Cyclic Deformation of Advanced High-Strength Steels: Mechanical Behavior and Microstructural Analysis
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INTRODUCTION
THE increased use of advanced high-strength steels (AHSS) in the automotive industry is based on the need for weight savings in vehicles while maintaining or improving safety. The AHSS are multiphase steels that provide an improved strength and ductility combination over conventional high-strength steels. To fully realize their potential in automotive applications, it is necessary to understand the properties of particular interest to the automotive industry including fatigue resistance for improved component life, crush resistance for energy absorption in vehicle damage, and a high level of formability for manufacturability. In this respect, dual-phase (DP) steels have demonstrated a good compromise between strength and ductility.[1,2] The DP steels exhibit a continuous yielding behavior, low yield point, and a high strain-hardening coefficient.[3,4] This has been attributed to an increase in the workhardening parameters through the formation of mobile dislocations due to the martensite formation during processing, and martensite twinning during forming.[5] Transformation-induced plasticity (TRIP) steels are another AHSS that have a unique strength ductility relationship due to the presence of stable retained austenite in the microstructure. The austenite is TIMOTHY B. HILDITCH and ILANA B. TIMOKHINA, Research Academics, LEIGH T. ROBERTSON, Postdoctoral Student, and PETER D. HODGSON, Professor, are with Centre for Material and Fibre Innovation, Deakin University, Waurn Ponds, VIC, Australia, 3217. Contact e-mail: [email protected] ELENA V. PERELOMA, Professor, is with School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, NSW, Australia, 2522. Manuscript submitted May 11, 2008. Article published online January 6, 2009 342—VOLUME 40A, FEBRUARY 2009
transformed to martensite in response to an applied load,[6,7] resulting in an increase of both the tensile strength and ductility. The macroscopic plastic strain accompanying the austenite to martensite phase transformation under the stress is defined as transformation plasticity. The strain can arise from transformation shape change or plastic accommodation process, which occur around the martensite islands as they form.[8] The fatigue performance of a vehicle is an important design and material selection consideration due to the repeated loads/strains that are experienced during normal use. While the vehicle is designed so that the stresses in a component are below the yield stress of the material, there are regions of stress concentration in the body structure that can generate small plastic strains. The presence of small inelastic strains, or microplasticity, in materials at stresses below the yield point may also make an important contribution to the fatigue properties. This makes it important to understand the effects of low-cycle fatigue (LCF) on the performance of automotive materials. Fatigue results in the alteration of the internal structure of the material; thus, the distribution of the strain within the micr
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