Mechanical behaviour and microstructure of Ti-CP foils with ultra-fine grains

The mechanical properties of 7 μm thick Ti-CP (grade 1) foils with ultra-fine grains were investigated through tensile tests performed at 25, 100, 200 and 300 °C and creep tests at 300 °C. In order to study the microstructure evolution, X-ray diffraction (XRD) measurements were carried out at the same temperature. The findings highlight significant size effects stemming from the large thickness-to-grain size ratio (t/D). Due to the nanometric grain size, both the yield stress (YS) and ultimate tensile strength (UTS) exhibit higher values compared to those of the corresponding bulk material, whereas total elongation (AR%) is considerably lower. Strain heterogeneity was observed in tensile tests because the grains in a surface layer are less restricted and deform easier than the inner grains. Moreover, in creep tests with applied stress up to 185 MPa, the samples exhibited an initial contraction instead of the expected elongation. Such anomalous phenomenon has been explained by considering the combined effects of the strong prismatic texture and the absorption of gas atoms (O and N) which occupy not only the octahedral sites in hcp lattice but also the hexahedral and crowdion ones.