Enlarging the palette of mechanical properties of TI64 by a quenching and partitioning approach

The usual bimodal microstructure of wrought TI64 is known to provide an excellent yield strength to density ratio. However, its poor work-hardening capacities brings about limited ductility. Moreover, the post-uniform elongation is also very limited. In the present work, we report the beneficial influence of dual-phase microstructures obtained by quenching from the alpha+beta phase field. Under certain circumstances, such dual-phase microstructure exhibit martensite reorientation induced plasticity, that was never reported in this well-known industrial alloy. The dual-phase microstructures are further annealed allowing the partitioning of V from the supersaturated martensite. Depending on the initial dual-phase microstructure and the parameters of the annealing treatments, a very large palette of mechanical properties is obtained. The strength levels, work-hardening behaviors, ductilities and post-uniform elongations are discussed

Crystallographic reconstruction study of the effects of finish rolling temperature on the variant selection during bainite transformation in C-Mn high-strength steels

The effect of finish rolling temperature (FRT) on the austenite- () to-bainite () phase transformation is quantitatively investigated in high-strength C-Mn steels. In particular, the present study aims to clarify the respective contributions of the conditioning during the hot rolling and the variant selection (VS) during the phase transformation to the inherited texture. To this end, an alternative crystallographic reconstruction procedure, which can be directly applied to experimental electron backscatter diffraction (EBSD) mappings, is developed by combining the best features of the existing models: the orientation relationship (OR) refinement, the local pixel-by-pixel analysis and the nuclei identification and spreading strategy. The applicability of this method is demonstrated on both quenching and partitioning (Q&P) and as-quenched lath-martensite steels. The results obtained on the C-Mn steels confirm that the sample finish rolled at the lowest temperature (829{\deg}C) exhibits the sharpest transformation texture. It is shown that this sharp texture is exclusively due to a strong VS from parent brass {110}, S {213} and Goss {110} grains, whereas the VS from the copper {112} grains is insensitive to the FRT. In addition, a statistical VS analysis proves that the habit planes of the selected variants do not systematically correspond to the predicted active slip planes using the Taylor model. In contrast, a correlation between the Bain group to which the selected variants belong and the FRT is clearly revealed, regardless of the parent orientation. These results are discussed in terms of polygranular accommodation mechanisms, especially in view of the observed development in the hot-rolled samples of high-angle grain boundaries with misorientation axes between and

The formation of plate martensite in a Fe-High Ni alloy: Crystallography and Variant Selection

Mainly two different morphologies of martensite can be obtained in steels depending on the amount of alloying elements. The first morphology, referred to as lath martensite, forms in low alloy, low carbon steels. It is, by far, the most extensively studied form of martensite due to its industrial applications. The second morphology of martensite, referred to as plate martensite, forms in highly alloyed and in high carbon steels and in particular in Fe-High Ni alloys. In this case, the transformation product is disc shaped and internally twinned. This morphology is the only form of martensite that has the potential to exhibit shape memory properties. It is therefore of great interest to understand the mechanisms of its formation. This is investigated in the present dissertation through the study of the martensitic transformation occurring in a Fe-30.5%Ni-0.155%C alloy. More precisely, the influence of stress and grain size on the crystallography of plate martensite is discussed in the general framework of the phenomenological theory of martensite crystallography. This theory allows associating a unique shape deformation to each orientational variant. In this way, the experimental observations carried out at different length scales by means of optical microscopy, EBSD and TEM can be used to infer the transformation path followed under different conditions. Firstly, the burst configurations of variants observed in coarse-grained austenite under stress free conditions are rationalized by considering the mechanical couplings between the variants. It is shown that self-accommodating and autocatalytic couplings are responsible for the formation of hierarchical configurations of variants. More precisely, the transformation is shown to occur through the alternate formation of perpendicular plate groups of variants. Self-accommodation is the dominant coupling between variants of the same plate group while autocatalytic couplings are responsible for the transfer of the transformation from one generation to the next. It is suggested that the plastic accommodation of the shape deformation plays a dominant role in propagating the transformation to a lower length scales. Secondly, the influence of a uniaxial stress state on the transformation is studied. It is seen experimentally that only the most favoured variants are systematically formed in coarse Cube grains while coarse non-Cube grains generally transform into plate groups of variants that are only moderately favoured by the stress. These observations are well explained by considering the interaction energy between the applied stress and the shape deformation associated with the transformation. Thirdly, the influence of the austenitic grain size on the transformation is also studied. A decrease in grain size is seen to decrease the martensite start temperature. For a grain size below about 10µm, the thermal transformation in liquid nitrogen is indeed suppressed in the present alloy. This observation is related to the increasing yield strength of austenite as the grain size is reduced. A noticeable change in the morphology of martensite also accompanies the decrease in grain size. Indeed, martensite forming in coarse-grained austenite is mostly lens shaped and partially twinned while it appears plate shaped and fully twinned in smaller grains. Furthermore, martensite forming in fine-grained austenite develops self-accommodating configurations suggesting that most of the transformation deformations are elastically accommodated in this case. This is believed to be related to the observance of a shape memory effect in the present alloy in its fine-grained condition.Doctorat en Sciences de l'ingénieur et technologieinfo:eu-repo/semantics/nonPublishe

