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

Phase transformation of non-BCC delta-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 delta-A15 Cr phase to the equilibrium alpha-BCC Cr phase is an irreversible, exothermic, first-order transition. At a heating rate of 10 degrees C min(-1), the onset and peak temperatures of transformation were determined to be 428 and 437 degrees C, respectively. The enthalpy change of the transformation, Delta H, was estimated to be in a range from -6 to -12 kJ mol(-1). The uncertainty of Delta H was attributed to an ill-defined transition completion temperature and an incorporation of substrate material into the DSC him samples analyzed. Our in situ DSC results confirmed the metastable and allotropic nature of delta-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 delta-A15 Cr phase is of an oxide phase.