Influence of the ageing conditions and the initial microstructure on the precipitation of α phase in Ti-17 alloy

The precipitation of α phase during ageing was investigated in the near-β titanium alloy Ti-17 considering either a fully βmetastable initial microstructure or a 35% αprimary + 65% βmetastable initial microstructure. In-situ electrical resistivity and high energy X-ray diffraction measurements revealed the influence of the initial microstructure, with different α morphologies (size and distribution of αprimary), as well as the heating rate on the precipitation sequences and kinetics following the decomposition of the β-metastable phase. Various amounts of metastable phases (ωisothermal and α″isothermal) precipitate in temperature ranges that increase with the heating rate. From temperatures about 500 °C, the orthorhombic α″isothermal structure evolved towards the hexagonal close-packed α as temperature increased. SEM microstructure characterisations showed that slow heating rates promoted a fine and dense α precipitate distribution through the formation of ωisothermal and/or α″isothermal, leading to higher hardness values. A higher heating rate restricted the precipitation of α″isothermal and shifted to the one of α at a higher temperature, leading to coarser precipitates. Furthermore, precipitation kinetics of α″isothermal/α were quicker considering an initial intragranular α precipitation as compared to α colonies

On the complementarity between resistivity measurement and ultrasonic measurement for in-situ characterization of phase transitions in Ti-alloys

We present the results of in-situ characterization of the phase transitions in metastable β-Ti alloy Ti5553 by contactless laser-based resonant ultrasound spectroscopy method and electrical resistance measurement in a four probe configuration. Phase transformations were studied during continuous heating from the room temperature to 700 °C with various heating rates. We showed that both methods provide complementary results and can be successfully used for observation of phase transitions in metastable β-Ti alloys

Carbon content evolution in austenite during austenitization studied by in situ synchrotron X-ray diffraction of a hypoeutectoid steel

Using in situ high energy X-ray diffraction study of austenite formation in hypoeutectoid steel with three differ- ent initial microstructures (ferrite-pearlite, tempered martensite and bainite), the lattice parameters of ferrite, cementite and austenite are examined on heating at 0.25, 10 and 100 °C/s. The lattice parameters of ferrite, cementite and austenite do not vary linearly with the temperature, especially, in the temperature range where the austenitization takes place. For the austenite, it is suggested that the deviation from the linearity is mainly associated to the carbon content variation. Using Dyson and Holmes equation, the carbon content in austenite is evaluated for any moment of the austenite formation for each initial microstructure and all heating rates. For the ferrite-pearlite microstructure heated at 0.25 °C/s, the carbon content in austenite after complete cementite dissolution corresponds to that of pearlite. Moreover, a rapid decrease in carbon content in the austenite is observed during the first stage of the austenitization (simultaneous dissolution of ferrite and cementite) followed by a slow further decrease during the transformation of the remaining ferrite. The obtained results are discussed using thermodynamic calculations

In Situ Stress Tensor Determination during Phase Transformation of a Metal Matrix Composite by High-Energy X-ray Diffraction

In situ high-energy X-ray diffraction using a synchrotron source performed on a steel metal matrix composite reinforced by TiC allows the evolutions of internal stresses during cooling to be followed thanks to the development of a new original experimental device (a transportable radiation furnace with controlled rotation of the specimen). Using the device on a high-energy beamline during in situ thermal treatment, we were able to extract the evolution of the stress tensor components in all phases: austenite, TiC, and even during the martensitic phase transformation of the matrix

Ferrite precipitation in quaternary Fe–C–X1–X2 systems using high-throughput approaches

This study investigates the effect of composition on ferrite growth kinetics in quaternary Fe–C–X1–X2 systems (X: Ni, Cr, Mo) using a high-throughput methodology. This study provides the largest dataset to date on ferrite growth kinetics in multi-component steels, offering novel insight into the behavior of these complex systems. To this end, high-energy X-ray diffraction is utilized to gather kinetic data in situ along composition gradients, leading to the measurement of phase transformation kinetics maps in compositional space. The obtained data is compared to predictions from various models describing ferrite growth kinetics in low-alloy steels. The modified "three-jump" solute drag model is shown to describe best the ferrite growth kinetics in these quaternary systems, without the need for additional calibration or fitting parameters. The success of this model is attributed to its consideration of individual solute interactions with the interface and inter-elemental interactions. The findings of this study provide valuable insight for robust modeling of phase transformations and microstructural evolution in multi-component steels, a critical tool in accelerating alloy optimization and in enhancing process control

Study of harmonic microstructure development during Spark Plasma Sintering (SPS) of β-CEZ titanium alloy

