Austenite stability in TRIP steels studied by synchrotron radiation

Abstract

TRIP steel is a material providing great mechanical properties. Such steels show a good balance between high-strength and ductility, not only as a result of the fine microstructure, but also because of the well-known TRIP effect. The Transformation Induced-Plasticity (TRIP) phenomenon is the transformation of the soft metastable austenite phase to the hard martensite phase due to a mechanical or a thermal stimulus. Already developed in the 1980s, these materials gained new interest since their fabrication has been achieved via a considerable reduction of relevant alloying element concentrations. The mechanical response of such steels, intended to be used in the automotive industry to decrease the gas emission of vehicles, remains difficult to predict. The reason is that the stability of austenite simultaneously depends on a number of intrinsic and extrinsic parameters. As a result, limited in-depth information can be obtained using conventional characterization techniques. Some progress has been made by means of micromechanical models developed to tailor the TRIP effect. These advanced multi-scale models that consider both the macrostructure and microstructure are valuable tools, but have so far been based on limited experimental input for validation and are likely to be not correct. Improved physical characterization methods based on synchrotron radiation make it possible to study the transformation behaviour of the metastable phase in-situ during mechanical or thermal deformation both macroscopically and at the level of individual grains. The aim of this thesis is to quantify the microstructural parameters controlling the mechanical and thermal stability of austenite at an individual grain level using synchrotron X-ray diffraction. This thesis presents the results from experiments probing the transformation behaviour at a macroscopic scale, at a grain level scale and even at a sub-grain scale. In Chapter 3 I report in-situ magnetization and high-energy X-ray diffraction measurements on two aluminum-based TRIP steels during cooling from room temperature down to 100 K in order to evaluate amount and stability of the retained austenite for different heat treatment conditions. I found that the bainitic holding temperature affects the initial fraction of retained austenite at room temperature but does not influence significantly the rate of transformation upon cooling. In Chapter 4 the stability of the retained austenite has been studied in-situ in low-alloyed TRIP steels using high-energy X-ray diffraction during tensile tests at variable temperatures down to 153 K. A detailed powder diffraction analysis has been performed to probe the austenite to martensite transformation by characterizing the evolution of the phase fraction, load partitioning and texture of the constituent phases simultaneously. Our results show that at lower temperatures the mechanically induced austenite transformation is significantly enhanced and extends over a wider deformation range, resulting in a higher elongation at fracture. Low carbon content grains transform first, leading to an apparent increase in average carbon concentration of the remaining austenite. At higher deformation levels the average carbon content saturates while the austenite still continues to transform. In the elastic regime the probed {hkl} planes develop different strains reflecting the elastic anisotropy of the constituent phases. The observed texture evolution indicates that the austenite grains oriented with the {200} along the loading direction are transformed preferentially as they experience the highest resolved shear stress. For increasing degrees of plastic deformation the combined preferential transformation and grain rotation results in the standard deformation texture for austenite with the {111} component along the loading direction. The mechanical stability of retained austenite in TRIP steel is found to be a complex interplay between carbon concentration in the austenite, grain orientation, load partitioning and temperature. In Chapter 5 the microstructure evolution during shear loading of a low-alloyed TRIP steel with different amounts of the metastable austenite phase and its equivalent Dual Phase (DP) grade has been studied by in-situ high-energy X-ray diffraction methods. A detailed powder diffraction analysis has been performed to probe the austenite-to-martensite transformation by characterizing simultaneously the evolution of the austenite phase fraction and its carbon concentration, the load partitioning between the austenite and the ferritic matrix and the texture evolution of the constituent phases. My results show that for shear deformation conditions the TRIP effect extends over a significantly wider deformation range than for simple uniaxial loading. A clear increase in average carbon content during the mechanically-induced transformation proves that austenite grains with a low carbon concentration are least stable during shear loading. The observed texture evolution indicates that under shear loading the orientation dependence of the austenite stability is relatively weak. Earlier work had shown that under a tensile load the {110} component transforms preferentially. The mechanical stability of retained austenite in TRIP steel is found to be a complex interplay between the interstitial carbon concentration in the austenite, the grain orientation and the load partitioning. Chapter 6 focuses on the determination of the local retained austenite-to-martensite transformation behaviour in an inhomogeneous yet carefully controlled shear loaded region of double notched TRIP and DP steel samples. A detailed powder analysis has been performed to simultaneously monitor the evolution of the phase fraction and carbon enrichment of metastable austenite and the local strain components in the constituent phases as a function of the macroscopic stress and location with respect to the shear band. The metastable retained austenite showed a mechanically-induced martensitic transformation in the localized shear zone, which is accompanied by an apparent carbon enrichment in the remaining austenite. At the later deformation stages the geometry of the shear test samples results in the development of an additional tensile component. The experimental strain field within the probed sample area is in good agreement with the predictions from finite-element calculations. The strain development observed in the low-alloyed TRIP steel with metastable austenite is compared to that of steels with the same chemical composition yet containing either no austenite (a DP grade) or a stable (i.e. non-transforming) retained austenite fraction (a TRIP grade produced at a long bainitic holding time). The transformation of metastable austenite under shear load is a complex interplay between the local microstructure and the evolving strain fields. In Chapter 7 the stability of individual metastable austenite grains during tensile loading has been studied in-situ. A new analysis method based on Friedel diffraction pairs has been developed to correlate the macroscopic behaviour of the material to the microstructural parameters of individual grains. The carbon concentration, grain volume and orientation have been determined from single peaks of the diffraction pattern. My results show that these three parameters control the mechanical stability, while the grain volume was found to be the dominant parameter. The orientation-dependent stability of the austenite grains with respect to the tensile axis shows a transformation sequence that is in line with their Schmid factor. It has been observed that for increasing tensile load most austenite grains transform into martensite in one step. In Chapter 8 the martensitic transformation behaviour of the meta-stable austenite phase in low-alloyed TRIP steels during deformation has been studied in more detail. The stability of austenite has been studied at different length scales during tensile tests. A powder diffraction analysis has been performed to correlate the macroscopic behaviour of the material to the observed changes in the volume phase fraction. Moreover, the austenite transformation behaviour has been studied at the length scale of individual grains, where an in-depth characterization of four selected grains has been performed including grain volume, local carbon concentration and grain orientation. For the first time, a high resolution far-field detector was used to study the initial and evolving structure of individual austenite grains during uniaxial tensile deformation of the sample. The sub-grain size in austenite is found not to change significantly during the deformation. The final transformation to martensite occurred in either one or two loading steps.Radiation Science and TechnologyApplied Science