Abstract
Deformation mechanisms of cold drawn and electropolished nickel microwires are studied by performing in-situ monotonous and cyclic tensile tests under synchrotron radiation. X-ray diffraction tests allow probing elastic strains in the different grain families and establishing a link with the deformation mechanisms taking place within the microwires. The measurements were carried out on several microwires with diameters ranging from as-drawn 100 µm down to 40 µm thinned down by electropolishing. The as-drawn wires exhibit a core-shell microstructure with <111> fiber texture dominant in core and heterogeneous dual fiber texture <111> and <100> in the shell. Reduction of specimen size by electropolishing results in a higher yield stress and tensile strength along with reduced ductility. In-situ XRD analysis revealed that these differences are linked to the global variation in microstructure induced by shell removal with electropolishing, which in turn affects the load sharing abilities of grain families. This study thus proposes a new way to increase ductility and retain strength in nickel microwires across different diameters by tuning the microstructure architecture.
Highlights
Literature in the field of multi-crystalline samples is contradictory with two prominent observations: the so-called “smaller is stronger”[31,32,33] and “smaller is weaker”[22,29,34,35]
These studies have shown that cold drawn Ni microwires seem to be sensitive to extrinsic size effect, with increase in tensile strength and reduction of ductility when reducing their diameter by electropolishing, i.e. without changing their internal microstructure
An example of cross-sectional electron back-scatter diffraction (EBSD) map as well as pole figures obtained using both EBSD and high energy X-ray diffraction is shown in Fig. 3: according to the inverse pole figure (IPF) map shown in Fig. 3(a), it can be seen that the micro-texture of 100 μm wire exhibits a ‘core-shell’ type microstructure, that will be commented below
Summary
Literature in the field of multi-crystalline samples is contradictory with two prominent observations: the so-called “smaller is stronger”[31,32,33] and “smaller is weaker”[22,29,34,35]. Similar tests (Agepati et al.49) on a second batch of Ni microwires from the same commercial source yielded a huge scatter in tensile strength As a summary, these studies have shown that cold drawn Ni microwires seem to be sensitive to extrinsic size effect, with increase in tensile strength and reduction of ductility when reducing their diameter by electropolishing, i.e. without changing their internal microstructure. Tensile tests on Ni microwires are performed in combination with XRD to derive the deformation mechanisms taking place in the different grain families These mechanisms are discussed in view of the initially observed microstructure (grain size and crystallographic micro-texture) and the effect of diameter change by electropolishing. Guidelines are proposed to tailor the strength and ductility of Ni microwires, these considerations being general enough to be extended to other FCC metals
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