Browsing by Subject "Interstitial-free steel"
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Item Development of an elevated-temperature tensile testing instrument for investigating dynamic grain growth in an interstitial-free steel(2022-04-29) Bennett, Thomas James, IV; Taleff, Eric M.; Fahrenthold, Eric PExposure to elevated temperatures during plastic deformation, termed dynamic conditions, produces microstructures distinct from static annealing for a titanium interstitial-free (Ti-IF) steel sheet material. For similar annealing times, dynamic conditions produce larger grains than static annealing and a crystallographic texture that is unique. These differences are attributed to dynamic grain growth (DGG), which occurs by mechanisms different from those of static conditions. The mechanisms of DGG are not yet well understood. The objective of this work is to design and construct a testing instrument suitable for investigations that probe the mechanisms of DGG in Ti-IF steels. This instrument is required to deform a specimen in tension at elevated temperatures in a reducing atmosphere and then rapidly quench that specimen to preserve the microstructure developed during deformation. The instrument was constructed using a servohydraulic tensile test frame, a split tube furnace, and an original elevated-temperature testing enclosure with reducing gas and air quenching systems. It enabled elevated-temperature tensile tests of Ti-IF steel specimens without significant specimen oxidation. The mechanical data obtained from these tests are in agreement with existing data for the same material. Scanning electron microscopy revealed the presence of preserved subgrain structures in the microstructures of tested specimens. These experimental results confirm that the new instrument satisfies the stated design requirements and is suitable to advance the study of DGG phenomena in Ti-IF steel.Item Dynamic normal grain growth in BCC interstitial-free steel during hot deformation(2018-08-15) Rupp, Ryann Elizabeth; Taleff, Eric M.; Bourell, David L.; Juenger, Maria C.G.; Kovar, Desiderio; Seepersad, Carolyn C.Concurrent plastic deformation at elevated temperature can produce dynamic normal grain growth (DNGG). DNGG significantly impacts microstructure by accelerating grain growth and/or evolving texture. Both of these were observed in an interstitial-free steel deformed in uniaxial tension at 850°C at a constant true-strain rate of 10⁻⁴ s⁻¹. Two complementary analysis methodologies were used to investigate DNGG and its mechanisms. The first analysis method determined the contributions from (i) lattice rotation during slip and (ii) DNGG to texture evolution during high-temperature deformation. Lattice rotation and DNGG were isolated by determining the texture of three states: (i) undeformed (recrystallized), (ii) deformed at room temperature, and (iii) deformed at high temperature. The second analysis method segmented EBSD data into individual grains, which included their sizes and orientations. This analysis method probed the relationships between crystallographic orientation and changes in grain area fraction, number fraction, and size with strain. A statistical analysis identified the characteristics of grains that were preferred for growth in order to investigate DNGG and its mechanisms. DNGG preferentially grew grains of specific orientations at the expense of other orientations. Grains that preferentially grew by DNGG were observed to be from orientations with specific crystallographic planes parallel to the plane of the sheet material. Grains oriented with the {111}, {332}, or {322} planes parallel with the plane of the sheet material were preferred for growth. Among grains with these planes parallel to the sheet plane, a specific subset of orientations were most preferred for growth. The reference orientations most favored for growth depended on the initial recrystallized texture relative to the specimen tensile direction. Subgrains were observed to occur in grains during hot deformation and concurrently with DNGG. High-resolution EBSD (HR-EBSD) resolved subgrains and associated hot deformation substructure that is hypothesized to be important to DNGG.Item The influence of high-temperature tensile deformation on microstructure evolution in select BCC metals(2015-12) Noell, Philip James; Taleff, Eric M.; Bourell, David L; Engelhardt, Michael; Ferreira, Paulo; Kovar, DesiderioHigh-temperature plastic deformation can enhance the rate of grain boundary migration in some metals. In the body-centered-cubic (BCC) refractory metals molybdenum and tantalum, this increased rate of grain boundary migration produces abnormal grain growth at temperatures significantly lower than is possible by static annealing. This phenomenon is termed dynamic abnormal grain growth (DAGG). The influence of microstructure on DAGG is studied by examining the morphology of DAGG grains produced in two Mo sheet materials. DAGG grain propagation in these materials is not uniform throughout the sheet thickness. Variations through the sheet thickness in texture and grain size are explored as causes of these behaviors. DAGG grains in both materials preferentially grow into the finest-grained polycrystalline regions of the sheet. Direct effects of local crystallographic texture variation are not evident in microstructures containing DAGG grains. The initiation of abnormal grains in Mo materials by plastic straining at elevated temperatures is investigated. The minimum strain necessary to initiate DAGG, termed the critical strain, decreases approximately linearly with increasing temperature. The variation in critical strain values observed at a single temperature and strain rate is well described by a normal distribution. An increased fraction of grains aligned with the <110> along the tensile axis, a preferred orientation for DAGG grains, appears to decrease the critical strain for DAGG initiation. Uniaxial tension tests at temperatures between 1650 and 1950 °C were conducted with commercial-purity Mo rods to determine if DAGG can be used to produce large single crystals of Mo. Necking in these tensile rod specimens during the production of DAGG grains presents a potential issue with translation to commercial application. High-temperature tensile tests were also performed on another BCC metal, an interstitial-free steel. Grain growth in this interstitial-free steel during high-temperature plastic deformation is significantly accelerated above that of the static case. Grains generally oriented between {112}<110> and {111}<113> grow more readily than other grains during dynamic grain growth in this material.