Surface and Interface Energies of Complex Crystal Structure Aluminum Magnesium Alloys
- PDF / 1,506,484 Bytes
- 11 Pages / 593.972 x 792 pts Page_size
- 100 Downloads / 160 Views
ION
THE mechanical properties of materials are determined by their atomic structure, and atomistic simulations are increasingly capable of computing such properties from first principles.[1,2] Most such calculations have been applied to materials with relatively simple crystal structures. Some intermetallic alloys, however, are not so simple. A technologically important example is the Al-Mg system. The Mg serves to strengthen the Al through solid-solution hardening,[3,4] as well as to enhance seawater corrosion resistance,[5,6] at concentrations that allow the material to remain in the face-centered cubic (fcc) a-Al structure. However, if exposed to moderate temperatures for long enough times the alloy becomes ‘‘sensitized’’ as Mg diffuses to grain boundaries and forms b-Mg2Al3 (the Samson phase[7]) precipitates.[8,9] These precipitates are highly anodic with respect to the a-Al matrix,[10] and with sufficient coverage of the grain boundary by b-Mg2Al3, the alloy can fail by intergranular corrosion.[11] Recent transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) studies of the evolution of grain boundary b-Mg2Al3 in an Al 5083 alloy[12,13] NOAM BERNSTEIN, Research Physicist, is with the Center for Computational Materials Science, Materials Science and Technology Division, Naval Research Laboratory Code 6394, Washington, DC 20375. Contact e-mail: [email protected] RAMASIS GOSWAMI, Senior Scientist, is with SAIC Inc., McLean, VA 22012, and is Resident at the Naval Research Laboratory. RONALD L. HOLTZ, Senior Physicist, is with the Multifunctional Materials Branch, Materials Science and Technology Division, Naval Research Laboratory Code 6351. Manuscript submitted August 10, 2011. Article published online February 7, 2012 2166—VOLUME 43A, JUNE 2012
have shown evidence of unusual elongated b structures on grain boundaries when partially sensitized, eventually merging into a continuous coverage of the boundary for fully sensitized material. Similar b-phase morphologies have been reported in an Al 5182 alloy, where it was suggested that this is the result of preferential growth of b-Mg2Al3 into particular crystallographic orientations of the a-Al grains, although the corresponding crystallographic directions of the b phase were not discussed.[14] It is expected that the nucleation of bMg2Al3 at a-Al grain boundaries[15,16] will be controlled by interfacial energies and strains, leading to particular favorable orientation relations that can be extracted from careful HRTEM measurements. Although the sensitized Al 5083 alloy is more vulnerable to corrosion fatigue in saltwater relative to the unsensitized material, this does not extend to fatigue crack growth in air.[17,18] In air, the crack faces indicate mixed intergranular and transgranular fracture, leading to a question about the microscopic damage mechanism in the absence of corrosion: Do the cracks propagate along grain boundaries through the Al matrix near the grain boundaries, through the b-Mg2Al3 phase, or at the a-Al–b-Mg2Al3 (a|b) interfa
Data Loading...
