The article focuses on the effect of alloying and microstructure on formability of advanced high strength steels (AHSSs) processed via quenching and partitioning (Q&P). Three different Q&P steels with different combination of alloying elements and volume fraction of retained austenite are subjected to uniaxial tensile and Nakajima testing. Tensile mechanical properties are determined, and the forming limit diagrams (FLDs) are plotted. Microstructure of the tested samples is analyzed, and dramatic reduction of retained austenite fraction is detected. It is demonstrated that all steels are able to accumulate much higher amount of plastic strain when tested using Nakajima method. The observed phenomenon is related to the multiaxial stress state and strain gradients through the sheet thickness resulting in a fast transformation of retained austenite, as well as the ability of the tempered martensitic matrix to accumulate plastic strain. Surprisingly, a Q&P steel with the highest volume fraction of retained austenite and highest tensile ductility shows the lowest formability among studied grades. The latter observation is related to the highest sum of fractions of initial fresh martensite and stress/strain induced martensite promoting formation of microcracks. Their role and ability of tempered martensitic matrix to accumulate plastic deformation during forming of Q&P steels is discussed.

In the present study, the effect of Niobium (Nb) on the hydrogen embrittlement resistance of Quenched and Partitioning (Q&P) steel is investigated. For this purpose, the hydrogen uptake level and its impact on the mechanical properties of a Nb-free and a 0.024 wt% Nb Q&P steel are thoroughly analysed. The hydrogen trapping capacity is evaluated via thermal desorption spectroscopy (TDS). In-depth analysis of the desorption kinetics at different heating rates allows identification and quantification of the available trapping sites. The hydrogen embrittlement sensitivity of both steels is characterized using static and dynamic tensile tests. The addition of Nb results in an increase of the hydrogen concentration by more than 25%. The larger hydrogen content in the Nb steel, as a result of the higher fraction of grain boundaries/interphases, gives rise to a more severe embrittlement of the Nb steel compared to the Nb-free one. In addition to the larger hydrogen fraction in the Nb Q&P steel, the larger retained austenite fraction of low stability is detrimental due to the larger fraction of high carbon martensite formed when straining. This results in higher susceptibility to hydrogen embrittlement of the Nb microalloyed steel due to the brittle character of the high carbon martensite that forms easily during straining. Under dynamic loading conditions, the hydrogen embrittlement of both steels is minimal, which is attributed to a reduced hydrogen diffusion and the suppression of the transformation induced plasticity (TRIP) effect due to adiabatic heating.

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.

The current investigation presents a breakdown analysis of the elastoplastic behavior of commercially pure aluminum pre-strained via severe plastic deformation (SPD) and tested in tension. The tensile samples selected, owing to their prior SPD, had the gradient microstructure from the fragmentation stage as well as a homogeneous equiaxed structure from the steady-state regime. Except for the regions of low deformation and steady-state of SPD, the microstructures of pre-strained material were largely dominated by the presence of geometrically necessary boundaries. During tensile straining, dislocation strengthening contributed more to the overall material strength in fragmentation stage samples, while grain boundary strengthening played a major role in the steady-state stage samples. The dislocation density evolution rate-based approach predicted a more active role for the dynamic annihilation/recovery events in the SPD material when compared to the fully recrystallized condition. This could explain the drastic drop in tensile elongation of the pre-strained material as well as the enhancement in uniform and post-necking elongation with the gradual increase in the amount of pre-strain. A plastic instability condition based on the dislocation density evolution approach successfully accounted for the observed stable deformation limits in the coarse- and fine-grained material.

Because of their excellent combination of strength and ductility, quenching and partitioning (Q & P) steels have a great chance of being added to the third generation of advanced high strength steels. The large ductility of Q & P steels arises from the presence of 10% to 15% of retained austenite which postpones necking due to the transformation induced plasticity (TRIP) effect. Moreover, Q & P steels show promising forming properties with favourable Lankford coefficients, while their planar anisotropy is low due to a weak texture. The stability of the metastable austenite is the key to obtain tailored properties for these steels. To become part of the newest generation of advanced high strength steels, Q & P steels have to preserve their mechanical properties at dynamic strain rates and over a wide range of temperatures. Therefore, in the present study, a low-Si Q & P steel was tested at temperatures from -40 °C to 80 °C and strain rates from 0.001 s^-1 to 500 s^-1. Results show that the mechanical properties are well-preserved at the lowest temperatures. Indeed, at -40 °C and room temperature, no significant loss of the deformation capacity is observed even at dynamic strain rates. This is attributed to the presence of a large fraction of austenite that is so (thermally) stable that it does not transform in the absence of deformation. In addition, the high stability of the austenite decreases the elongation at high test temperatures (80 °C). The additional adiabatic heating in the dynamic tests causes the largest reduction of the uniform strain for the samples tested at 80 °C. Quantification of the retained austenite fraction in the samples after testing confirmed that, at the highest temperature and strain rate, the TRIP effect is suppressed.

