John S. Vetrano

Direct observation of the behavior of grain boundaries during continuous dynamic recrystallization in an Al–4Mg–0.3Sc alloy

The micromechanisms operating during dynamic continuous recrystallization of an Al-4Mg-0.3Sc alloy have been studied in real time by reloading pre-deformed samples at temperature in situ in the transmission electron microscope. The processes responsible for the evolution of the microstructure include the migration and breakup of subgrain boundaries, the destruction of triple junctions, and large-scale volume rotations. These observations are considered in terms of the micromechanisms proposed previously to account for dynamic continuous recrystallization.

Stress corrosion cracking of Al-Mg and Mg-Al alloys

Abstract Al- and Mg-based alloys are being used for reducing the weight of automobiles. For structural applications, they must have adequate stress corrosion resistance, and yet, under some circumstances, stress corrosion cracking can occur in both alloy systems. Precipitation of the Mg-rich β-phase (Al3Mg2) at grain boundaries of Al-Mg alloys and the γ-phase (Mg17Al12) at grain boundaries of the Mg-Al alloys is a critical factor in their stress corrosion performance. In Mg-Al, the γ-phase is cathodic (noble) to the matrix, while in the Al-Mg case, the β-phase is anodic (active) to the matrix. These phases produce localized galvanic-induced corrosion that leads to intergranular stress corrosion cracking and crack growth rates of 350 and 1,800 times faster than the solution-treated condition, for Al-Mg and Mg-Al, respectively.

Influence of the particle size on recrystallization and grain growth in Al-Mg-X alloys

Abstract Recrystallization and grain growth studies of Al-Mg-X (where X is Mn, Zr or Sc) alloys have been carried out using a combination of optical and transmission electron microscopy of interrupted heating tests, changes in hardness, and in-situ observations in the transmission electron microscope. Nucleation sites have been identified as typically large (> 1 μm diameter) eutectic constituent particles, but also occasionally clusters of sub-micron dispersoids. By altering the density and size of the Mn-, Sc- and Zr-based particles with selected thermomechanical treatments, recrystallization could be suppressed up to temperatures near the melting point. These results can be explained by examination of the stored energy from cold work and the particle-substructure interactions due to the precipitate morphology. Implications of these results for grain size control in Al-Mg based alloys will be discussed.

Superplastic behavior in a commercial 5083 aluminum alloy

When considering the forming and post-forming properties required of a superplastic material, attractive candidates are commercial Al-Mg-Mn weldable alloys such as AA5083. There have been several investigations of hot deformation of 5083-type alloys in the literature. Only two studies evaluated commercial-purity 5083 and they achieved tensile elongations of 150% and 200%. Alloy modification has produced improved behavior in three 5083-type alloys developed specifically for SPF. Two were deemed high-purity 5083 (low Fe and Si) and achieved elongations of 450% and 630%. Engineering strains up to 700% were measured by Watanabe et al. in a 5083-based alloy with the addition of 0.6% Cu as a grain refiner. These results suggest that alloy modifications such as reduced Fe and Si contents or Cu additions may be required to improve superplastic response. Unfortunately, specific SPF-grade 5083 alloys are substantially more expensive than the commercial grade, and the addition of Cu decreases the corrosion resistance of the base material. The purpose of this work is to examine the effect of prior degrees of cold work on the SPF behavior of a standard-grade 5083 alloy. Superplastic behavior of this material at 510[degree]C is assessed and compared to published results for the SPF-grade alloys.

Influence of Mg on the corrosion of Al

This article summarizes a series of experiments to determine the influence of Mg on the corrosion and electrochemical behavior of Al. Magnesium is commonly added to increase the strength of lightweight nonheat treatable Al alloys. However, these alloys are susceptible to grain boundary dissolution, stress corrosion cracking, or hydrogen induced embrittlement due to changes in the alloy structure and elemental distribution during processing, welding, or in-service exposure to elevated temperatures. Auger electron spectroscopy and transmission electron microscopy measurements show that alloys having a distribution of Al3Mg2 (β phase) precipitates and segregated Mg on grain boundaries are more susceptible to cracking. To understand the roles of Mg on the cracking process we compared the corrosion potential and film formation of pure Al, Al implanted with Mg, a 7 wt % Mg–Al alloy and pure Al3Mg2 phase. The surfaces of the specimens were cleaned and prepared in a surface analysis system and transferred in a va...

