Transformation Induced Plasticity Steels - TRIP steels
The microstructure of Transformation-Induced Plasticity (TRIP) steels contains retained austenite embedded in a matrix of ferrite, bainite or martensite depending on alloy composition and preceding processing.
Also mixtures of these matrix compunds are frequently used.
TRIP steels are typically hypoeutectoid iron carbon alloys often with 0.1 – 0.4 mass% carbon.
The metastable austenite is typcially in the form of retained and partitioned austenite but reversed auistenite can be used too. Typically TRIP steels contain 5-10 vol.% of metastable retained austenite or sometimes more.
Upon mechanical loading TRIP steels first undergo conventional dislocation plasticity of the soft ferrite portions of the microstructures and with further increase in deformation the retained austenite fraction progressively
transforms into martensite, thereby increasing the work hardening rate at higher strain levels.
Acta Materialia 111 (2016) 262-272
Acta Materialia 111 (2016) 262 martensit[...]
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Introduction of interlath reverted austenite is an effective method to design ductile lath martensitic steels. The challenge in this concept is that all reverted austenite films have similar
mechanical stability, hence, they all undergo transformation-induced plasticity (TRIP) at the same strain level. Here we
propose a new thermo-mechanical treatment route to activate the TRIP effect over a broad strain regime and refer to it as ‘spectral TRIP effect’. It aims at spreading the micro-mechanical stability of reverted austenite grains by widening the austenite nucleation barrier in martensite. To validate the proposed
thermo-mechanical treatment route, an as-quenched medium-Mn martensitic steel was cold rolled prior to the reversion treatment at 600 C. Microstructure characterization was carried out by electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). Mechanical tests show that the approach is effective. The spectral TRIP effect improves both, the strength and the ductility due to the well dispersed size distribution and the associated size-dependent deformation and phase transformation behavior of the reverted austenite grains, extending TRIP-related work hardening over a broad strain range.
Acta Materialia 113 (2016) 19-31
Belde Acta Materialia vol 113 (2016) pag[...]
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The mechanical performance of multi-phase steel microstructures critically depends on the constituents’ chemical and morphological constitutions, which in combination determine the composite
hardness, the onset of plasticity, internal load and strain-partitioning, as well as the stability and transformation kinetics
of retained austenite in case of TRIP steels. The novel approach of utilising temporary vessel phases, hence termed vessel microstructure design, enables the tuning of constituent phase properties by linking their formation to a controllable landscape of chemical gradients. This approach hinges on the
introduction of alloy carbides as a temporary container, or ‘vessel’ phase, deliberately producing localised enrichment of alloying elements in a structure predetermined by preliminary heat treatments, referred to as conditioning and accumulation stages. These vessel carbides, which act as reservoirs for specific
alloying elements, are then partially dissolved through flash heating, leading to a self-organising landscape of alloying elements in the vicinity of the dissolving particles. The resulting three- or multiple phase microstructures then consist of confined laminates incorporating retained carbides, enveloped by retained austenite shells, embedded within a martensitic matrix. Such complex yet entirely self-organized microstructures offer unique opportunities for strain and load partitioning which we refer to as core-shell micromechanics. Different variants of these core-shell composite structures are produced and examined together with reference microstructures by tensile testing, hardness mappings, impact toughness, X-ray measurements, as well as by electron microscopy. It is found that these novel microstructures, when tempered, exhibit ultra-high strength and delayed necking, enabled by a combination of gradual strain-hardening and transformation-induced plasticity that is tuneable via control of the
initial carbide structure.
