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Austenitic stainless steels have a wide area of applications, including spring steels in their hardened condition. A preceding forming process mainly achieves the necessary high yield strength (YS) for spring steel. During strain hardening, strain-induced α′-martensite formation results in high strength characteristics. The driving forces behind this transformation include low stacking fault energy of austenitic base material and high chemical driving force of α′-martensite formation. Both parameters are strongly dependent on temperature. This research represents the temperature dependent mechanical properties in a temperature range between −40 °C and 200 °C of a) nickel reduced manganese and nitrogen alloyed stainless steel X5CrMnNiMoN16-4-4 (4Mn) and b) CrNi steel X10CrNi18-8 (1.4310) at a strain rate of 4.6 × 10−4 s−1. Additionally, the effect of a moderate increase in strain rate to 9.2 × 10−4 s−1 and 6.7 × 10−3 s−1 on quasi-adiabatic heating of the steels was investigated. For both steels, by increasing the strain rate, the adiabatic temperature increases and subsequently decreases the strain-induced α′-martensite formation. The increase in the strain rate results in a strong drop of ultimate tensile strength (UTS) for both steels. Nevertheless, the uniform elongation (UE) remains almost constant at a high level for 4Mn. The UE decrease sharply in case of 1.4310. Independent of strain rate value, a larger strain-induced α′-martensite fraction generated in 4Mn compared with 1.4310. Consequently, under all experimental conditions, higher UTS values were achieved for 4Mn. The strain-induced α′-martensite was quantified by magnetic saturation analysis (MSAT). Subsequently, it was characterised by scanning electron microscopy (SEM) using electron channelling contrast imaging (ECCI) and electron backscatter diffraction technique (EBSD). [ABSTRACT FROM AUTHOR]

The accurate prediction of the elastic modulus of aluminum foam is important for the application in high-tech fields, e.g. in aerospace industry. However, the scaling law based on the Gibson-Ashby model did not achieve the precise prediction of elastic modulus, in absence of complex porous structural features. In this study, we extended the scaling law by introducing the topological and morphological parameters into the multivariate equation. The 3D numerical images of aluminum foam specimens were obtained by X-ray tomography method, and the nine structural parameters were calculated and analyzed. The results show that as the porosity increases from 77% to 91%, the highest frequency of sphericity and solid material thickness of the cellular structure remains practically constant, although the distribution of cell size becomes more dispersed. The edges connections and the Inter-Branches Angle (IBA) of aluminum foam remain unchanged for all the specimens. Seven structural parameters were introduced into the extended scaling law to predict the elastic modulus with a higher residual value R 2 of 0.932, in contrast to the R 2 of 0.702 for the conventional scaling law. Through a stepwise linear regression method, the extended scale method was simplified by removing redundant parameters, and still achieved the elastic modulus predictions with a residual value R 2 of 0.929. The remaining structural parameters in the extended scaling law, i.e. relative density, solid material thickness, and sphericity, were supposed to independently influence the elastic modulus, according to the covariance analysis. The three independent parameters offer the possibility to accurately predict and regulate the mechanical properties of aluminum foam. • The scaling law is extended by combining the cellular structure parameters. • Accurate prediction of the elastic modulus of Al foam is achieved with R 2 of 0.932 • Three of these structural parameters independently influence the elastic modulus. • The node connectivity features of aluminum foam coincide with trabecular bone. [ABSTRACT FROM AUTHOR]

Solidification and segregation behavior of a novel X3CrNiCuN17-6-4 (concentrations in wt.%) cast steel was studied by thermodynamic calculations and corresponding microstructure investigations. In addition, resulting mechanical properties were examined by tensile tests in a temperature range of −60 to 200 °C and compression tests were performed with varying strain rates ranging from 1 s−1 to 10−4 s−1 at room temperature. The investigated steel showed an anomalous temperature dependence of tensile elongation, which is a typical characteristic of high alloy TRIP/TWIP steels. In the as-cast state before heat treatment, a few micrometer-sized copper precipitates were visible, which were exclusively located in the interdendritic regions. A solution annealing was performed at 1050 °C for 30 min to dissolve nitrides and carbides likely to be present in the as-cast microstructure. However, in areas with a high copper content further nanometer-sized copper precipitates were formed after heat treatment. EDS point measurements and EPMA were used to determine the chemical composition of dendrites, interdendritic regions and copper precipitates before and after solution annealing. While the enrichment of chromium and copper in interdendritic regions could be verified by the thermodynamic calculations, the enrichment of nickel in interdendritic regions contradicted the predictions according to Scheil calculation. [ABSTRACT FROM AUTHOR]