Evidence for excess vacancies at sliding grain boundaries during superplastic deformation

Rapid quenching of Al-Mg alloys during superplastic deformation has revealed the presence of nano-scale cavities along many grain boundaries. They were observed only under deformation conditions where grain boundary sliding was the dominant mechanism. Fine-probe compositional measurements revealed that the cavity surface is enriched in Mg, and in situ heating in the transmission electron microscope demonstrated that they are not stable above 175 C. Kinetic analysis of cavity formation during a quench concludes that the cavities did not exist during deformation but were formed as the sample cooled. It is proposed that these cavities are evidence for a localized excess of vacancies during grain boundary sliding.

Microstructure Characterization of Dislocation Wall Structure in Aluminum Using Transmission Electron Microscopy

The configuration of dislocation wall structures and the interactions between dislocations and dislocation walls play a significant role in the understanding of deformation processes in metals. Samples of single-crystal aluminum deformed by tensile-straining (15%) were analyzed using TEM. In tensile-deformed (15%) single crystal aluminum, a cell structure is well developed and dislocations in the cell boundaries consist of either one set of Burgers vector or two sets of Burgers vector. The three-dimensional image of cell wall structure, misorientation angle across the cell boundaries and the Burners vectors of dislocations in the cell boundaries are characterized.

Pinning effect of solute atoms on grain boundary dislocation dissociation

The dissociation of extrinsic grain boundary dislocations (EGBDs) in Al, Al-Mg and Al-Mg-Sn alloys was studied at various temperatures. While EGBDs were commonly observed in low- and high-angle boundaries in the alloy materials deformed to small strains, they could only be seen in low-angle grain boundaries in Al. Annealing of the lightly deformed alloy samples at 100 or 150°C led to the disappearance of the EGBDs with critical disappearance kinetics depending on grain boundary composition. A modified EGBD dissociation mechanism incorporating a solute drag effect has been developed in this work. Predictions based on the model agree reasonably well with the observed thermal behavior of the EGBDs in the three Al alloys.

Development of Superplasticity in 5083 Aluminum with Additions of Mn and Zr

The superplastic behavior of the 5083 aluminum alloy with additions of Mn and Zr was studied by uniaxial tensile testing and microstructural evaluations. Additions of up to 0.2% Zr and 0.8% Mn were made to a base 5083 aluminum alloy to decrease the grain size and improve superplastic behavior. Constant strain-rate tensile test data were used to determine strain-rate sensitivity (m values) and elongations-to-failure for the alloys at strain rates ranging from 4 {times} 10{sup {minus}4} to 1 {times} 10{sup {minus}1} s{sup {minus}1} at temperatures of 450 to 550C. Elongations-to-failure of up to 400% at 1 {times} 10{sup {minus}2} s{sup {minus}1} were achieved for the modified alloys. The strain-rate sensitivity for the alloys as a function of strain was determined and two distinct behaviors were observed. For the alloys having composition close to the base 5083 alloy, the m value steadily decreased with increasing strain; however, in alloys with higher levels of Zr, the m value remained stable. A maximum m value of 0.65 was achieved at 0.7 strain for the 1.6% Mn and 0.2% Zr alloy at 1 {times} 10{sup {minus}3} s{sup {minus}1}.

Temperature and strain-rate effects on deformation mechanisms in irradiated stainless steel

Analysis of the deformation microstructures in ion-irradiated stainless steel shows twinning to be the predominant deformation mode at room temperature. Dislocation channelling also occurs under slow strain rate conditions. Stresses required for twinning were calculated by the model of Venables and are compatible with observed yield stresses in neutron-irradiated material if loops are the principal twin source. Computation of the expected radiation hardening from the defect structure, based on a simple model, is consistent with yield strengths measured on neutron-irradiated steels. Lower yield stresses and greater thermal energy at 288 C lessen the probability of twinning and dislocation channeling becomes the primary deformation mode at the higher temperature. However, preliminary early results show that some twinning does occur in the irradiated stainless steel even at the higher temperature when higher strain rates are used.