Depending on the alloy composition, intercritical annealing may provide different phases in the microstructure. For low-alloyed dual-phase (DP) steels it is usually ferrite and martensite, while for ...medium-Mn steels retained austenite is also formed. In a present study, a wide intercritical temperature range was applied to a 5% Mn steel to investigate possible microstructure combinations: ranging from fully ferritic, through ferritic-austenitic, multiphase, to fully martensitic, which were next investigated in terms of mechanical properties to clarify the behavior of this type of material. The obtained results together with technological issues and economic indicators were next compared to mechanical properties of typical DP steels in order to assess the possibility of replacing this material in car production. The mechanical properties were evaluated using static tensile and hardness tests. The phase composition was determined qualitatively and quantitatively using dilatometry, X-ray diffraction measurements, and electron backscatter diffraction analysis. The results suggest that both initial austenite and martensite fractions have a decisive influence on the yielding and elongation of steel; however, the tensile strength depends mainly on the sum of martensite initially present in the microstructure and the strain-induced martensite formed from the plastically deformed austenite regardless of the initial retained austenite-martensite ratio. The results indicate superior total elongation of medium-Mn steels reaching 30% compared to DP steels with a similar strength level in the range between 900 and 1400 MPa. However, medium-Mn steels could be a significant competitor to dual phase steels only if some technological problems like discontinuous yielding and serrations are significantly reduced.
The strength and ductility of two tungsten products, developed for application in nuclear fusion environment, are studied before and after neutron irradiation using uniaxial tensile tests. The first ...product is a commercially pure tungsten, produced by AT&M company according to ITER specification, and the second one is reinforced with zirconium carbide (W–ZrC) particles. The addition of ZrC particles leads to a reduction of the ductile to brittle transition temperature (DBTT) in non-irradiated conditions down to 50–100 °C without loss of strength and of other attractive properties of tungsten.
The neutron irradiation was performed in the range 625–700 °C up to 1.125 dpa. The tests were performed to screen the shift of the DBTT as well as to characterize the evolution of the strength and ductility at the irradiation temperature. In addition, a series of interrupted tensile tests were performed in order to determine the variation of the yield strength as a function of temperature using an original single specimen test method.
The neutron irradiation causes the reduction of the total elongation of both tungsten products. The DBTT range, which was evaluated from the tensile test results, of W–ZrC lies in the 300–500 °C range (while it is ∼100 °C in non-irradiated state). The DBTT range of pure tungsten is between 400 and 575 °C i.e. higher than that of W–ZrC. The irradiation hardening, measured at ∼600 °C, which is close to the irradiation temperature, leads to an increase of the proof stress by a factor of two in both studied grades. Despite that the irradiation induced hardening, both products retain a total elongation of about 10% prior to fracture. W–ZrC exhibits a similar total elongation at 500 °C, thus maintaining a significant ductility resource, while pure W becomes brittle at 500 °C and below.
The evaluation of the fracture toughness of tungsten is required for the design of plasma-facing components in order to ensure safe and durable operation in ITER reactor, being under construction in ...France. During operation, plasma facing materials will be exposed to cyclic thermo-mechanical loads combined with high energy neutron flux, which, in general, reduce the fracture toughness. Characterization of the degradation of the mechanical properties after exposure to the neutron flux involves time consuming and expensive procedures due to nuclear activation and special handling. Subsequently, development of sample miniaturization and protocols to reduce the volume of material under inspection is critical to speed up the progress in R&D. In this work, we propose a combined approach for the reconstruction of the fracture toughness – temperature curve, which is applied in the ductile to brittle transition temperature range. The approach consists of two steps: (i) application of the three point bending tests using miniaturized samples to reveal the transition temperature range on the basis of flexural strain data; (ii) execution of standardized fracture toughness tests at the upper temperature of the transition regime. The results allow the determination of the fracture toughness as a function of temperature with a reasonable accuracy. The validity of the approach has been demonstrated on two commercial tungsten grades produced according to ITER specification and tested in the as-fabricated state. The conclusions are supported by microstructural analysis performed on both standardized and miniaturized samples.
