Interfacial segregation and deformation of superplastically deformed Al-Mg-Mn alloys

Microstructural and microchemical studies have been carried out on superplastically deformed Al-Mg-Mn (AA5083-type) alloys. Grain boundary composition was measured using a Scanning Auger Microprobe (SAM) and an Analytical Transmission Electron Microscope (ATEM), while conventional TEM was used for microstructural evaluation. Non-equilibrium segregation of Si to grain boundaries following deformation was measured by both techniques. Significant interfacial Si enrichment was only detected in gage sections of tensile specimens after uniaxial strains from 50 to 200%. Grip regions which experience identical thermal histories, but without plastic deformation, did not reveal Si segregation. Selected samples also showed a slight depletion of Mg at grain boundaries after deformation. The only reproducible observation of equilibrium segregation was in Zr-modified alloys, where Sn was detected by SAM in both the deformed and undeformed sections of the sample. Microstructural analysis documented subgrain formation and subgrain-precipitate interactions during superplastic deformation. In addition, many grain boundaries and precipitate interfaces contained small (5 to 20 nm) voids. Compositional analysis of these nano-voids revealed that they were enriched in Mg with the adjacent boundary regions correspondingly depleted

Interfacial precipitation, segregation and deformation in alloy 600: Implications on primary-side IGSCC

A great many unknowns still exist concerning the mechanisms controlling intergranular stress corrosion cracking (IGSCC) of alloy 600 in high-temperature, deaerated water environments. Any proposed mechanism must involve the microstructure, microchemistry and mechanical properties of grain boundary regions. To facilitate basic understanding, specific aspects of alloy 600 metallurgy are reviewed and discussed. Interfacial carbide precipitation, chromium depletion, impurity segregation and local deformation characteristics are examined and related to IGSCC behavior. Purpose of this paper is to provide information, and prompt discussion, on these various issues for the EPRI Alloy 600 Experts Meeting

Metastable phase formation in Be-Nb intermetallic compounds

Amorphous structures or metastable crystalline phases are produced in sputter-deposited Beryllium-Niobium (Be-Nb) alloys (5-15 at. % Nb) depending on the substrate temperature. The metastable phases transform to the stable Be{sub 12}Nb, Be{sub 17}Nb{sub 2}Nb phases on annealing at temperatures >800{degree}C. No Be{sub 5}Nb phase was found and the Be{sub 17}Nb{sub 2} phase is stable to low temperature. The Be{sub 12}Nb phase appeared to have a stoichiometric range of about 5.5 to 7.7 at. % Nb. The formation of the metastable phases is consistent with current models and theories. 17 refs., 1 fig., 2 tabs

Phase stability in Be-Nb and Be-Nb-Zr intermetallics

Sputter deposition of Be-Nb alloys at low temperature (30{degrees}C) produces an amorphous phase for compositions >5 at.% Nb. A metastable crystalline phase which can be considered a highly faulted form of the Be{sub 12}Nb occurs at higher deposition temperatures or by low-temperature annealing of the amorphous phase. Because of structural similarities, this metastable phase is a precursor to the formation of either Be{sub 12}Nb or Be{sub 17}Nb{sub 2} upon high temperature annealing. There was no evidence of the Be{sub 5}Nb phase which has been postulated on some phase diagrams. The Be{sub 12}Nb phase can accomodate considerable Zr in the structure and the Be{sub 13}Zr can accomodate Nb into its structure. The Be{sub 13}Zr becomes the predominant phase when the Zr/Nb composition ratio > 1. High temperature annealing of the ternary results in dual-phase regions of Be{sub 12}(Nb,Zr) + Be{sub 17}(NbZr){sub 2} or Be{sub 13}(Zr,Nb) + Be{sub 17}(Zr,Nb){sub 2}, but the coexistence of Be{sub 12}(Nb,Zr) + Be{sub 13}(Zr,Nb) has not been observed. 10 refs., 5 figs

Synthesis and properties of nanostructures formed by sputter deposition

Nanoscale grain structures can be created in many materials by high rate sputter deposition upon an appropriate substrate. By controlling the sputtering parameters of substrate temperature, deposition rate, and substrate bias, the scale of microstructure can vary from amorphous to grain sizes typical of conventionally treated material. The morphology and size of the microstructure will also be controlled by post-deposition heat treatment, particularly in the heat treatment of amorphous and metastable phases. Examples are given of nanostructural development in sputter-deposited pure metals, stainless steel alloys, and intermetallic compounds. Some properties of these nanostructures such as microhardness and yield strength have been evaluated and shown to be a strong function of the grain size. The structure and properties of nanolayered Cu-Mo composites produced by sputter deposition are discussed. Synthesis of materials at low temperature using displacement reactions of layered composite is demonstrated. 16 refs., 10 figs