Correlation of stress state and nanohardness via heat treatment of nickel-aluminide multilayer thin films
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Peter M. Anderson Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210
Jennifer L. Hay MTS Systems Corporation, Oak Ridge, Tennessee 37830 (Received 5 April 2004; accepted 10 August 2004)
Heat treatment of ␥-Ni(Al)/␥⬘-Ni3Al multilayer thin films demonstrates that multilayer hardness correlates with the magnitude of biaxial stress in alternating layers. Films with a columnar grain morphology and (001) texture were fabricated over a range of volume fraction and bilayer thickness via direct current magnetron sputtering onto NaCl (001) substrates at 623 K. The films were removed from substrates, heat-treated at either 673 K or 1073 K in argon, and then mounted for nanoindentation and x-ray diffraction. The biaxial stress state in each phase was furnished from x-ray diffraction measurement of (002) interplanar spacings. The 673 K treatment increases the magnitude of alternating biaxial stress state by 70 to 100% and increases hardness by 25 to 100%, depending on bilayer thickness. In contrast, the 1073 K heat treatment decreases the stress magnitude by 70% and decreases hardness by 50%. The results suggest that the yield strength of these thin films is controlled, in part, by the magnitude of internal stress. Further, thermal treatments are demonstrated to be an effective means to manipulate internal stress.
I. INTRODUCTION
The extraordinary plastic strength displayed by multilayer thin films1–3 has generated significant interest to determine the mechanisms by which bulk plastic deformation is suppressed at such large values of stress. Josell et al.4 have documented the suppression of plasticity as the bilayer thickness of Al/Ti multilayers is decreased from 900 to 120 nm. The experimental data clearly shows that for bilayer thickness below ∼230 nm, the tensile constitutive response is essentially linear up to fracture, with a large scatter in ultimate tensile strength. Scanning electron microscopy (SEM) of fractured Ni(Al)/Ni3Al multilayer thin films indicates that such macroscopically brittle fracture may actually consist of narrow shear bands through the film thickness,5 suggesting that plastic deformation can be quite localized near the fracture plane. Novel transmission electron microscopy (TEM) studies by Foecke and Kramer6 document a crack tip plastic zone in Cu/Ni multilayer thin films that
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0435 3374
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 11, Nov 2004 Downloaded: 24 Mar 2015
is highly localized along the fracture path and orders of magnitude smaller than the plastic zone in the adjoining Cu substrate. Such studies have also documented the propagation of dislocations in a confined layer slip mode in Cu/Ni multilayer thin films7 and transmission of partials across interfaces during indentation of W/NbN multilayer thin films.8 In both of these systems and in substrate curvature testing of Cu and Al thin films on ␣alumina substrates,9 interface
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