A 0.3C-1.6Si-3.5Mn(wt%) steel was subjected to different Q&P treatments, leading to different combinations of initial martensite, bainite, secondary martensite, and retained austenite. In this study, ...initial martensite refers to the martensite formed during the initial quenching step and then subjected to an isothermal treatment at 400°C; secondary martensite refers to martensite formed during quenching from 400°C to room temperature. The yield strength of each constituent phase was determined by applying physical models to the data obtained from detailed microstructural characterization. The yield strength (uncertainty of 5%) of the Q&P microstructures was calculated by using a composite law to account for the contribution of each constituent phase. The dependence of the yield strength on the microstructural features of the Q&P microstructures was revealed by using the approach developed in this work. For example, initial martensite (which has a high yield strength and is the dominant phase in the microstructures) had the greatest effect on the yield strength of the Q&P microstructures. Furthermore, the phase fraction and dislocation density of this phase increased with decreasing quenching temperature, leading to an increase in the yield strength of the material.
Restrictions on fuel consumption and safety in the automotive industry have stimulated the development of quenching and partitioning (Q&P) steel. This steel is expected to have very high strength in ...combination with acceptable ductility owing to its microstructure consisting of martensite with a considerable amount of retained austenite. The effect of retained austenite on the mechanical properties and its transformation stability were determined by stepwise uniaxial micro-tensile testing and subsequent electron backscatter diffraction (EBSD) study of a pre-selected region. The austenite fraction evolution with increasing plastic deformation and the influence of fresh martensite on the local strain distribution were quantified based on the orientation data. The decrease of the retained austenite as a function of the applied strain was described by an exponential function with the pre-exponential and exponential factors related to the starting austenite fraction and its transformation stability respectively. It was proven that the presence of fresh martensite has a negative influence on this austenite transformation stability due to its constraining effect on the strain distribution. This effects the mechanical properties manifested by changes in the strain hardening behavior and total elongation. The results suggest that the ductility of the Q&P steels can be improved by an appropriate design of the heat treatment schedule in order to ensure high retained austenite fractions without the presence of fresh martensite in the final microstructure.
In present study, the potential of Niobium (Nb) to improve the mechanical properties of Quenched and Partitioning (Q&P) steels is investigated. To this purpose, the effect of the addition of Nb on ...the microstructure and mechanical properties of a Q&P steel was thoroughly analysed with special attention on the thermal stability of the retained austenite. The mechanical properties of the Nb Q&P steel were determined using static and dynamic tensile tests and compared to the properties of a Nb free Q&P steel. Additionally, the effect of test temperature was quantified by static and dynamic tests in the temperature range from −40 °C to +80 °C. It was found that the Nb induced grain refinement together with the stabilising effect of Nb on the retained austenite, resulted in an excellent low temperature behaviour of the Nb Q&P steel. Additionally, the high temperature, dynamic behaviour of the Nb micro-alloyed steel was improved with respect to the Nb free steel due to the presence of a large fraction of retained austenite with low thermal stability.
A large campaign of characterization of the ductile to brittle transition temperature (DBTT) and microstructure has been performed on several commercial and lab-scale pure tungsten grades, potassium ...doped tungsten alloys, and particle reinforced tungsten grades (with particles of TiC, Y2O3, or ZrC), all integrated in a large-scale neutron irradiation campaign. The DBTT is deduced based on miniaturized three-point bending tests to provide reference data for the assessment of the irradiation effects on the tungsten alloys. This miniaturized geometry is designed to minimize the operational cost of neutron irradiation, to speed up post-irradiation examination, and to reduce the amount of nuclear waste. The resulting DBTT ranges from around −15 up to 450 °C, depending on the material. The potassium doped tungsten alloys have the lowest DBTT, followed by rolled ZrC reinforced tungsten grade, commercial pure tungsten grades, lab-scale pure tungsten grades, and other particle reinforced tungsten grades. The crack plane orientation and microstructure with respect to grain shape and grain boundaries significantly affect the DBTT for forged/rolled tungsten products with elongated grains. The L-T orientation has a lower DBTT compared to the T-L orientation. Moreover, the DBTT difference in the L-T and T-L orientation raises with increasing the grain aspect ratio. An attempt is made to establish a relationship between the density of low and high angle grain boundaries and DBTT value. The obtained relationship is discussed in the frame of mechanical processing (i.e., rolling or forging) to optimize the DBTT by optimized manufacturing. The results are compared to recent computational predictions of the DBTT in tungsten.