The zig-zag pattern revisited in Fe-31Ni-0.155C

SCOPUS: bk.cinfo:eu-repo/semantics/publishe

On the relation between orientation relationships predicted by the phenomenological theory and internal twins in plate martensite

The phenomenological theory of martensite crystallography predicts two equivalent solutions for a particular habit plane in the case of a Fe–Ni–C alloy. Those two solutions differ in the magnitude of the inhomogeneous shear and in the orientation relationship (OR) they hold with austenite. Only the OR associated to the low shear solution has been observed experimentally so far. In the present study, the orientation relationship associated to the high shear solution is assessed experimentally using TEM measurements.info:eu-repo/semantics/publishe

On the relation between orientation relationships predicted by the phenomenological theory and internal twins in plate martensite

The phenomenological theory of martensite crystallography predicts two equivalent solutions for a particular habit plane in the case of a Fe-Ni-C alloy. Those two solutions differ in the magnitude of the inhomogeneous shear and in the orientation relationship (OR) they hold with austenite. Only the OR associated to the low shear solution has been observed experimentally so far. In the present study, the orientation relationship associated to the high shear solution is assessed experimentally using TEM measurements.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Microstructural evolution during spheroidization annealing of eutectoid steel :effect of interlamellar spacing and cold working

Eutectoid steels present a wide range of interesting mechanical properties (high strength, wear resistance, ductility and toughness) and could be a cheaper alternative to high strength low-alloyed steels (HSLA) in applications where weldability is not a critical requirement. The mechanical properties of pearlite are mainly dictated by the interlamellar spacing and the spheroidization of cementite. In this work, the spheroidization kinetics during annealing of a fully pearlitic steel produced in an electric arc furnace (EAF) is investigated. More specifically, the influence of a prior cold deformation and of the interlamellar spacing is studied using image analysis and hardness tests. It is shown that spheroidization is faster in fine pearlite than in coarse pearlite. Prior cold deformation strongly accelerates the spheroidization kinetics in fine and coarse pearlite. The tensile properties corresponding to different pearlite microstructure were measured and are compared to the hardness evolution during annealing. © (2010) Trans Tech Publications.info:eu-repo/semantics/publishe

Chemical, morphological and structural characterisation of electroless duplex NiP/NiB coatings on steel

Duplex electroless nickel coatings constituted of one layer of nickel-phosphorous and one of nickel-boron are a promising solution to provide combined wear and corrosion resistance to parts. Duplex systems were compared to systems of similar thickness constituted of only one material, in one or two layers. Duplex coatings present intermediate surface texture, but each layer keeps its typical cross-section morphology and structural features, even after heat treatment. The interfaces between the separate layers are sharp in the as-deposited state but not as much after heat treatment. When nickel-boron is deposited first, it influences slightly the grain growth of the subsequent nickel-phosphorous layer, but no influence can be observed when nickel-phosphorous is deposited first.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Quantitative analysis of variant selection and orientation relationships during the γ-to-α phase transformation in hot-rolled TRIP steels

SCOPUS: cp.kinfo:eu-repo/semantics/publishe