Severe plastic deformation imposed under high hydrostatic pressure introduces a considerable dislocation substructure in metals from the early stages of deformation, ultimately resulting in grain fragmentation. Characterization and quantification of the substructure require methods with a sufficiently high angular and spatial resolution to reveal the local heterogeneities in orientation differences and the length scales of the substructure. However, the statistical relevance of the observations should be assured which requires relatively large fields of view. In present work, the evolution of dislocation substructures during static and dynamic high pressure torsion processing of commercially pure aluminum is examined. Orientation data obtained by electron backscatter diffraction using two different mapping step sizes are utilized to assess the detection of the dislocation substructures and boundaries during the grain fragmentation stage. Accumulation of distortion in the crystal produces an increase in measurement noise at each pixel which is estimated using Kamaya's plots. The storage of dislocations and related angular misfits reduces the peak height of the probability density distribution of misorientation gradients, moves the peak to higher misorientation gradients and widens the distribution. Superposition of double Rayleigh distributions over the combined dislocation boundary data predicts a slightly higher median for the frequency of geometrically necessary boundaries and larger misorientation gradients across these boundaries in dynamically deformed material. In incidental dislocation boundaries, higher misorientation gradients are only observed at lower equivalent strains. Buildup of shear strain leads to the deterioration in the quality of the fitting to a double Rayleigh distribution and is linked to the complex evolution pattern of the dislocation boundaries. Finally, in statically deformed material, anisotropy in the substructure evolution is observed in the shear and radial planes.

Ultrafast annealing (UFA) is considered a feasible alternative processing route for third-generation advanced high strength steels (AHSSs). In fact, significant grain refinement occurs in low carbon steels (0.1 wt%) when heating rates (example, 1000 K/s) considerably higher than the conventional heating rates are applied. Additionally, high heating rates result in multiphase microstructures containing ferrite, retained austenite, and martensite/bainite. The mixture of structural constituents is induced by carbon gradients resulting from the short time available for diffusion during ultrafast annealing of the steel. Quasi-static and high strain rate tensile tests, with strain rates ranging from 0.0033 s⁻¹ to 600 s⁻¹, revealed that the tensile strength of both conventional and UFA samples increases with increasing strain rates, whereas the strain to fracture decreases. However, compared with the conventionally heat-treated samples (heating rate: 10 K/s), the UFA samples exhibited less ductility deterioration at high strain rates. The samples were investigated using light optical microscopy, scanning electron microscopy, electron backscatter diffraction (EBSD), and X-ray diffraction. The results indicated that the improved tensile properties of the UFA steels (compared with those of the conventional steels) resulted mainly from the presence of retained austenite (up to 5%) in conjunction with a lower carbon martensite/bainite fraction. Owing to adiabatic heating associated with dynamic conditions, the stability of retained austenite shifted to higher strain values than those encountered under static conditions. The TRIP effect of the UFA samples appeared at higher strain values than in the conventionally heat-treated samples. The delayed transformation is considered essential for realizing improved preservation of the deformation capacity and, in turn, improved crashworthiness.

Bainitic steels, as a third generation of advanced high strength steels, are potential steel grades for automotive applications. Two grades of bainitic steels with low and high silicon content, with three different thermal treatments per grade and therefore different second phase constituents, are examined under quasi-static and high strain rate deformations. Microstructures are studied by advanced characterization techniques, including X-ray diffraction and scanning electron microscope equipped with an electron backscatter diffraction detector. Subsequently, the quasi-static and dynamic mechanical responses of the steels are correlated to the microstructures. A positive effect of the strain rate is observed for all the examined materials: when the strain rate is increased, both the tensile stress and deformation levels increase, thus also the energy absorption capacity. However, it is shown that the higher the fraction of second phase constituents, the lower the effect of strain rate becomes. In addition, the grain size directly correlates to the strain rate effect too. The phenomenological hardening model of Johnson-Cook is used to simulate the quasi-static and dynamic flow behaviors, allowing to quantify the strain rate sensitivity for each material. A comprehensive literature survey on the strain rate sensitivity of various steel grades reveals that steels with higher strength demonstrate a lower strain rate sensitivity factor. This trend can be approximated by a power law function which clearly is followed by the materials under consideration in this study.

Severe plastic deformation (SPD) of metals to obtain ultra-fine or even nano-sized grains has proven to be an interesting concept explored over the last few decades. However, the mechanical behavior of SPD metals and the underlying microstructural phenomena are not fully understood yet. In present work, commercially pure aluminum was subjected to high pressure torsion (HPT) deformation with strains ranging from very low levels to values well in the steady-state microstructure regime. The mechanical properties of the HPT processed samples were determined using tensile tests on miniature samples using full-field strain mapping. Orientation imaging microscopy (OIM) was utilized to follow the progression of grain refinement and texture as a function of imposed SPD. Local orientation based misorientation gradients helped to perform statistical boundary analysis and determine the fractions of incidental and geometrically necessary dislocation (GND) boundaries and local GND densities. From probability density distributions of the misorientation gradients two different stages of microstructural evolution, namely, fragmentation and saturation, could be discerned. The strength increased monotonously and the uniform elongation, though lower than the value of the annealed material, enhanced with the imposed strain in HPT. The post-necking response was observed to be highly microstructure dependent, where a lower grain size augmented the resistance for micro-crack propagation and enhanced the elongation-to-failure. In addition, the work hardening response corresponding to the yield point displayed maxima coinciding with the onset of the saturation stage. Anisotropy in fracture strain, observed between the axial and radial directions in a disk-like HPT sample, reduced with the randomization of shear texture, while higher intensities of the C {100}<110> orientation was considered responsible for the lower elongation-to-failure along the radial direction.