Investigation of discontinuous precipitation upon age-hardening of invar-based Sn alloy
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Olga Fabrichnaya Institute of Materials Science, TU Bergakademie Freiberg, Freiberg 09599, Germany
Olena Volkova Institute of Iron and Steel Technology, TU Bergakademie Freiberg, Freiberg 09599, Germany (Received 31 May 2017; accepted 21 August 2017)
Age-hardening of homogenized and cold-rolled invar-based Sn alloys results in the development of continuously-formed (CP) and discontinuously-formed (DP) Ni3Sn2 precipitates. In situ investigation of the DP reaction front (RF) velocity (V) revealed a nonsteady-state behavior upon early aging stages followed by a constant V after prolonged aging. The reason for the initial nonsteady-state behavior was experimentally studied and attributed to the reduction of matrix Sn-supersaturation ahead of the DP RF as a result of the simultaneous CP coarsening (in homogenized specimen) or the CP increased volume fraction (in cold-rolled specimen). A similar trend of V variation in the homogenized specimen was obtained after the modification of the original Hornbogen model for the nonsteady-state DP growth kinetics. In general, variations of the transformed matrix fraction via the DP reaction suggest the faster kinetics of this reaction in cold-rolled specimen as compared to the homogenized one due to the existence of more nucleation sites induced by the cold deformation.
I. INTRODUCTION
Fe–36 wt% Ni alloy, so-called invar, is well known for its dimensional stability upon temperature changes between 20 and 100 °C.1 Depending on the type and alloy composition, the coefficient of thermal expansion (CTE) of this alloy lies in the range between 1.3 and 5 ppm/°C.2,3 From the practical point of view, this alloy cannot be used under the conditions where a high strength is demanded. Therefore, several researches have been performed on the age hardening of this alloy by addition of the alloying elements such as Al, Ti, Be, and Sn or the interstitial elements such as C to obtain a modified hardness and strength.4–8 Upon agehardening, the developed precipitates from the ternary solid-solution of the invar-based alloy increase the hardness and ultimate tensile strength.9–11 Few investigations have been conducted on the agehardening of invar-based Sn alloys to study the microstructure of the precipitates as well as the variation of CTE upon aging.12–16 The age-hardening treatment of invar-based Sn alloys brings about an increased hardness as a result of the formation of Ni3Sn2 precipitates of the hexagonal crystal structure. Additionally, the depletion of Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.364
Ni atoms from the solid-solution upon aging gives rise to a raised CTE.12 The Ni3Sn2 precipitates can form continuously (CP) in the form of needle-type precipitates or discontinuously in the form of lamellar/cellular precipitates.12,13,16 Upon the later type of discontinuous precipitation (DP), the supersaturated matrix of the invar-based Sn alloy decomposes into the lamellae of the less Sn-saturated matrix a
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