Strengthening metals by narrowing grain size distributions in nickel-titanium thin films
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David T. Wu Department of Materials Science and Engineering, Institute of High Performance Computing, A*STAR, Singapore, 138632, Singapore
Derek Zhao and Ainissa G. Ramireza) Department of Mechanical Engineering & Materials Science, Yale University, New Haven, Connecticut 06520 (Received 3 September 2012; accepted 26 March 2013)
Grain size influences the mechanical strength of materials. In polycrystalline materials, strength increases with decreasing average grain size (for grains larger than 100 nm). This well-known Hall–Petch relationship typifies a strengthening mechanism, in which dislocation motion is impeded by grain boundaries. As grains become smaller, higher stresses are required to deform them. However, this formalism only considers the role of the “average” size of grains. Heterogeneous materials, however, have a broad “distribution” of grain sizes. Here we show that materials with narrowed grain size distributions have mechanical properties that differ from Hall–Petch predictions. Narrower distributions show increased strength, as their homogeneously sized grains yield at higher loads than the large grains in materials with broader grain size distributions. Plastic deformation depends on the coarsest grains, which yield first. These results suggest new routes for tailoring material properties.
I. INTRODUCTION
A fundamental understanding of the role of microstructure on properties provides researchers a pathway to control them in a predictable way. One well-known example of the linkage between microstructure and properties is the Hall–Petch relationship,1,2 which correlates the strength of materials to the average grain size. Since the mid-20th century, this relationship has coupled the yield strength of polycrystalline materials to the inverse square root of their mean grain size.1–5 This expression, based on experimental findings on the behavior of steels, embodies the strengthening of materials by grain boundaries, which impede dislocation motion, and has been applied to metals, ceramics, and intermetallics.6 The role of grain size continues to be of interest today, as researchers explore the limits of this expression for decreasingly small grains7 and an inverse behavior for nanostructured materials.8 While the discussion of the role of the “average” grain size on properties continues in the literature,7 the role of the grain size “distribution” has been largely overlooked. This consideration is important as the models that explicate the material behavior are based on an assumption that all grains in the material have the same size. 9 However, heterogeneous materials rarely meet this criterion. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.83 J. Mater. Res., Vol. 28, No. 10, May 28, 2013
Therefore, to achieve a more robust understanding of material behavior, knowledge of the role of the distribution of grain size on properties is fundamental. A limited number of theoretical works have considered the role of grain size distributions on properti
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