Phase transformation of A15 crystal structure chromium thin films grown by the sputter deposition

Phase transformation of non-BCC δ-A15 Cr thin films prepared by r.f. magnetron sputtering has been characterized. Using differential scanning calorimetry (DSC), we demonstrated for the first time that the phase transformation of the δA15 Cr phase to the equilibrium α-BCC Cr phase is an irreversible, exothermic, first-order transition. At a heating rate of 10 °C min −1 , the onset and peak temperatures of transformation were determined to be 428 and 437 °C, respectively. The enthalpy change of the transformation, Δ H, was estimated to be in a range from −6 to −12 kJ mol −1 . The uncertainty of ΔH was attributed to an ill-defined transition completion temperature and an incorporation of substrate material into the DSC film samples analyzed. Our in situ DSC results confirmed the metastable and allotropie nature of δ-A15 Cr phase, which has been proposed by other prior studies using conventional ex situ X-ray and electron diffraction methods. Our X-ray photoelectron spectroscopy analyses on as-deposited and annealed films showed that while the chrome oxides or carbides were formed inevitably in annealed films the Cr and O were mostly in the unbound states. Hence, it is unlikely that the δ-A15 Cr phase is of an oxide phase.