The present study focuses on the formation of harmonic microstructures in a metastable β titanium alloy, the β-Cez alloy (Tβ =890°C). Previous studies showed that harmonic structures obtained by a powder metallurgy route led to an increase in mechanical properties. In this study, the harmonic structure was obtained after a Mechanical Milling of the Initial Powder followed by Spark Plasma Sintering. The phase transformations occurring in the Initial Powder and Mechanical Milled powders during a heat treatment similar to the SPS one were studied. Electrical resistivity, high energy XRD and SEM-EBSD were used to characterize the evolution of phases and microstructures and highlight the effect of the thermal treatment and the milling. It was shown that after thermal treatment of Mechanical Milled powders, a harmonic α + β microstructure is obtained consisting of nodular α grains in the powder shell and α lamellae in the powder core. The stress/strain induced martensite formed during the milling associated with the heavier deformation at the powder surface areas contributes highly to the formation of network arrangement of nodular α grains by a recovery/recrystallization phenomenon of β and α phases during the heating

Internal stresses in Metal Matrix Composites in relation with matrix phase transformations

Abstract. For metal matrix composites, internal stresses are a key factor for understanding the interactions between matrix and reinforcements and the mechanical properties of the composite. From in situ high energy X-ray diffraction on a steel matrix composite reinforced with TiC, the evolutions of the phase fractions and mean cell parameters of each phase during thermal treatment have been determined. In addition, a methodology is developed in order to get more information on the stress state evolutions in each phase during the treatment

Study of harmonic microstructure development during Spark Plasma Sintering (SPS) of β-CEZ titanium alloy

The present study focuses on the formation of harmonic microstructures in a metastable β titanium alloy, the β-Cez alloy (Tβ =890°C). Previous studies showed that harmonic structures obtained by a powder metallurgy route led to an increase in mechanical properties. In this study, the harmonic structure was obtained after a Mechanical Milling of the Initial Powder followed by Spark Plasma Sintering. The phase transformations occurring in the Initial Powder and Mechanical Milled powders during a heat treatment similar to the SPS one were studied. Electrical resistivity, high energy XRD and SEM-EBSD were used to characterize the evolution of phases and microstructures and highlight the effect of the thermal treatment and the milling. It was shown that after thermal treatment of Mechanical Milled powders, a harmonic α + β microstructure is obtained consisting of nodular α grains in the powder shell and α lamellae in the powder core. The stress/strain induced martensite formed during the milling associated with the heavier deformation at the powder surface areas contributes highly to the formation of network arrangement of nodular α grains by a recovery/recrystallization phenomenon of β and α phases during the heating

Nanostructured bainitic steels with carbide precipitations - a new steel class

European Congress and Exhibition on Avanced Materials and ProcessRecent work [1] on the tempering behaviour of nanostructured bainite has demonstrated the particularly interesting tempering resistance of this microstructure. Tempering of high Si nanostructured bainite at an adequate temperature will not destroy it´s unique nanostructure on the contrary supersaturated and metastable phase will tend to equilibrium und therefore reduce internal stresses. Taking these in to account tempering offers a new perspective in the ability to tailor the mechanical properties. The present work is about combining the increasingly well-known performances of nanostructured bainitic steels with a secondary precipitation mechanism. These are based on a previously investigated material (0.65C-1.25Mn-1.5Si-0.75Cr wt%) modified with additions of Mo and V. Mo and V addition does not influence nanostructured bainite formation but changes secondary hardening behavior strongly. Mechanical behavior esp. at elevated temperatures is enhanced due to addition of Mo and V. Steel and process design as well as mechanical properties up to 250°C of 5 different nanostructured bainitic steels grade with different Mo and V addition will be discussed. Results will be compared to conditions without secondary carbide formers and martensitic conditions. Conclusions: We will present the main achievements so far obtained under the auspices of a Research Fund for Coal and Steel (RFCS) project, STEELSECO

Final Technical. Report Part B

Project Number: RFCS-RPJ-2016-754070.The aim of this project is to investigate the microstructure evolution due to tempering of nanobainitic steels and their properties including fatigue properties at elevated service temperatures up to 250°C. Furthermore this project will be the first attempt to evaluate the potential of a new class of steel combining nanostructured bainitic steels with secondary precipitation. This entirely novel approach may indeed lead to a material that combines the excellent and increasingly well-known performances of nanostructured bainitic materials, with an improved potential for use at elevated temperatures. Intrinsic tempering resistance of nanostructured bainitic microstructure will be enhanced by adding secondary precipitating elements to create a new class of steel material with expected exceptional properties for service at moderate temperatures. Indeed, it is hoped that this yet untested combination will lead to an economical yet very high performance material for use at elevated temperatures. In comparison with nanostructured bainitic microstructure also steels with conventional QT matrix will be investigated.Research Fund for Coal and Steel under grant agreement No 754070.Peer reviewe