Modelling the effect of carbon on deformation behaviour of twinning induced plasticity steels

In this article, a physical model describing the deformation behaviour of Twinning Induced Plasticity (TWIP) steels has been extended to include the effect of carbon content. The experimental validation and the analysis show that carbon mainly controls the maximum number of dislocations piled up at the twin boundary, resulting in the increase of back-stresses (i.e. kinematic hardening) and therefore the work hardening rate. This explanation seems to be in agreement with recent TEM observations.     

Quality assessment of refined oil blends during repeated deep frying monitored by SPME–GC–EIMS,GC and chemometrics

The aim of this study was to investigate the effect of the refined palm oil addition (20%) on the fatty acid and sterol compositions of refined olive oil or refined soya bean oil and also to investigate the formation of total polar compounds and volatile compounds in these oil blends during fifty successive deep‐frying sessions of potato fries at 180 °C. The blend of refined olive oil and refined palm oil exhibited a higher chemical stability during the frying process than that of refined soya bean oil and refined palm oil. Indeed, the total polar compounds and volatile compounds formed, especially 2,4‐decadienal, were found to be relatively increased in the refined soya bean oil/refined palm oil blend reaching 36.50% and 46.70%, respectively, after fifty deep‐frying sessions. Moreover, the degradation of linoleic acid and β‐sitosterol was significantly (P < 0.05) observed for the refined soya bean oil/refined palm oil blend. The results have proven that the proper blending of monounsaturated refined olive oil with refined palm oil increases its stability and hence improves the quality of such olive oil during frying process.     

Phenolic composition, isolation, and structure of a new deoxyloganic acid derivative from Dhokar and Gemri-Dhokar olive cultivars

Two rare olive cultivars, Dhokar and Gemri-Dhokar, growing in the south of Tunisia were investigated to identify their phenolic profile and evaluate their sugar content and antioxidant activity. The highest value of phenolic content was detected in Gemri-Dhokar cultivar extract (6.97 g gallic acid equivalents/kg of fresh olive). In addition, sugar content was quantified; glucose (45.17 g/kg of fresh olive) was the predominant sugar in Dhokar cultivar, followed by fructose (40.83 g/kg of fresh olive). The identification of phenolic compounds was based on separation by high-performance liquid chromatography equipped with a diode array detector followed by liquid chromatography-mass spectrometry analysis. In both cultivars, Oleuropein aglycon derivatives and elenolic acid were the main phenolic components. Oleuropeins were the major compounds quantified in the Gemri-Dhokar drupes olives (61.04 mg/100 g of fresh olive), while 0.25 mg/100 g were found in fresh Dhokar. A new iridoid compound, isolated as pure compound, was not previously reported in the literature. Its structure was established by spectroscopic analyses (NMR, UV, MS, and IR). DPPH, ABTS, and FRAP assays showed that the most important antioxidant capacity of olive extracts was found in with Gemri-Dhokar cultivar.     

Evolution of microstructure and strength during the ultra-fast tempering of Fe–Mn–C martensitic steels

Ultra-fast tempering cycles, combining a rapid heating rate (R _H = 300 °C/s) from room temperature to a peak temperature within the range 400–700 °C and subsequent rapid cooling, were performed on a Fe–Mn–C martensitic steel. The influence of the peak temperature reached during the cycle was determined on both the tensile properties of the steel and on its microstructure. The mechanisms controlling the microstructural evolution occurring during rapid tempering were studied by combining both TEM observations and 3D reconstructions by FIB/SEM. A theoretical analysis coupled with the acquired experimental data was then proposed to explain the evolution of mechanical properties. The results obtained support the assumption that carbide precipitation during fast tempering plays a key role in the evolution of mechanical properties.     

