Permeability, magnetization, differential scanning calorimetry and X-ray powder diffraction measurements were carried out on the off-stoichiometric ferromagnetic shape memory alloys ...Ni(50+x)Mn(27-x)Ga(23) (-25<x<6) in order to establish the phase diagram over a wide range of concentration. The phase diagram was found to comprise of four regions, a paramagnetic austenite phase (Para-A), a paramagnetic martensite phase (Para-M), a ferromagnetic austenite phase (Ferro-A) and a ferromagnetic martensite phase (Ferro-M). Furthermore the martensitic transformation and the Curie temperatures crossed over at around x=2.5. The martensitic transformation was found to occur over a wide Mn-rich region. A linear increase in the martensitic transformation temperature with increasing number of valence electrons per atom, eja, was limited to a narrow concentration range around x=0.0. Overall the martensitic transformation temperature exhibited a concave behavior with a minimum occurring at approximately x=-8.
Microstructures of 2205 duplex stainless steel were examined using transmission electron microscopy. During isothermal heating at 950 degree C, M23C6 carbide was formed on the austenite grain ...boundaries with two types of morphologies: alarvaa and atrianglea. The orientation relationship between the M23C6 carbide and the austenite matrix is cubic-to-cubic. In addition, these two types of precipitates have a twin relationship with each other. Based on the STEM-EDS data, the silicon content of triangle M23C6 carbide is higher than that of the larva M23C6 carbide, revealing that the silicon content in the M23C6 carbide plays an important role in determining the orientation relationship between the M23C6 carbide and the austenite matrix.
Structure–mechanical property relationship studies were carried out on Gleeble simulated intercritically reheated coarse-grained heat affected zone (ICCGHAZ) of 700MPa linepipe steel microalloyed ...with Nb. The design of experiments was aimed at varying reheat temperature in the first pass to obtain different coarse grain size in the HAZ. This enabled the study of the effect of prior austenite grain size on martensite–austenite (M–A) constituent during the second pass reheating and its consequent influence on impact toughness. We elucidate here the role of phase transformation and the fraction, size, shape, distribution, and carbon content of M–A constituent on impact toughness. The data suggests that the fraction of M–A constituent is not influenced by grain size, but the size of M–A constituent is influenced by the prior austenite grain size, which consequently governs toughness. Coarse austenite grain size increases the size of M–A constituent and lowers the HAZ toughness. Coarse austenite grain associated with coarse M–A constituent along grain boundary is the dominant factor in promoting brittle fracture. The combination of fine prior austenite grain size and smaller M–A constituent is favorable in obtaining high toughness. Good toughness is obtained on refining the prior austenite grain size in the CGHAZ during first pass and hence ICCGHAZ in the second pass.
A novel approach to enhance the ductility of the transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steel was proposed in this work. Lamellar lath Mn-heterogeneity was introduced ...into the single austenite grain through austenite-reversion from martensite by slow-heating or intercritical annealing. Acicular austenite is formed accompanied with apparent Mn enrichment occurring during reversion, and such a lamellar lath Mn-inhomogeneity is remained even after full-austenitization. Highly stable retained austenite (RA) is formed at the original Mn-enriched region after austempering with higher Mn and carbon contents than those of traditional austempering from Mn-homogenized austenite. In addition, higher total fraction and larger ratio of film-like RA is obtained in the heterogeneous-austenite than homogeneous one. These contribute to the ductility improvement of the TBF steel.
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We propose a novel “Mn preservation-cold deformation-intercritical annealing” treatment for medium Mn steels. The process produces a large fraction of retained austenite with an enhanced stability, ...which increases the ductility by ~40% compared to the conventional single-step intercritical annealing in a model 10Mn steel, without sacrificing the ultrahigh strength level.
