Structural Materials for Aerospace and Defense: Challenges and Prospects: Magnesium Alloys
Sponsored by: MS&T Organization
Program Organizers: Roumiana Petrova, New Jersey Institute of Tech
Thursday 8:00 AM
October 20, 2011
Room: D242/243
Location: Greater Columbus Convention Center
Session Chair: Andrzej Wojcieszynski, AI Powder Metals
8:00 AM Invited
Mechanical Properties of Sensitized 5083 Al-Mg Structural Alloys: Ronald Holtz1; Peter Pao1; Robert Bayles1; Thomas Longazel1; Ramasis Goswami2; 1Naval Research Laboratory; 2SAIC
The effect of sensitized microstructures in Al-5083, produced by aging at 70, 100, and 175 °C, up to 10,000 hours, on SCC threshold stress intensity (K1SCC) and on high stress-ratio (R = 0.85) corrosion-fatigue crack growth threshold stress intensity (∆Kth) in saltwater are studied. Both K1SCC and ∆Kth initially decrease substantially with aging time, reach a minimum at certain aging time that depends on aging temperature, and then remain constant with further aging. The initial decrease in K1SCC and ∆Kth is correlated with the formation of discrete β phase at the grain boundaries. At the K1SCC and ∆Kth minimum, the grain boundaries of Al 5083 are covered by continuous β phase film. The observed stress-corrosion cracking and high stress-ratio corrosion-fatigue crack growth responses are discussed in terms of differences in grain boundary microstructure at the different aging temperatures and the interplay between stress-corrosion and corrosion fatigue cracking thresholds.
8:40 AM
Microstructural Influence on Impact Properties of a Friction Stir Processed Mg-Y-RE Alloy: Kumar Kandasamy1; Sushanta Panigrahi1; Rajiv Mishra1; Rick DeLorme2; Bruce Davis2; Ryan Howell3; Kyu Cho3; 1Centre of Friction Stir Processing and Department of Materials science and Engineering Missouri University of Science & Technology, Rolla, MO 65409; 2Magnesium Elektron North America Inc., Madison, IL 62060; 3U.S. Army Research Laboratory, Aberdeen Proving Grounds, MD 21005
Grain refinement has been one of the metallurgical methods adopted to improve the toughness by activating non-basal slip systems to neutralize deformation anisotropy and thus suppressing deformation twinning for subsequent crack nucleation. Friction stir processing (FSP) has been used extensively as a microstructural refinement tool and adapted at discrete material and at component levels. Recent developments in high strength Mg alloys applications in commercial and military sectors have prompted a wide range of investigations in Mg-Y-RE alloys. In this paper, we correlate coarse grained, fine grained FSP, and post heat treated Mg-Y-RE alloys microstructures respective to the measured impact toughness. The initial experimental results show that grain refinement enhances the impact toughness while post-ageing decreases the impact toughness of both parent and FSP alloys significantly. This phenomenon is discussed in term of an influence of precipitate size on the crack initiation and subsequent propagation during impact loading.
9:00 AM Student
Effect of Texture on the Formability and Mechanical Anisotropy of Severe Plastically Deformed Magnesium Alloys: Sonia Modarres-Razavi1; David Foley1; Majid Al-Maharbi2; Ibrahim Karaman1; Karl Hartwig1; 1Texas A&M University; 2Sultan Qaboos University
This study systematically investigates the equal channel angular processing (ECAP) using hybrid routes that generate excellent microstructural size uniformity, and prevent strain localization and cracking during processing and post-processing deformation of AZ31B magnesium alloy. The initial crystallographic texture, and its evolution during ECAP, was the main factor taken into account for the selection of the hybrid routes. The hybrid routes include the combination of known ECAP routes and reduced ECAP temperature between the certain number of passes. Such an approach allowed us to successfully ECAP process AZ31B at temperatures as low as 100°C. The flow stress anisotropy and tension-compression (T/C) asymmetry along the three orthogonal directions of the ECAP billets were monitored and great improvements in strength levels were achieved upon the use of hybrid routes. A visco-plastic self-consistent (VPSC) crystal plasticity model was employed to predict the texture evolution during ECAP.
9:20 AM
Low-Melting Point Metal Effects in the Ageing Response of Al-Mg-Si Alloys: Manuel Marya1; 1Schlumberger
Group III liquid metals like gallium and indium are well-known for degrading the properties of aluminum alloys, particularly mechanical and environmental properties. Despite disadvantages gallium has been explored for TLP bonding aluminum alloys and MMCs whereas indium has been added to improve machineability. The effects of both elements in age-hardenable aluminum grades is however unresolved. Since gallium readily wets aluminum and is greatly soluble in alpha aluminum, gallium’s contribution to the ageing responses of Al-Mg-Si alloys is as certain as strong chemical interactions with aluminum’s traditional solute elements like magnesium. Unlike gallium, indium does not solid-solution alloy with aluminum but it does interact with other alloying elements. To clarify effects of gallium and indium in Al-Mg-Si alloys, particularly on artificial ageing, over 26 ingots were induction melted. Alloys with strengths exceeding that of typical Al-Mg-Si alloys were surprisingly identified, thereby leading the way to novel aluminum alloys.
9:40 AM Break