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Title: | Microstructural Evolution and Mechanical Properties of a Medium Manganese Transformation Induced Plasticity Third Generation Advanced High Strength Steel |
Authors: | Patel, Vivek |
Advisor: | McDermid, Joseph |
Department: | Materials Engineering |
Publication Date: | 2019 |
Abstract: | Third-generation advanced high strength steels (3G AHSS) are being developed to facilitate vehicle lightweighting in order to improve their fuel efficiency without compromising passenger safety. This thesis will examine the development of microstructure and mechanical properties of a prototype medium Mn TRansformation- and TWinning-Induced Plasticity (TRIP/TWIP) 3G-AHSS with a composition of 0.15C-6Mn-1Si-1Al. Heat treatments consisting of intercritical annealing and overageing stages compatible with the continuous galvanizing process were investigated in terms of their effects on the microstructure and the mechanical properties from the as-received, tempered martensite (TM) microstructure and a fully martensitic (M) starting microstructure. Samples intercritically annealed from the M starting microstructure exhibited a lamellar morphology of retained austenite/martensite laths and faster austenite reversion kinetics. Samples intercritically annealed from the TM starting microstructure exhibited a globular/equiaxed morphology and slower austenite reversion kinetics. Though they had high (~0.3 volume fraction) initial amounts of retained austenite, samples intercritically annealed from the TM starting microstructure did not meet the 3G-AHSS mechanical property envelope due to insufficient mechanical stability of retained austenite and its resulting transformation to α’-martensite at low strains. These samples did, however, show impressive UTS values (up to 1600 MPa), despite lower tensile elongation values. Despite having lower fractions of retained austenite in the as-annealed microstructure, the samples intercritically annealed from the M starting microstructure achieved combinations of strength and tensile elongations that met the 3G-AHSS mechanical property envelope. In particular, the M-690-120 sample nearly met the specific U.S. DOE mechanical property target of 1200 MPa UTS and 30% TE exhibiting 1150 MPa UTS and 29% TE. TEM results from strained and fractured tensile samples showed that samples intercritically annealed from the TM and M starting microstructures both exhibited deformation twinning in the retained austenite, which suggests that the combination of the TRIP and TWIP effect resulted in an excellent combination of strength and tensile elongation. Based on experimental results, this work demonstrates that medium Mn TRIP/TWIP alloys are capable of achieving the target 3G-AHSS mechanical properties employing heat treatments compatible with the continuous galvanizing process. |
URI: | http://hdl.handle.net/11375/24276 |
Appears in Collections: | Open Access Dissertations and Theses |
Files in This Item:
File | Description | Size | Format | |
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Patel_Vivek_J_2019April_M.A.Sc.pdf | 6.93 MB | Adobe PDF | View/Open |
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