Endre Romhanji

Stress corrosion cracking susceptibility of Al–Mg alloy sheet with high Mg content

Slow strain rate testing (SSRT) was used to study the effect of the microstructure on the stress corrosion cracking (SCC) susceptibility of Al– Mg alloy sheet containing 6.8 wt.% Mg. In the cold-rolled and fully annealed conditions, high SCC susceptibility was experienced. In those cases the ductility was strongly affected by the presence of corrosive environment (for hard temper: Elair ¼ 13:6% ,E l SCC ¼ 0:6%; for annealed condition: Elair ¼ 24:1–25.3%, ElSCC ¼ 3:2–4.2%) and the elongation loss was great, Elloss ¼ 81:7–95.6%. It is supposed that the high SCC susceptibility results from a continuous network of the b-phase (Mg5Al8) precipitate at grain boundaries for the annealed temper, and heavy precipitation ofb-phase along the planes of localized deformation for the hard temper. High SCC resistance attained after thermal exposure at the temperature range 225–285 8C (stabilized condition). The ductility was almost unaffected by the presence of corrosive environment (Elair ¼ 12:8–23.2%, ElSCC ¼ 12:8–22%) and the elongation loss was small, Elloss < 7%. High SCC resistance was related to the stabilized structure, which causes discontinuous b-phase (Mg5Al8) precipitation in a globular form, uniformly distributed throughout the structure. # 2002 Published by Elsevier Science B.V.

Deformation pattern and strain hardening in a highly textured 8090 Al-Li alloy sheet

Abstract Structure-deformation pattern analysis was made on a highly textured 8090 Al–Li alloy sheet deformed in tension. It is shown that the sharp β-fiber type crystallographic texture induces a strong orientation variation in the mechanical properties and in the deformation structure. The soft orientation is 55° in respect to the rolling direction (RD), while the hard orientation is parallel to the RD. Both results are predicted from the detected texture by the relaxed constrained Taylor deformation model (RC). Cellular dislocation substructure was observed in the samples deformed parallel to the RD. However, in the samples tested 55° to the RD, coplanar dislocation arrangement was preserved even up to 30% elongation. In the hard orientation, the strain hardening ability is quickly exhausted. This is due to the effect of dynamic recovery occurring from the onset of deformation. In the soft orientation, the strain hardening is better sustained as a result of easy glide of the coplanar dislocations, on the primary slip systems, through the grain structure. The pattern of surface slip traces is compatible with the dislocation substructure arrangement observed by transmission electron microscopy (TEM).

Formability of a high-strength Al-Mg6.8 type alloy sheet

Room temperature formability testing was performed on an AlMg6.8 type alloy sheet with a fully recrystallized structure (average grain diameter ∼18 μm) and after partial annealing with a retained deformed structure. The yield strengths attained after full recrystallization and after partial annealing, were 175 and 283 MPa respectively. Such an increase in strength is followed by formability degradation, maximized around the plain strain state to either 42%, as obtained using the limiting dome height test (LDH), or 35% after using forming limit curves (FLC). A comparison with known high-strength formable alloys has shown that the tested alloy in the recrystallized condition has a better stretch formability (at the same or even higher yield stress level), while in the unrecrystallized-partially annealed condition it has a lower formability, limiting its application to moderate forming requirements for very high-strength parts.

Effect of the Thermo-Mechanical Treatment on IGC Susceptibility of AA 5083 Alloy

This work reports on the effect of thermo-mechanical treatment on IGC susceptibility of the AA 5083 alloy. Specimens underwent varied amount of cold work and final annealing was conducted at 240°C. Extent of the cold work affected the IGC susceptibility of the alloy significantly. Microstructure characterization showed that depending on the amount of the cold work different deformation substructure was created, which, in turn, influenced morphology of precipitated β-phase (Al3Mg2). Formation of continuous film of the β-phase at the grain boundaries was observed in the specimens that were subjected to lower degree of the cold work and which were IGC susceptible. Better corrosion resistance characterized the specimens that underwent higher degree of the cold work (over 30–40%) due to β-phase precipitation in the form of discrete particles at the grain boundaries and in grain interiors.

Study of Homogenization Treatments of Cast 5xxx Series Al-Mg-Mn Alloy Modified with Zn

Microstructural changes of DC-cast 5xxx series Al-Mg-Mn alloy modified with Zn addition occurring during the homogenization treatments were studied through the thickness of a rolling ingot. The homogenization treatments included: low temperature annealing at 430 °C for 12h and high temperature homogenization at 550 °C for 16h. Microstructure evolution was followed by electrical resistivity measurements, optical microscopy, SEM and TEM characterization and microanalysis. During the homogenization, decomposition of the supersaturated solid solution, present in the as-cast state, occurred. Distribution of the precipitates was dictated by segregations and could be related to the partition coefficients of alloying elements. It was found that during the high temperature homogenization not only dissolution processes occurred, but also precipitation of new phases.

Kinetics of internal oxidation in Ag-6Sn-2In-1Te-0.2Ni and Ag-6Sn-0.6Cu-1Bi-0.2Ni type alloys

Internal oxidation process was followed in Ag-6Sn-2In-1Te-0.2Ni and Ag-6Sn-0.6Cu-1Bi-0.2Ni alloy types in free air and oxygen at 650 °C and 680 °C. It was found that the Cu and Bi bearing alloy had a higher rates of internal oxidation in all of the tested conditions, whish was due to Bi effect accelerating the internal oxidation. In the early stage of internal oxidation of the Ag-6Sn-2In-1Te-0.2Ni alloy, an inhomogeneous structure revealed, which changed to a homogenous pattern after longer oxidation times. Such a structure was suppressed in the Bi bearing alloy. The applied 6 bar oxygen atmosphere brought on a 5–6 times higher rate of internal oxidation, with a homogenous structure compared to the free air oxidation. In free air after long time oxidation (200 h–250 h), the rate of internal oxidation was found to have almost doubled in both alloys.

Precipitation of the β-phase and Corrosion Behavior of an Al-6.8 wt.% Mg Alloy

This study was performed on the Al-Mg alloy with the chemical composition: Al-Mg6.8-Mn0.51-Fe0.2-Si0.1. As-received cold rolled material (O-temper) was subjected to (i) cold rolling and annealing at 265°C and 320°C, followed by (ii) sensitization treatment at 100°C. Microstructure characterization showed that the preferential sites for β-phase precipitation were grain boundaries and preexisting Mn-rich particles. The annealing temperature had significant effect on the β-phase morphology: microstructure of sensitized specimens annealed at 265°C was characterized by formation of discontinuous particles, while annealing at 320°C resulted in the formation of the thin film at grain boundaries. Presence of the thin film induced corrosion resistance degradation and also affected grain boundary morphology: initially smooth, curved grain boundaries became strongly faceted.