Pct Zn Alloy Research Articles

Effect of stress reductions on the stress exponent and subgrain size in an Al-Zn alloy

Stress change and uninterrupted tests were performed on an Al-5 wt pct Zn alloy over the high temperature range (0.7 to 0.99T m, whereT mis the melting point) and at a normalized stress range extending from 10−5 to 2 × 10−4. Two techniques, etch-pit (EP) and transmission electron microscopy (TEM), were used to examine dislocation substructure developed during creep. The results of stress reduction tests, when compared with those of uninterrupted tests, lead to two findings: (a) there is no difference between the stress exponents determined from stress reduction tests and uninterrupted tests, and (b) after a stress reduction the subgrain size, measured by both EP and TEM, coarsens, reaching the steady-state value determined in an uninterrupted test at the reduced stress. TEM micrographs show that subgrain coarsening during the transient period after a stress reduction involves substructural activities such as subgrain boundary dissolution. Also, TEM examination of the interiors of subgrains occasionally reveals the presence of a cellular substructure which resembles very small subgrains. The characteristics of this cellular substructure are discussed with reference to recent conflicting evidence concerning the ability of the subgrain size to coarsen after a stress reduction.

Stress corrosion of Cu- Zn and Cu- Zn- Ni alloys: The role of dealloying

Evidence is presented to demonstrate the occurrence of strain induced dealloying during stress corrosion of Cu-Zn alloys with 15-30 pct Zn and a Cu-28 pct Zn-12 pct Ni alloy. The binary Cu-Zn alloys cracked intergranularly in the ammoniacal solutions used, whereas cracking was transgranular in the ternary alloy. Dealloying was thus found to be a common feature of both modes of cracking. The results further indicate that this dealloying occurs only during crack propagation and not during the incubation period before crack initiation. Additional support for these observations was provided by slow strain rate stress corrosion tests on the Cu-30 pct Zn alloy. These results are consistent with a dealloying model for stress corrosion cracking. The possible mechanisms for the enhanced room temperature transport of solute atoms, required in this model, are discussed.

Variations of zinc content in solid solution during reversion treatment in an Al-4.4 At. Pct Zn Alloy

In a previous work 1 it has been shown that the thermoelectric power (TEP), measured at 20 °C, is insensitive to GP zones existing in Al-Zn alloys. This result has been verified for large precipitates sizes (o = 40 A). The TEP seems to depend only on the zinc content remaining in solid solution in the matrix. Therefore this TEP property allows to follow, especially, the reversion of GP zones. This has been already verified in a qualitative way. 2 Thus the purpose of the present work is to follow quantitatively the evolution of the zinc content in solid solution by TEP measurements and to compare these results to those obtained by the measurement of the integral intensity of X-Ray small angle scattering (XSAS). Moreover it will be shown that the temperature at which TEP measurements are carried out, has no influence on zinc content determination.

Effect of grain size and texture on the mechanical properties of warm worked cadmium-1.5 Pct zinc alloy

The structural changes occurring during warm working of Cd-1.5 pct Zn alloy and their effect on the subsequent mechanical properties are studied. It is observed that changes in grain size and preferred orientation are important to a large extent in controlling the mechanical strength. The Hall-Petch slope,R decreases in the warm worked material while the friction stress, σo increases. The lowerR values are attributed to the development of a (101l) texture and the higher σo values are interpreted on the basis of changes in the basal texture.

Morphological and structural studies of thermoelastic martensites in a Ag-38 At. Pct Zn alloy

Thermoelastic martensites formed upon subzero cooling a Ag-38 at. pct Zn alloy have been studied by means of optical and electron microscopy, electron diffraction, and electrical resistivityvs temperature measurements. Two different morphologies, slender banded and spear shaped, are observed in cooled bulk specimens, both being thermoelastic. The slender banded martensite is retained at room temperature in thin foils after subzero cooling and possesses the 9R type long period stacking order structure with internal stacking faults on the basal plane. This 9R martensite is “normal” with an orthorhombic lattice:a=4.81,b=2.82 andc=20.5A (a:b:c=1.71:1:7.27). The spear shaped martensite is also retained at room temperature in subzero cooled thin foils and in shape memory cycled ribbons as well. This morphology exhibits a monoclinic 3R type stacking order structure with internal twins and stacking faults. Lattice parameters of the 3R martensite area=4.83,b=2.87,c=6.94A and β=87.4 deg (a:b:c=1.68:1:2.41). This structure is equivalent to that of AuCuI (L10, fct) with lattice parametersa=4.07,c=3.89, andc/a=0.96. The twin and stacking fault planes in the 3R martensite are the (201) and (001) planes respectively. The basal planes of the 9R and 3R martensites are common, being generated from the {110} plane of the CsCl type parent phase, and the two martensites differ only in the shuffling mode on the {110} plane upon transformation. The thermelastic nature of the 3R martensite is slightly different from that of the 9R one.

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 ast1/3 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.

Intergranular fracture in an Al-15 Wt Pct Zn alloy

Crack propagation rates for the intergranular fracture of an Al-15 wt pct Zn alloy tested in air, distilled H2O, and 0.5M NaCl have been studied using a double cantilever beam specimen. This technique has been applied to two different types of specimens: polycrystals with large equiaxed grains and bicrystals. The susceptibility to intergranular fracture in air and 0.5M NaCl increases as the volume fraction of G.P. zones in the matrix increases. The microstructure which is most susceptible to fracture exhibits intense planar slip traces while the more resistant microstructures have a more diffuse surface slip pattern. The dependence of cracking rates on the microstructure is explained in terms of the blocking of inhomogeneous plastic flow in the matrix by grain boundaries, resulting in locally severe stress concentrations at the boundaries. Bicrystal tests substantiate these results and show that orientations favoring partial continuity of slip bands across grain boundaries are less susceptible to stress corrosion cracking than arbitrarily oriented boundaries with no slip continuity.