To elucidate the response of austenite reversion and subsequent bainite transformation behavior to the prior austenite grain (PAG) size, the transformation kinetics and amount of reverted austenite ...in the intercritical region, the subsequent bainite transformation kinetics, and the tensile properties were investigated by dilatometers, scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and the mechanical tests. The results show that, when annealed at 730 °C, fine-grained specimens exhibit faster reverted austenite transformation rate than coarse-grained specimens due to the presence of more PAG boundaries. As the intercritical temperature rises to 760 °C, more intragranular globular austenite forms in the coarse-grained specimen, which accelerates the reverted austenite transformation rate. Compared to fine-grained specimens, coarse-grained specimens exhibit faster bainite transformation and higher amount of bainitic ferrite. The microstructure after austempering consists of intercritical ferrite, bainitic ferrite, RA and fresh martensite. With increasing the austempering time, more bainitic ferrite is formed and the amount of RA and fresh martensite decreases. With increasing the austempering time, the increase in the amount of bainitic ferrite leads to an increase in the yield strength, while the decrease in the amount of RA leads to a decrease in the elongation. Moreover, compared with coarse-grained specimens, the finer microstructure in fine-grained specimens improves yield and tensile strengths, while higher amount of RA contributes to larger uniform and total elongations.
•In fine-grained specimens, more globular austenite is formed along prior austenite grain boundaries.•Fine-grained specimens exhibit finer microstructures such as packets, blocks, and laths.•Fine-grained specimens exhibit higher yield and tensile strengths, and larger uniform and total elongations.
Deformation behavior was studied in cold-rolled 0.2C-1.6Al-6.1Mn-Fe transformation-induced plasticity (TRIP) steel subjected to intercritical annealing. The steel intercritically hardened at 650℃ ...exhibited excellent mechanical properties, and the excellent ductility was primarily associated with the discontinuous TRIP effect. Moreover at 650℃, the formation of Lüders bands was associated with TRIP effect and cooperative dislocation glide. The length of Lüders strain was gradually reduced with increasing pre-strain, and was eventually eliminated when the pre-strain was increased to 10%. The increased average stability of retained austenite and increased dislocation density in ferrite induced by pre-strain was responsible for decrease and ultimate elimination of Lüders bands. While in steel intercritically annealed at 600℃, ferrite and austenite was predominantly deformed, which was responsible for poor work hardening rate and inferior tensile properties.
The factors leading to the room temperature stabilization of austenite were investigated for an ultrafine-grained 6
mass% Mn transformation-induced plasticity steel. The size effect of ultrafine ...austenite grain and the partitioning of Mn to austenite during intercritical annealing were the two main contributions to the austenite stability. Mechanical stabilization of the austenite was not a factor contributing to the austenite stability due to the very low dislocation density of the austenite grains.
•Fine film-like stable retained austenite was obtained in a low alloyed steel.•Stabilization of retained austenite was studied.•Intercritical partition of C, Mn and Ni was revealed by TEM ...study.•Effect of retained austenite on toughness was investigated.•Fracture process of the steel was studied by instrument impact test.
Fine film-like stable retained austenite was obtained in a Fe–0.08C–0.5Si–2.4Mn–0.5Ni in weight percent (wt.%) steel by the two-step intercritical heat treatment. The first step of intercritical annealing creates a mixed microstructure of preliminary alloy-enriched martensite and lean alloyed intercritical ferrite, which is called as “reverted structure” and “un-reverted structure”, respectively. The second step of intercritical tempering is beneficial for producing film-like stable reverted austenite along the reverted structure. The stabilization of retained austenite was studied by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), dilatometry and X-ray diffraction (XRD) analysis. The two-step austenite reverted transformation associated with intercritical partition of C, Mn and Ni is believed to be the underlying basis for stabilization of retained austenite during the two-step intercritical heat treatment. Stable retained austenite is not only beneficial for high ductility, but also for low temperature toughness by restricting brittle fracture. With 10% (volume fraction) of retained austenite in the steel, high low temperature toughness with average Charpy impact energy of 65J at −80°C was obtained.
We elucidate here the impact of grain size and manganese concentration on the austenite stability and the deformation behavior of a cold-rolled transformation-induced plasticity (TRIP) steel with a ...nominal chemical composition of Fe-11Mn-4Al-0.2C (wt.%). Intercritical hardening at 770 degree C led to a ferrite-austenite mixed microstructure, which was characterized by an excellent combination of ultimate tensile strength of 1007MPa and total elongation of 65% and a three-stage work-hardening behavior. The grain size was a critical factor in governing the stability of austenite and the optimal grain size for maximum stability was observed to be 0.6 mu m. The superior mechanical properties are attributed to the discontinuous TRIP effect and the cooperative deformation of ferrite, where the discontinuous effect is a consequence of the non-uniform distribution of manganese, which is responsible for introducing varying degrees of stability in the austenite phase.
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