•Miniaturized three-point bending testing method to determine the DBTT for neutron irradiated samples is developed.•The DBTT for L-T and T-L orientation in tungsten depends on the grain aspect ratio.•Low-angle grain boundary density has a strong positive effect on the reduction of the DBTT.•Particle reinforcement does not have a strong effect on the DBTT reduction.
The effect of the microstructural characteristics of retained austenite on its transformation stability in steel after Quenching and Partitioning (Q&P) was studied via interrupted tensile tests and ...Electron Backscatter Diffraction measurements on a pre-determined zone of a micro-tensile test sample. The evolution of the retained austenite fraction was obtained as a function of the plastic strain. The dependence of the austenite transformation stability on the corresponding grain size, morphology, and local crystallographic orientation was discussed. Furthermore, the importance of the parameters on the austenite stability was analysed and it was shown that the austenite grains rotated, in addition to being transformed, constituting therefore an additional contribution to the ductility of Q&P steel.
Laser powder bed fusion (L-PBF), categorized as additive manufacturing technique, has a capability to fabricate NiTi (Nitinol) shape memory alloys with tailorable functional properties and complex ...geometries. An important processing parameter, hatch distance (h), is often related to macroscale structural defects; however, its role on controlling the microstructure and functional properties is usually underestimated in L-PBF of NiTi. In this work, equiatomic NiTi (50.0 at% Ni) parts were fabricated with various hatch distances to tailor the microstructure and their shape memory characteristics. Contrary to what is observed in Ni-rich NiTi alloys, in this work, we demonstrate that phase transformation temperatures of L-PBF equiatomic NiTi do not decrease proportionally with hatch distance but rather relate to a critical hatch distance value. This critical value (120 μm) is derived from the synergistic effect of thermal stress and in situ reheating. Below this value, epitaxial grain growth and in situ recrystallization are enhanced, while above, irregular grains are formed and dislocations induced by thermal stresses decrease. However, the critical value found herein is characterized by high dislocation density and fine grain size, resulting in a superior thermal cyclic stability. The proposed finite element model is proven to be an effective tool to understand and predict the effect of hatch distance on grain morphology and dislocation density evolutions in L-PBF NiTi SMAs. In the present study, we provide a comprehensive understanding for in situ controlling L-PBF NiTi microstructure and functional characteristics, which contributes to designing 4-dimensional shape memory alloys.
This research presents a coupled thermomechanical modelling procedure for the wheel-rail contact problem and computes the flash-temperature and stress-strain responses when thermal effects are ...present. A three-dimensional elasto-plastic finite element model was built considering the wheel-track interaction. When the wheel is running on rail, frictional energy is generated and converted into heat. To evaluate the contribution of thermal effects and plasticity, five different material models were studied among them TEPS was nonlinear and temperature-dependent including thermal softening. Discussions were made on the effect of solution type and material type. The rail temperature, calculated for a critical creepage case, confirmed the potential of martensitic phase transformation. Thermal effects were also important at lower creepages, where a synchronization effect causes earlier damage.
•A coupled thermomechanical finite element modelling procedure is developed.•The frictional energy is directly converted into heat and no heat function has to be defined.•Various material models are studied among them the temperature-dependent-elasto-plastic material is the most advanced one.•Temperature gradients and thermomechanical stresses in the rail are calculated.•The potential of martensitic phase transformation and the fatigue behaviour at elevated temperatures are evaluated.
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