Bainite and stress-induced martensite in an AISI type 300 steel: an X-ray diffraction study of the microstructure by the Rietveld method (original) (raw)
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Materiali in tehnologije, 2016
The performed investigations concerned the influence of the degree and temperature of deformation on the microstructure of metastable austenite in the stainless steel X5CrNi18-10 after its strain-induced martensitic transformation. Samples of steel strip were cold rolled within a degree of deformation from 20 % to 70 % and stretched at a low temperature of-196°C. The microstructure was observed by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM, HREM). It wasen found that after cold rolling with a small degree of deformation (20 %) in the tested steel, generally a single-phase microstructure of the matrix g is found with a high density of dislocations and numerous deformation bands morphologically characteristic of stainless steel with a low stacking-fault energy. After rolling with a 50 % thickness reduction, however, the microstructure displayed deformation twins as well as refined morphologic formations of the phase a', mostly localized in the vicinity of the grain boundaries of the metastable matrix g, and also trace amounts of carbide precipitates. In samples stretched at a temperature of-196°C the microstructure of the matrix displayed a considerable density of dislocations with lath areas of the martensite a' and precipitations of the carbides M23C6. Moreover, the tested steel revealed a crystallographic dependence of the planes and directions on the identified phases g and a', corresponding to dependences of the Kurdjumov-Sachs type, independent of the method and temperature of the plastic deformation. Tests carried out in the TEM proved that the typical sites of nucleation induced by the plastic deformation of martensite are the shear bands, particularly their intersection. The preferred mechanism of transformation, observed in the conditions of cold rolling is, however, a direct transformation of the type g (fcc) ® a' (bcc).
Effect of prior martensite on bainite transformation in nanobainite steel
Nanobainite transformation behavior was comparably studied using in situ neutron diffraction measurements, scanning electron microscopy and electron backscatter diffraction observations for two heat treatments: with and without partial quenching before isothermal holding at 523 or 573 K. Prior martensite transformation was found to accelerate the subsequent nanobainite transformation. Bainitic lathes formed adjacent to a pre-existing martensite plate exhibited an almost identical orientation. Dislocations introduced in austenite due to stress relaxation of transformation strains are believed to assist bainite transformation accompanying variant selection. Diffraction profiles of austenite were found to show symmetric broadening with martensite transformation whereas nonsymmetric broadening occurred with nanobainite transformation, indicating the generation of two populations of austenite. Diffraction line broadening analysis using the convolutional multiple whole profile method provided a dislocation density of 1.51 Â 10 15 m À2 in austenite after partial martensite transformation.
Japanese Journal of Applied Physics, 2000
The present study concerns X-ray characterisation of the microstructures of the martensites of Fe–23Ni–3.6Mn alloy, transformed isothermally at low temperature. Along with the austenized powder and bulk forms of the alloy, coldworked powders have also been analysed. The methodology adopted is Rietveld's whole profile fitting technique which incorporates correction for preferred orientation of the crystallites. The results reveal important information on the crystallite (domain) sizes, residual microstrains, preferred orientation, stacking and compound fault probabilities, dislocation density etc., for both the austenite and martensite phases of the alloy. The martensite has smaller crystallite sizes and larger microstrain values, both of which are isotropic in nature for the transformed bulk and the austenized powders but anisotropic for the coldworked powders. The transformed matrix revealed high percentage of martensites in coldworked powder and bulk whereas annealed powder re...
Micro-tensile Behaviour of Low-alloy Steel with Bainite/martensite Microstructure
ISIJ International, 2016
Micro-tensile testing and numerical analysis using a crystal plasticity finite element method (CPFEM) were employed to elucidate the deformation behaviour of bainite/martensite structures of a low-alloy steel. The bainite single-phase specimens exhibited habit-plane-orientation-dependent yielding, similar to the martensite single-phase specimens. In the bainite/martensite dual-phase specimen, deformation concentrated in the bainite region oriented favourably for inhabit plane slip, leading to low-ductility fracture. With consideration of the habit-plane-orientation-dependent yielding, the present CPFEM analysis successfully reproduced the anisotropic plastic deformation behaviour of the single-phase steels observed in the experiments. The numerical results for the bainite/martensite specimen showed slip localization in the bainite region and stress concentration near the interphase boundary. This suggests that the interphase boundary can be a site for the fracture origin.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 1999
The present study attempts to characterise and compare the microstructures of martensite in powders prepared by hand-filing Fe-23Ni-3.3Mn alloys at room temperature with those of the martensites formed at low temperature in the austenite powder and bulk forms of the alloy. The X-ray diffraction line profiles obtained from the powder and the bulk specimens of the material have been analysed by Rietveld's whole profile fitting method incorporating preferred orientations of the crystallites. The results reveal that the martensite formed in cold-worked powder has higher value of the stacking fault density compared to that of the martensite formed at 160 K in annealed powder of Fe-23Ni-3.3Mn. The martensites have higher value of dislocation density (10 11 m − 2) with smaller crystallite sizes and larger r.m.s. strain values. The bulk martensite has higher hardness value. The volume percentage of martensites formed are 24, 79, 84% for the austenite powder, cold worked powder and bulk sample, respectively.