The deformation-induced formation of α′ martensite was investigated by tensile testing of a Fe-19Cr-3Mn-4Ni-0.15C-0.17N cast austenitic steel between −196 °C and 400 °C. The steel did not exhibit spontaneous α′ martensite formation at temperatures as low as −196 °C. Therefore, the critical driving force for the formation of α′ (−2780 J/mol) was obtained by determining the complementary mechanical energy necessary to trigger the deformation-induced α′ martensite at 0 °C. Driving forces for the γ→α′ transformation at other tensile test temperatures associated with the deformation-induced α′ formation were then obtained by subtracting the mechanical energies applied to trigger the martensitic transformation from the critical driving force. The triggering mechanical energies were obtained from in-situ magnetic measurements which enabled to mark the onset of the γ→α′ transformation. The driving forces for the γ→α′ transformation calculated using the preceding method indicated an increase in the stability of austenite which was attributed to changes in the mechanical and physical properties of austenite in the vicinity of the Néel temperature. The method can be used to calculate modified driving forces for the occurrence of the γ→α′ phase transformation. [ABSTRACT FROM AUTHOR]

This paper presents a systematic study of quenching and partitioning (Q&P) processing of a (C + N)-containing austenitic stainless steel with a special focus laid on the influence of partitioning conditions on the mechanical properties and its comparison with C-containing Q&P-treated stainless steels. Similar to C-containing stainless steels, martensite phase in the studied steel exhibited pre-precipitation phenomena such as interstitial segregation to defects below 250 °C and precipitation reactions such as the formation of paraequilibrium M 3 C carbides (M representing Fe and substitutional elements) above 300 °C. The interstitial partitioning between martensite and austenite and the dependence of mechanical properties on the partitioning conditions resembled those of C-containing stainless steels. With increasing partitioning temperature, interstitial enrichment in the austenite and ductility were enhanced while tensile strength decreased. Similar observations were made upon prolonged partitioning at 250 °C. In contrast, the ductility of steels partitioned at 350 °C and 450 °C barely changed with partitioning time. For partitioning at 450 °C, this was because most of the interstitial partitioning at 450 °C took place within the first 3 min of partitioning. For partitioning at 350 °C, the interstitial enrichment of fine austenite regions was completed rapidly as well and the later interstitial partitioning into coarse austenitic regions gave rise to little increase in austenite stability due to their large size. The optimal product of ultimate tensile strength and total elongation, 47 GPa%, was obtained after partitioning at 450 °C for 3 min. The sufficiency of a short partitioning to obtain superior mechanical properties is of great practical significance in the context of growing global concerns about energy demand and climate change. • Quenching and partitioning of a (C + N)-containing austenitic stainless steel. • Reactions competing with interstitial partitioning. • Mechanical properties insensible to partitioning time at specific temperatures provide possibilities for energy efficiency. • Presence of isothermal martensite upon subzero quenching. • Comparison with quenching and partitioning of C-containing stainless steels. [ABSTRACT FROM AUTHOR]

Fe–TiC metal matrix composite (MMC) material is fabricated by EB-PBF (Electron Beam Powder Bed Fusion) additive manufacturing technique. Herewith, gas atomized model alloy powder containing in-situ formed reinforcement phase is utilized. Powder properties (i.e., particle size distribution, morphology, flowability, tap, and apparent density) are discussed in relation to composite powder applicability for EB-PBF. As a result, samples with over 99% density were fabricated. Cross-section samples investigated via SEM (scanning electron microscopy) and EBSD (electron back-scattered diffraction analysis) reveal homogeneously distributed submicron TiC reinforcements of two morphologies – blocky TiC precipitates and fine needle-shaped ones. Fabricated composite demonstrates enhanced tensile strength of 537 MPa, while preserving the sufficient level of ductility (35.0%), which correlates with the low hardness of 153.9 HV10. Additionally, heat treatment of as-built samples is investigated by implementing differential scanning calorimetry (DSC), dilatometry analysis, and austenitization-water-quenching techniques. Applied heat treatment induces a significant grain refinement, which increases the hardness of treated samples up to 246.3 HV10 by water quenching, while TiC precipitates undergo minor changes in morphology and size. Fabricated MMC material demonstrates the successful application of the gas atomized powder with an in-situ formed carbide reinforcement phase for additive manufacturing. [ABSTRACT FROM AUTHOR]