ISIJ International, Vol. 49 (2009), No. 10, pp. 1601–1609
ISIJ International 49 (2009) 1601.pdf
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The crystallographic orientation relationships that are active during the transformation of austenite to bainite are studied for two TRIP steels by means of Electron BackScatter Diffraction
(EBSD). A detailed evaluation of about 360 retained austenite grains and their BCC neighbours was performed. Three relationships were considered, namely Kurdjumov–Sachs,
Nishiyama–Wassermann and Pitsch. It was found that the majority of the austenite grains had at least one neighbour that could be related with one of the three orientation
relationships. The Kurdjumov–Sachs relationship appeared to be dominant and no strong indication for variant selection could be retrieved from the studied data. It was, however, also demonstrated that some precautions need to be made since a clear distinction between the evaluation of a small region of the microstructure and conclusions made for the complete material is necessary.
Acta Materialia 85 (2015) 216-228
Acta Materialia 85 (2015) 216 Wang et al[...]
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Conventional martensitic steels have limited ductility due to insufficient microstructural strain-hardening and damage resistance mechanisms.
It was recently demonstrated that the ductility and toughness of martensitic steels can be improved without sacrificing the strength, via partial reversion of the martensite back to austenite. These improvements were attributed to the presence of the transformation-induced plasticity (TRIP) effect of the austenite phase, and the precipitation hardening (maraging) effect in the martensitic matrix. However, a full micromechanical understanding of this ductilizing effect requires a systematic investigation of the interplay between the two phases, with regards to the underlying deformation and damage micromechanisms. For this purpose, in this work, a Fe–9Mn–3Ni–1.4Al–0.01C (mass%) medium-Mn TRIP maraging steel is produced and heat-treated under different reversion conditions to introduce well-controlled variations in the austenite–martensite nanolaminate
microstructure. Uniaxial tension and impact tests are carried out and the microstructure is characterized using scanning and transmission electron
microscopy based techniques and post mortem synchrotron X-ray diffraction analysis. The results reveal that (i) the strain partitioning between austenite
and martensite is governed by a highly dynamical interplay of dislocation slip, deformation-induced phase transformation (i.e. causing the TRIP effect) and mechanical twinning (i.e. causing the twinning-induced plasticity effect); and (ii) the nanolaminate microstructure morphology leads to enhanced damage resistance. The presence of both effects results in enhanced strain-hardening capacity and damage resistance, and hence the enhanced ductility.
Acta Materialia 79 (2014) 268-281
Wang et al Acta Materialia 79 (2014) 268[...]
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Steels containing reverted nanoscale austenite islands or films dispersed in a martensitic matrix show excellent strength, ductility and toughness. The underlying microstructural mechanisms
responsible for these improvements are not yet understood, but are observed to be strongly connected to the island or film size. Two main micromechanical effects are conceivable in this context,
namely: (i) interaction of gamma with microcracks from the matrix (crack blunting or arresting); and (ii) deformation-induced phase transformation of gamma to martensite (TRIP effect).
The focus here is on the latter phenomenon. To investigate size effects on gamma transformation independent of other factors that can influence austenite stability (composition,
crystallographic orientation, defect density, surrounding phase, etc.), a model (TRIP-maraging steel) microstructure is designed with support from diffusion simulations (using
DICTRA software) to have the same, homogeneous chemical composition in all gamma grains. Characterization is conducted by in-situ tension and bending experiments in conjunction with
high-resolution electron backscatter diffraction mapping and scanning electron microscopy imaging, as well as post-mortem transmission electron microscopy and synchrotron X-ray diffraction
analysis. Results reveal an unexpected “smaller is less stable” effect due to the size-dependent competition between mechanical twinning and deformation-induced
Scripta Materialia 60 (2009) 1141–1144
Scripta Materialia 60 (2009) 1141 maragi[...]
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We present mechanical and microstructure results on precipitation-hardened ductile high-strength martensitic and austenitic–martensitic steels (up to 1.5 GPa strength) with good ductility. The alloys have a low-carbon content (0.01 wt.% C), 9–12 wt.% Mn, and minor additions of Ni, Ti and Mo (1–2 wt.%). Hardening is based on transformation-induced plasticity and the formation of intermetallic nanoprecipitates in the martensite during heat treatment (aging). The approach leads to an unexpected simultaneous increase in both strength and total elongation.