Atomic-scale investigations of isothermally formed bainite microstructures in 51CrV4 spring steel
Materials Characterization, 2019
Atomic-scale investigation was performed on 51CrV4 steel, isothermally held at different temperatures within the bainitic temperature range. Transmission electron microscopy (TEM) analysis revealed three different morphologies: lower, upper, and inverse bainite. Atom Probe Tomography (APT) analysis of lower bainite revealed cementite particles, which showed no evidence of partitioning of substitutional elements; only carbon partitioned into cementite to the equilibrium value. Carbon in the bainitic ferrite was found to segregate at dislocations and to form Cottrell atmospheres. The concentration of carbon remaining in solution measured by APT was more than expected at the equilibrium. Upper bainite contained cementite as well. Chromium and manganese were found to redistribute at the cementite-austenite interface and the concentration of carbon in the ferritic matrix was found to be lower than the one measured in the case of lower bainite. After isothermal treatments close to the bainite start temperature, another austenite decomposition product was found at locations with high concentration of Mn and Cr, resembling inverse bainite. Site-specific APT analysis of the inverse bainite reveals significant partitioning of manganese and chromium at the carbides and at the ferrite/martensite interfaces, unlike what is found at isothermal transformation products at lower temperatures.
Journal of Alloys and Compounds, 2002
The present study considers X-ray characterization of the microstructures of deformation-induced martensites of Fe-Mn-C alloy powders of grain size |50 mm (hand-filed) having compositions 5.6, 5.8 and 6.0 Mn and 1.0 C (mass%). The cold-worked powders were further subjected to transformation at low temperatures close to M and the evolved phases were again characterized microstructurally. s The methodology applied for characterization involves Rietveld's whole X-ray profile fitting technique adopting the most recently developed software, MAUD (Materials Analysis Using Diffraction) which incorporates Popa model for crystallite (domain) size and microstrain (root mean square, r.m.s.) and preferred orientation of the crystallites. The analysis also considers lattice defect-related features of the microstructure viz. stacking, twin, compound fault probabilities and dislocation density value. The cold-worked powders (hand-filed at room temperature) revealed the highest degree of transformation with 47, 43 and 42% volume fractions of martensites with increasing Mn concentration which for the bulk state of the same alloys transformed at low temperatures are 36, 40 and 47%. The same deformed alloy powders when subjected to low temperature transformation, evolved a maximum of 60, 68 and 62% volume fractions of martensites at 170, 175 and 190 K. The analysis reveals the occurrence of a high propensity of stacking faults in the deformed austenites 22 23 1 2 1 3 22
Acta Materialia, 2013
The bainite transformation behavior after plastic deformation of austenite, i.e., ausforming was studied by in situ neutron diffraction and ex situ experiments, and the effects of ausforming temperature was made clear. Ausforming, at a low temperature (573 K) was found to accelerate bainite transformation and produce a characteristic microstructure, whereas at a high temperature (873 K), ausforming had little influence. The reason for the different results stems from the dislocation structure introduced in austenite; planar dislocations remaining on the active slip planes are believed to assist bainite transformation, accompanied by strong variant selection. The variant selection rule that focuses on Shockley partial dislocation was verified from electron backscatter diffraction results.
To achieve safety and reliability in pipelines installed in seismic and permafrost regions, it is necessary to use linepipe materials with high strength and ductility. The introduction of dual-phase steels, e.g., with a bainite and dispersed martensite–austenite (MA) constituent, would provide the necessary ingredients for the improvement of the strain capacity (as required by a new strain-based linepipe design approach) and toughness. To fine-tune the alloy design and ensure these dual-phase steels have the required mechanical properties, an understanding of the governing deformation micromechanisms is essential. For this purpose, a recently developed joint numerical–experimental approach that involves the integrated use of microscopic digital image correlation analysis, electron backscatter diffraction, and multiphysics crystal plasticity simulations with a spectral solver was employed in this study. The local strain and stress evolution and microstructure maps of representative microstructural patches were captured with a high spatial resolution using this approach. A comparison of these maps provides new insights into the deformation mechanism in dual-phase microstructures, especially regarding the influence of the bainite and MA grain size and the MA distribution on the strain localization behavior.
Estimation of dislocation density in bainitic microstructures using high-resolution dilatometry
Scripta Materialia, 2009
It is possible by means of high resolution dilatometry together with a model based on isotropic dilatation and atomic volumes, to estimate the dislocation density introduced in the microstructure as a consequence of the isothermal decomposition of austenite into bainitic ferrite. The relatively high dislocation density associated with this microstructure is attributed to the fact that the shape deformation accompanying this displacive transformation is accommodated by plastic relaxation.