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dc.contributor.advisorFindley, Kip Owen
dc.contributor.authorLehnhoff, Gregory R.
dc.date.accessioned2007-01-03T05:53:36Z
dc.date.accessioned2022-02-09T08:39:56Z
dc.date.available2007-01-03T05:53:36Z
dc.date.available2022-02-09T08:39:56Z
dc.date.issued2013
dc.identifierT 7338
dc.identifier.urihttps://hdl.handle.net/11124/80128
dc.description2013 Fall.
dc.descriptionIncludes illustrations (some color).
dc.descriptionIncludes bibliographical references (pages 129-136).
dc.description.abstractAdvanced high strength steels (AHSS) for automobile light-weighting utilize Si and Al alloying to retain austenite in the microstructure during thermal partitioning treatments. This research project utilized fully austenitic steels with varied Si and Al compositions to understand the effect of these elements on austenite deformation response, including deformation induced martensite formation and deformation twinning. Specific focus was directed at understanding austenite deformation response during fatigue loading. Independent alloying additions of 2.5 wt pct Si and Al were made to a base steel composition of 15 Ni - 11 Cr - 1 Mn - 0.03 C (wt pct). Weak beam dark field transmission electron microscopy (TEM) imaging of dissociated dislocations was implemented to experimentally determine the influences of Si and Al on austenite stacking fault energy (SFE). The 2.5 wt pct Si alloying addition decreased the SFE by 6.4 mJ/m2, while the 2.5 wt pct Al alloying increased the SFE by 12 mJ/m2. Fully reversed, total strain controlled, low cycle fatigue (LCF) tests indicated that all four alloys underwent primary cyclic hardening and stabilization. Secondary cyclic strain hardening was correlated to BCC martensite formation using Feritscope magnetic fraction measurements of LCF specimens; the formation of 1 pct martensite led to 7 MPa of secondary hardening. TEM showed that martensite predominantly formed as parallel, irregular bands through strain induced nucleation on austenite shear bands. The austenite shear bands consisted of austenite {111} planes with concentrated dislocations, stacking faults, and/or HCP [epsilon]-martensite. Aluminum alloying promoted martensite formation during LCF, while Si suppressed martensite. Therefore, the strain induced nucleation process was not suppressed by the increased SFE associated with Al alloying. Tensile testing indicated that Si alloying promoted deformation twinning by lowering the SFE. Similarly to LCF loading, Al promoted martensite formation and Si suppressed martensite formation during tensile loading. Both twinning and martensite formation increased tensile work hardening, but deformation twinning led to better combinations of strength and ductility. X-ray diffraction (XRD) measurements indicated that 1 wt pct of Al alloying expands the austenite lattice equivalently to 0.15 wt pct C. Therefore, XRD measurements of retained austenite C-content in Al-alloyed AHSS should include the effect of Al.
dc.format.mediumborn digital
dc.format.mediumdoctoral dissertations
dc.languageEnglish
dc.language.isoeng
dc.publisherColorado School of Mines. Arthur Lakes Library
dc.relation.ispartof2010-2019 - Mines Theses & Dissertations
dc.rightsCopyright of the original work is retained by the author.
dc.subjectaustenite stability
dc.subjectsteel
dc.subjectstacking fault energy
dc.subjectmartensite
dc.subjectfatigue
dc.subject.lcshSteel alloys -- Fatigue
dc.subject.lcshDeformations (Mechanics)
dc.subject.lcshAustenitic stainless steel -- Fatigue
dc.subject.lcshMartensitic stainless steel
dc.subject.lcshSilicon alloys
dc.subject.lcshMartensitic stainless steel
dc.titleInfluence of silicon and aluminum on austenite deformation behavior during fatigue and tensile loading, The
dc.typeText
dc.contributor.committeememberMatlock, David K.
dc.contributor.committeememberSpeer, J. G.
dc.contributor.committeememberDe Moor, Emmanuel
dc.contributor.committeememberBerger, John R.
thesis.degree.nameDoctor of Philosophy (Ph.D.)
thesis.degree.levelDoctoral
thesis.degree.disciplineMetallurgical and Materials Engineering
thesis.degree.grantorColorado School of Mines


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