Strain-induced phase transformation of a thin Co film on flexible substrates

The strain-induced FCC to HCP phase transformation of a two phase Co film on polyimide was investigated by performing a tensile test in an X-ray diffractometer. During straining of the 2 mu m thick film, the intensity of the (002)(Fcc) peak continuously decreases at engineering strains between 2 and 8% and remains constant at higher strains. Complementary in situ tensile tests under an optical light microscope showed crack formation at 6.7% and crack saturation at around 10% engineering strain. The strain induced phase transformation starts before the first cracks form leading to a maximum lattice strain of approximately 0.9% as initiation strain measured from the (10 (1) over bar1)(HCP) peak with the sin(2)psi method which converts to a film stress of approximately 1270 +/- 150 MPa. It could be revealed that a strain induced phase transformation can enhance the ductility and therefore delay the crack onset of a thin cobalt film. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.