Microstructure and Mechanical Properties After Shock Wave Loading of Cast CrMnNi TRIP Steel
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DUCTION
HIGH-ALLOY steels showing the transformation-induced plasticity (TRIP) effect are known for their outstanding combination of strength and ductility and were subject to a number of studies in the past.[1–5] Their plastic deformation behavior is characterized by an increase of both strength and ductility, due to the strain-induced transformation of the austenitic c-phase to e- and a¢-martensite within deformation bands.[6–8] Especially in the automotive sector, dynamic deformation data are required to satisfy concurrent demands for weight reduction and crash safety. Therefore, efforts were recently made to characterize CrMnNi TRIP steels and their deformation behavior in the strain rate regime up to 103 s 1.[9,10] Although at even higher strain rates, fewer data are available, due to the increasing experimental complexity and high safety requirements, especially when working with explosives. Nonetheless, such data are of importance for military applications as well RALF ECKNER, Research Associate, Doctoral Student, and L. KRU¨GER, Full Professor, Head of Institute, are with the Institute of Materials Engineering, Technische Universita¨t Bergakademie Freiberg, Gustav-Zeuner-Str. 5, 09599 Freiberg, Germany. Contact e-mail: [email protected] C. ULLRICH, Research Associate, Doctoral Student, and D. RAFAJA, Full Professor, Head of Institute, are with the Institute of Materials Science, Technische Universita¨t Bergakademie Freiberg. T. SCHLOTHAUER, Research Associate, Doctoral Student, and G. HEIDE, Full Professor, Head of Institute, are with the Institute of Mineralogy, Technische Universitt Bergakademie Freiberg, Brennhausgasse 14, 09599 Freiberg, Germany. Manuscript submitted February 4, 2016. Article published online August 2, 2016 4922—VOLUME 47A, OCTOBER 2016
as for high rate metalworking, e.g., shot peening, explosive welding, or explosive hardening. The triggering mechanism for the strain-induced phase transformation is the formation of stacking faults, the amount of which depends strongly on the stacking fault energy (SFE) of the TRIP steel.[11,12] When testing at high strain rates (>100 s 1), martensite formation is suppressed to some extent, due to quasi-adiabatic heating during plastic deformation.[9,10] On the one hand, higher temperature facilitates dislocation glide at the expense of the martensite formation. On the other hand, the increasing temperature lowers the chemical driving force for the c fi a¢ transformation.[9–13] When the loading rate is increased to 104 s 1 or beyond, the processes within the material’s microstructure become more complex and depend on the specific type of experiment (e.g., flyer-plate impact, rod impact, and laser shock). In such cases, the deformation of a specimen is associated with the passage of a shock wave through the material, which reaches or exceeds the speed of sound.[14] A shock wave corresponds to a compressive wave, in which the material is elastically and plastically strained and densified under high pressure and elevated temperature.[14,15] Behind t
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