Constitutive model of the TWIP effect in a polycrystalline high manganese content austenitic steel

The TEM study of our steel with a high manganese content reveals that mechanical twining (TWIP effect) occurs during the deformation at room temperature. Microtwins are organised into parallel stacks and two systems are sequentially activated in each grain. They participate to the deformation and they are strong obstacles for the dislocations and for other twins, leading to the decrease of the effective grain size. Thus, TWIP provides our alloy a very good ductility and a high hardening rate. Our constitutive modelling deduced from a model proposed by Bouaziz and Guelton [4] integrates this typical organisation of microtwins. Twinning is quantified in each grain by the partial volume fraction of twins in each system. A nucleation law for the microtwins is introduced which depends on the local stress and the stress relaxation due to pre‐existing twins. The flow stress is deduced from the dislocation density, which evolves with the dynamical recovery and the decrease of the mean free path (MFP). The MFP takes into account the grain and twin boundaries and the forest dislocations. The strain is calculated by adding the contributions of dislocation glide and twinning accounting the orientation of the grain. To treat the polycristal, the behaviours of different grain orientations are mixed by assuming at each strain step that the increment of elastic energy stored is the same in each grain. The model was successfully applied to describe the mechanical properties of our alloy, for two different grain sizes. Some microstructural parameters are yet fitted. This leads to an insufficient prediction of the evolution of the microstructure. In further developments, we expect to introduce numerical simulation results on local characteristics of microtwins (thickness, critical resolved shear stress for twinning) and experimental results on the rate of twin nucleation.     

An acoustic–structural interaction modelling for the evaluation of a gearbox-radiated noise

Gearbox power transmissions are widely used in the automotive industry. They have a complex vibro-acoustic behaviour that is influenced by the various acoustic–structural interaction mechanisms. This paper concentrates on modelling these systems using a three-dimensional finite-element (FE) approach for the structure combined with a Rayleigh integral (RI) method for the acoustic radiation process. A modal analysis method is used to evaluate the elasto-acoustic modal characteristics for the coupled system. Effect of the fluid inside the gearbox on the vibration response is discussed. The combined use of this FE/RI model enables evaluation of the acoustic response. A case study of a simplified gearbox internally excited with gear mesh stiffness fluctuation is presented. Vibratory analysis allows concluding that the vibro-acoustic coupling between elastic housing, air-cavity and free acoustic field have not to be neglected.     

The effect of prior ferrite formation on bainite and martensite transformation kinetics in advanced high-strength steels

Most advanced high-strength steel products contain complex phases, including ferrite, bainite and martensite, which form successively during elaboration. It is essential to understand the effect of prior ferrite transformation on the subsequent bainite and martensite transformation kinetics to achieve precise control of the final microstructure. Nevertheless, the effect of the interface between the prior formed ferrite and the residual austenite (α/γ), together with the related chemical heterogeneity at the interface, on the subsequent phase transformations has been studied only rarely, and remains unclear. This study pays particular attention to the effect of the α/γ interface and its related concentration gradients on bainite and martensite transformation. It is shown that the interface and its related concentration gradients can play a very significant role on the subsequent bainite or martensite transformation kinetics: it retards bainite transformation whereas it accelerates martensite transformation. It is revealed from microprobe wavelength-dispersive spectrometry analysis and model calculations that there are both manganese and carbon gradients in front of the α/γ interface at the end of the ferrite transformation holding. The subsequent bainite transformation kinetics is controlled by the competition between the acceleration effect of the interface boundary itself and the retardation effect of the higher alloying concentration near the interface. Martensite transformation should initiate at the pre-existing dislocations in the center of the residual austenite grains, where the C and Mn contents are the lowest. A simple martensite transformation kinetics model taking into account C heterogeneity is proposed that can describe well the martensite transformation kinetics following the prior ferrite transformation.     

Influence of the sintering temperature on Young's modulus and the shear modulus of tricalcium phosphate – fluorapatite composites evaluated by ultrasound techniques

The mechanical properties of the tricalcium phosphate sintered between 1100 °C and 1450 °C for 1 h with different percentages of fluorapatite (13.26 wt%; 19.9 wt%; 26.52 wt%; 33.16 wt% and 40 wt%) have been characterized and evaluated using the ultrasound techniques. Young's modulus and the shear modulus were calculated from the point of the longitudinal and the transversal ultrasonic velocities. Young's modulus and the shear modulus of tricalcium phosphate increased with the sintering temperature and with the addition of the fluorapatite additive into the tricalcium phosphate matrix. At 1300 °C, the shear modulus and Young's modulus of the tricalcium phosphate – 40 wt% fluorapatite composites registered optimum values: 26 GPa and 66.2 GPa, respectively. Above 1300 °C, the mechanical properties of the tricalcium phosphate – fluorapatite composites were hindered by the tricalcium phosphate allotropic transformation and the formation of both the intragranular porosity and the cracks.