1997 TMS Annual Meeting: Wednesday Session

CHEMISTRY AND PHYSICS OF NANOSTRUCTURES AND RELATED NONEQUILIBRIUM MATERIALS: Session VI: Mechanical Properties

Session Chairperson: Philip Nash, Illinois Institute of Technology, METM Dept., Chicago, IL 60616

2:00 pm INVITED

MECHANICAL BEHAVIOR OF BULK NANOSTRUCTURED Fe/Cu ALLOYS: Walter W. Milligan, John E. Carsley, Stephen A. Hackney, Metallurgical and Materials Engineering, Elias C. Aifantis, Mechanical Engineering and Engineering Mechanics, Michigan Technological University, Houghton, MI 49931

The mechanical behavior of bulk iron alloys containing 10% copper was investigated. Grain sizes were varied by processing, and ranged from 45 nm to 900 nm. Deformation in all cases occurred by intense localized shear banding as the first and only mechanism of plastic deformation, from the yield point until fracture or buckling. Mechanical response was elastic-perfectly plastic. Shear band angles and an asymmetry of the yield strength in tension and compression both indicated the possibility of a pressure-sensitive yield criterion. Continuum models based on this approach proved promising. Shear band widths increased with microstructural coarseness, and were correlated successfully with a gradient plasticity approach. A number of similarities between the behavior of these alloys and amorphous materials such as metallic glasses and amorphous polymers will be discussed.

2:30 pm INVITED

DEFORMATION MECHANISMS IN METALLIC NANOLAMINATES: Tim Foecke, Metallurgy Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-0001

It has been seen that the introduction of a nanolayered microstructure can increase the yield strength and hardness of a material to many times that of a conventional microstructure. The deformation mechanisms that operate (or fail to operate) at such a small length scale is not understood and is only beginning to be investigated. An in situ TEM deformation experiment has revealed a dislocation generation mechanism that operates in Cu/Ni single crystal nanolaminates. Dislocation loops nucleated at several locations within a Ni layer at the Cu/Ni interface, and were seen to expand until they intersected the next interface. Two Orowan bows were then seen to expand in opposite directions along the layer. Discussion will include possible nucleation sites and thin film artifacts, as well as presentation of other TEM observations of fracture and deformation in metallic nanolaminates.

3:00 pm INVITED

DEFORMATION AND FRACTURE BEHAVIOR OF HIGH-STRENGTH Al94(V2,Ti)4Fe2 ALLOYS CONSISTING OF NANOGRANULAR AMORPHOUS AND Al PHASES: Akihisa Inoue, Hisamichi Kimura and Kenichiro Sasamori, Institute for Materials Research, Tohoku University, Sendai 980-77, Japan

A new mixed structure consisting of nanogranular amorphous and fcc-Al phases was formed in melt-spun Al94V4Fe2 and Al94V2Ti2Fe2 alloys and the complete replacement of V by Ti causes the change into a mixed structure of nanogranular Al phase surrounded by an amorphous phase. The structural change appears to result from the difference in solidification processes, i.e., the formation of the amorphous phase as a primary phase, followed by the Al phase for the V-containing alloys and the primary formation of the Al phases and then the amorphous phase for the V-free alloy. The former type is a unique process which is observed in the limited alloys containing V with icosahedral-forming ability and low diffusivity. The coexistence of the nanogranular amorphous phase is expected to affect significantly the mechanical strength, deformation behavior and fracture mode. These mixed phase alloys have good bending ductility and exhibit high tensile strength of 1390 MPa for Al94V4Fe2, 1370 MPa for Al94V2Ti2Fe2 and 1320 MPa for Al94Ti4Fe2 at room temperature. The fracture mode is analogous to that for amorphous alloys. At the meeting, we will report the detailed results on the temperature dependence of mechanical strength, deformation behavior and fracture mode for the nanogranular amorphous alloys.

3:30 pm BREAK

3:45 pm

FATIGUE BEHAVIOR OF NANOCRYSTALLINE AND ULTRAFINE-GRAINED Cu: S.R. Agnew1, R.Z. Valiev2, J.R. Weertman1, 1Northwestern University, Evanston, Illinois, 2Institute for Metals Superplasticity Problems, Russian Academy of Sciences, Ufa, Russia

One of the tests that can give insight into the deformation mechanisms of nanocrystalline and ultrafine-grained metals is fatigue. In the past, we investigated the microstructural stability of nanocrystalline (~20nm) Cu under cyclic loading. We observed good stability of the microstructure using XRD. An SEM study revealed extrusions on the surface, similar to those of persistent slip bands, oriented in the direction of maximum shear. The presence of extrusions suggests deformation by a shearing process. To augment our understanding of this phenomenon, we have begun studying the fatigue behavior of UFG Cu with a grain size of about 200nm. There we have observed similar extrusions as in the previous study. We are investigating the microstructural stability of this metal using the TEM. We will discuss our experimental results and their contribution to our understanding of the deformation mechanisms operative in pure FCC metals with ultra-fine microstructures.

4:05 pm

COHERENT SOLUBILITY LIMITS OF '-TYPE PHASES IN FOUR BINARY Ni-BASE ALLOYS: F. Li and A.J. Ardell, Department of Materials Science and Engineering, University of California, Los Angeles, CA 90095-1595

The equilibrium solubilities of coherent phases can be affected by the initial solute concentration of the alloy, Xo. This differs completely from incoherent equilibrium, wherein the solubility limits are independent of Xo. To investigate the relationship between coherent equilibrium solubility and Xo, Ni-Al, Ni-Ga, Ni-Ge and Ni-Ti alloys, each with several different values of Xo, were prepared by arc melting, rolling into sheet, solution, treating, and aging for times up to 1632h at 500°C. The variation of solute concentration in the Ni-rich matrix, X, with aging time, t, was followed using measurements of the ferromagnetic Curie temperature. The equilibrium solubilities, Xeq, were estimated by plotting the data as X vs. t-1/3 and extrapolating to t-1/3=0 (t=), thus presupposing that coarsening of the expected '-type phases in all four alloys is responsible for the variation of X. Xeq is unequivocally dependent on Xo, but whereas Xeq in the Ni-Al, Ni-Ga and Ni-Ti alloys increases with increasing Xo as predicted theoretically by Ardell and Maheshwari, Xeq in the Ni-Ge alloy system decreases. TEM is currently in progress to confirm the identity and state of coherency of the precipitate phases present. This work is supported by the National Science Foundation

4:25 pm INVITED

SYNTHESIS, STRUCTURE AND PROPERTIES OF Cu/Nb NANOLAYERED COMPOSITES: H. Kung, A.J. Griffin, Jr., Y.C. Lu, M.F. Hundley, T.-E.Mitchell, M. Nastasi, Los Alamos National Laboratory, MS K765 Los Alamos, NM

The effect of composition wavelength () on the structure, electrical resistivities and mechanical properties of Cu/Nb nanolayered composites were evaluated. The as- sputtered multilayers, with varying between 25Å and 1000Å, exhibit a strong Kurjudmov-Sachs orientation relationship between the close packed planes and directions of the fcc Cu and bcc Nb: <110>{111}Cu//<111>{l10}Nb. As ~decreases to 11Å, the entire multilayer shows bcc structure. The Cu grows pseudomorphically on the bcc Nb which acts as a template for the normally fcc Cu. The layers are heavily strained and there is a high density of dislocations present. As increases from 11Å to 25Å, it is suspected that there is enough loss of coherency to transform bcc Cu martensitically to fcc Cu. The mechanical and electrical properties of nanolayered Cu/Nb composites with Cu in either fcc or bcc structure will be reported.