Critical factors that determine face-centered cubic to body-centered cubic phase transformation in sputter-deposited aus
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Bulk austenitic stainless steels (SS) have a face-centered cubic (fcc) structure. However, sputter deposited films synthesized using austenitic stainless steel targets usually exhibit body-centered cubic (bcc) structure or a mixture of fcc and bcc phases. This paper presents studies on the effect of processing parameters on the phase stability of 304 and 330 SS thin films. The 304 SS thin films with in-plane, biaxial residual stresses in the range of approximately 1 GPa (tensile) to approximately 300 MPa (compressive) exhibited only bcc structure. The retention of bcc 304 SS after high-temperature annealing followed by slow furnace cooling indicates depletion of Ni in as-sputtered 304 SS films. The 330 SS films sputtered at room temperature possess pure fcc phase. The Ni content and the substrate temperature during deposition are crucial factors in determining the phase stability in sputter deposited austenitic SS films. I. INTRODUCTION
The family of bulk austenitic stainless steels (304, 310, 316, 330 SS, etc.) has been known to have face-centered cubic (fcc) structure at room temperature. It is well known that body-centered cubic (bcc) ferrite normally does not form in bulk 304 SS without subzero temperature treatment. However, by physical vapor deposition (PVD), such as magnetron sputtering deposition or ebeam evaporation, metastable bcc 304 SS, with compositions similar to bulk materials, can be formed.1–3 Furthermore, bcc 304 SS film is usually found to be ferromagnetic. The magnetic properties of metastable bcc 304 SS have been used to monitor phase transformations during or after annealing.1,4,5 Upon annealing above 550 °C, the metastable bcc 304 SS was reported to transform to a paramagnetic fcc phase. This transformation completed after annealing at 700 °C for 1 h5 or at 800 °C for 20 min.4 The fact that ferrite forms in several types of austenitic SS films (304 and 316) by PVD technique and in smaller particles of rapid solidified 303 SS6 lead to a conclusion that rapid quenching, from liquid to solid or from vapor to solid, results in the formation of metastable ferrite (bcc SS).1,6 There are, however, several issues that have not been resolved on the origins of the formation of metastable bcc SS. First, is the bcc structure reported in SS films really a body-centered tetragonal martensitic phase? If yes, given the well known influence of stress on the martensitic DOI: 10.1557/JMR.2004.0215 1696
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transformation temperature MS,7 can the thin film residual stresses affect the stability of the observed bcc structure in sputtered 304 films? Residual stress in films can be either due to coherency between the film and the substrate or growth stress (stress induced by island growth and coalescence) or both.8 Homo- or heteroepitaxial growth conditions have resulted in the stabilization of the fcc structure in sputtered SS films. For example, fcc 316L SS was formed on acid cleaned 316LSS substrates,9 while 1–5 nm fcc 304 SS was formed within Cu/304 SS multilayers due to small latti
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