Ni alloy

Isothermal transformation from austenite in an Fe-9.14 pct Ni alloy has been studied by optical metallography and examination by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the temperature range 565 °C and 545 °C, massive ferrite (α _q ) forms first at prior austenite grain boundaries, followed by Widmanst?tten ferrite (α _W ) growing from this grain boundary ferrite. Between 495 °C and 535 °C, Widmanst?tten ferrite is thought to grow directly from the austenite grain boundaries. Both these transformations do not go to completion and reasons for this are discussed. These composition invariant transformations occur below T ₀ in the two-phase field (α+γ). Previous work on the same alloy showed that transformation occurred to α _q > and α _W on furnace cooling, while analytical TEM showed an increase of Ni at the massive ferrite grain boundaries, indicating local partitioning of Ni at the transformation interface. An Fe-3.47 pct Ni alloy transformed to equiaxed ferrite at 707 °C ±5 °C inside the single-phase field on air cooling. This is in agreement with data from other sources, although equiaxed ferrite in Fe-C alloys forms in the two-phase region. The application of theories of growth of two types of massive transformation by Hillert and his colleagues are discussed. This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part of the Fall 2000 TMS Meeting held October 16–19, 2000, in St. Louis, Missouri, under the auspices of the ASM Phase Transformations Committee.     

Equiaxed dendritic solidification with convection: Part II. Numerical simulations for an Al-4 Wt pct Cu alloy

The multiphase model developed in part I for equiaxed dendritic solidification with melt convection and solid-phase transport is applied to numerically predict structural and compositional development in an Al-4 wt pct Cu alloy solidifying in a rectangular cavity. A numerical technique combining a fully implicit control-volume-based finite difference method with a multiple time-step scheme is developed for accurate and efficient simulations of both micro- and macroscale phenomena. Quantitative results for the dendritic microstructure evolution in the presence of melt convection and solid movement are obtained. The remarkable effects of the solid-liquid multiphase flow pattern on macrosegregation as well as the grain size distribution are illustrated.     

Cavitation damage in the superplastic Zn-22% Al-0.5% Cu alloy

A quantitative analysis of cavitation damage in a superplastic ZnAl alloy has been made, and results contrasted with theoretical models. Metallography indicates that cavities nucleate in Fe-rich particles and that fracture occurs at relatively low cavitation levels (6–12%) after local strains of about 6.5. The level of damage at any given strain is higher while cavities are larger and more round the lower the deformation rate. Experimental data can be reasonably fitted to mechanistic models for cavity growth assuming coupling between boundary diffusion and matrix plasticity as the mechanism of growth, if grain growth hardening and an enhancement of the diffusive contribution to growth due to the very small grain size are taken into account.     

Effect of impurity type on boundary sliding behavior in the superplastic Zn-22 pct Al alloy

The effect of impurity type on boundary sliding behavior in the superplastic Zn-22 pct Al alloy was investigated using two grades of the alloy: Zn-22 pct Al-0.13 pct Cu (grade Cu) and Zn-22 pct Al-0.14 pct Fe (grade Fe). In the investigation, boundary sliding offset measurements in both grades were made at strain rates ranging from 5×10⁻⁷/s to 10⁻¹/s. This range of strain rate covered region I (the low strain rate region), region II (the intermediate strain rate region), and region III (the high strain rate region) of the sigmoidal plot between stress and strain rate that was previously reported for grade Fe. The experimental results show that the contributions of boundary sliding to the total strain, ξ, in the two grades of Zn-22 pct Al are about 20 and 52 pct at high (region III) and intermediate (region II) strain rates, respectively. By contrast, the experimental data reveal that ξ in grade Cu at low strain rates (52 pct) is essentially equal to that at intermediate strain rates (region II), while ξ in grade Fe at low strain rates (24 pct) is considerably lower than that at intermediate strain rates (56 pct). It is demonstrated that the difference in sliding behavior between grade Fe and grade Cu at low strain rates corresponds well with the difference in superplastic behavior between the two grades. In addition, consideration of the present and earlier data on sliding behavior in Zn-22 pct Al provides a correlation between two roles played by boundaries during superplastic deformation: the ability of boundaries to contribute to deformation through the process of boundary sliding and their ability to serve as favorable sites for the accumulation of impurities, i.e., boundary segregation.     

Atom-probe analysis of GP zones in an Al-1.7 At. pct Cu alloy

Guinier-Preston (GP) zones in an Al-1.7 at. pct Cu alloy aged at 383 K for 8000 minutes were analyzed by atom-probe field ion microscopy (AP-FIM). Layer-by-layer concentration profiles in the 〈100〉 direction were obtained from several GP zones. Generally, the GP zones were found to consist of several copper-enriched layers, with the highest measured concentration of copper in any of these layers being 50 at. pct. Although some uncertainty exists as to the copper content of the GP zones, these results appear to be inconsistent with the existence of layers that approach 100 pct copper. Some GP zones showed two well-separated copper-enriched regions; others showed a broad single peak in copper concentration. Therefore, it was concluded that aging at 383 K for 8000 minutes causes a transition from GP 1 to GP 2. The present results together with the previous atom-probe data indicate that there is a distinct stage for GP 2 zones which have two separated copper-enriched layers of 200 planes. Formerly Graduate Student, Department of Materials Science and Engineering, The Pennsylvania State University     

The effect of two-step aging on the quench sensitivity of an Al-5 Pct Zn-2 Pct Mg alloy with and without 0.1 Pct Cr

The effect of two-step aging on the quench sensitivity of an Al-5 pct Zn-2 pct Mg alloy with and without 0.1 pct Cr has been studied. Results show that the quench sensitivity effect can be eliminated in thin samples of these alloys by two-step aging if the slow cooling during quenching does not allow the precipitation process to proceed too far. Lack of achievement of full strength in the aged condition due to a slow quench rate can be attributed to 1) loss of vacancies during quenching and 2) formation of incoherent-type precipitates during quenching. The trend to lower strength due to the loss of vacancies can be reversed by two-step aging; however, if incoherent-type precipitates form, some strength potential of the alloy is permanently lost. A 0.1 pct Cr addition increases the quench sensitivity effect by accelerating incoherent-type precipitation during quenching. These incoherent precipitates, which appear in the form of bands within the grains and in the grain boundaries, lead to an increase in ductility. Formerly Research Assistant, M.I.T., Cambridge, Mass.     

On the relationship between ductility and fracture toughness in an Al-6.0% Zn-2.5% Mg alloy

The fracture toughness (K_IC) of an Al-6.0%Zn-2.5%Mg alloy heat-treated variously was examined in relation to tensile properties using the notched and unnotched specimens. As usual trends, the fracture strain (ϵ_f) showed minimum at the peaks of yield stress (σ_0.2) and ultimate tensile stress (σ_uts). The K_IC and ϵ_f when aged isothermally varied similarly, but when isochronally aged K_IC and σ_0.2 (and σ_UTS) showed a similar trend. An intimate relationship was found between the dimple size at the portion fractured intergranularly and the fracture strain (ϵ_f). An equation showing the relation between k_ic and tensile properties such as σ_0.2. ϵ_UTS (the strain up to ultimate tensile stress) and n (the work hardening parameterl could be introduced, based on a new model of the plastic zone. The model was verified experimentally.     

Combined effects of Pb and Sn additions on the corrosion of Zn-12 pct am pct Cu-0.02 pct Mg sand cast alloy in wet steam

The effects of varying the Sn content from 0 to 0.01 pct at 0.005 and 0.015 pct Pb on the corrosion of sand-cast Zn-12 pct Al-1 pct Cu-0.02 pct Mg alloy in saturated water vapour at 95°C were investigated. Within the relevant specification limits there was no interaction between Pb and Sn, the combined effect being equivalent to that of Sn alone. The tolerance of the alloy for Sn was so low that the maximum for this impurity was considered to be 0.001 pct. For Pb, 0.01 pct produced substantially the same corrosive attack as 0.001 pct and would be considered as the upper limit for this alloy.