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The microstructural evolution and deformation mechanisms of 316L stainless steel (SS) have been investigated at 297, 173, 50 and 15 K by in situ neutron diffraction during tensile loading and correlative transmission electron microscopy (TEM). The yield strength and ultimate tensile strength of 316L stainless steel are significantly improved at cryogenic temperatures. In contrast to room temperature, deformation-induced martensitic transformation was observed at all cryogenic temperatures. The γ-austenite (FCC) content decreases and α′-martensite (BCC) increases with increasing strain, a fraction of this γ to α′ transformation is accompanied by the transient appearance of ε-martensite (HCP) as an intermediate phase. The maximum volume fraction of ε-martensite increases with decreasing deformation temperature and reaches 13% for deformation at 15 K. TEM results confirm that γ → ε → α′ and γ → α′ martensitic transformations occur during cryogenic deformation, while mechanical twins were observed only at 173 K. The evolution of lattice strain, phase volume fraction, stacking fault probability (SFP) and stacking fault energy (SFE) were quantified to investigate the correlation between deformation mechanisms and mechanical behaviour of 316L SS as a function of deformation temperature. [ABSTRACT FROM AUTHOR]

In-situ neutron diffractometry is performed to study the visco-plasticity in a nickel-base single crystal superalloy during in-situ cooling from high temperatures. It is found that visco-plastic deformation has two contributions from creep and stress relaxation, which are subject to the accumulation of dislocation activity and dislocation annihilation, respectively. Use has been made of the lattice strain evolution of the (200) γ+γ′ peak to confirm this effect. The decrease in lattice strain and macro-stress during in-situ cooling has been observed and confirms that there was softening taking place before thermal strain dominates at lower temperatures. Therefore, in-situ isothermal cyclic loading and relaxation tests under strain control, akin to thermal contraction during casting, have been carried out. A visco-plasticity law was then developed based on macro-strain development during creep and lattice strain evolution during stress relaxation within an appropriate timescale, where transient effects are captured. The constitutive law developed has been used to independently determine the evolution of stress and strain during in-situ cooling. The implementation of these findings into thermo-mechanical modelling during cooling from solidification is also discussed. [ABSTRACT FROM AUTHOR]

The effects of strain rate on the mechanical behaviour and microstructure evolution of extruded Mg AZ31 alloy are investigated here for strain rates ranging from 0.0001 s −1 to 4000 s −1 . High strain rate compression testing was performed using a split Hopkinson pressure bar apparatus. The microstructure and texture evolution were characterised by using electron backscatter diffraction (EBSD) and neutron diffraction. The results show that the yield strength increases by roughly 55% from the quasi-static to the high strain rate regime while the plateau stress increases by roughly 35%. After compression along the extrusion direction, the deformed textures of both quasi-static and high strain rate loading samples are similar, showing that a large portion of grains have been reoriented through the activation of tensile twinning. However, a detail EBSD analysis suggests that there is a higher fraction of secondary compressive twins in high strain rate deformed specimens at 16% strain. A visco-plastic self-consistent (VPSC) model, which incorporates the composite grain scheme to account for twinning, is employed to model the mechanical response and texture evolution, and to elucidate the collaborative nature of the slip-twinning deformation. Using only a single set of parameters, the VPSC model successfully reproduces the stress-strain curves at all tested strain rates, and more importantly, captures the texture and twin volume fraction evolutions. The simulation results suggest that under quasi-static and high strain rate loadings, the dominant slip systems are different in both untwinned grains and tensile twins. The results also suggest that twins are formed at a lower strain at high strain rates when compared to quasi-static loading. [ABSTRACT FROM AUTHOR]

High manganese steels are emerging as promising structural materials for cryogenic applications due to their low production cost and great potential in achieving excellent strength-ductility combinations. Micro-alloying serves as a desirable method in tailoring stacking fault energy (SFE) of the steels and thus tailoring the mechanical performance. In this study, we investigated the dedicate role of Cu addition played on the mechanical and microstructural responses of high manganese steels at the low-temperature range (293, 173, and 77 K) via in situ neutron diffraction and microscope characterizations. The addition of 1 wt%Cu to the steel not only effectively improved the yield strength (YS) and elongation but also increased the SFE thus postponing the martensite formation. For both high Mn steels, as deformation temperature decreased, the tensile strength was increased linearly, the formation of stacking faults and dislocation was promoted, and the SFE almost linearly decreased with a slope of about 0.06 mJm−2·K−1. The contributions to YS and flow stress from lattice friction, grain boundary, dislocation, deformation twins, and phase transformation were determined based on neutron diffraction results and previously validated models. The work revealed the critical roles of Cu micro-alloying in tailoring the SFE of TWIP steels and the resulting deformation mechanisms, paving the way in adapting new high manganese steels for cryogenic applications. • Cryogenic mechanical properties of two TWIP steels were measured. • Stacking fault energy (SFE) was determined theoretically and experimentally. • Deforming mechanisms were studied at cryogenic temperatures. • Adding Cu can enhance yield strength, tailor SFE, and change deformation behaviors. • Strengthening effects from a variety of hardening mechanisms were determined. [ABSTRACT FROM AUTHOR]

Abstract: In-situ neutron diffraction was employed to monitor the evolution of nano-bainitic ferrite during low temperature isothermal heat treatment of austenite. The first 10 peaks (austenite, γ and ferrite, α) were monitored during austenization, homogenisation, rapid cooling and isothermal holding at 573K. Changes in the α-110 and γ-111 peaks were analysed to determine the volume fraction changes and hence the kinetics of the phase transformation. Asymmetry and broadening in the α-200 and γ-200 peaks were quantified to lattice parameter changes due to carbon redistribution as well as the effects of size and dislocation density. Atom Probe Tomography was then used to confirm that, despite the presence of 1.5 mass% Si, carbide formation was evident. This carbide formation is the cause of poor ductility, which is lower than expected in such steels. [Copyright &y& Elsevier]

Neutron and high-energy X-ray diffraction methods have been used to investigate deformation mechanisms (slip and twinning activities) during uniaxial tensile tests of Grade 1 commercially pure titanium (Ti-α). These two diffraction techniques have provided valuable and complementary information. The predictions made by an Elasto-Plastic Self-Consistent (EPSC) model, taking into account grain reorientation and stress relaxation induced by twinning activity, have been compared with the experimental data. Results clearly demonstrate that there is a good agreement between simulations and diffraction measurements. The EPSC model enables accurate predictions of: (i) the lattice strains developed in variously oriented {hk.l} grain families within the bulk material, (ii) the grain reorientation induced by twinning activity and (iii) the macroscopic stress-strain response. Two different textured products were studied in order to evaluate the influence of texture on the mesoscopic and macroscopic responses of Ti-α alloy. [ABSTRACT FROM AUTHOR]

Primary creep regeneration (PCR) is observed during cyclic creep deformation in many engineering alloys. PCR is a phenomenon in which reverse inelastic strain fully, or partially, resets the early creep strain hardening memory of the material. The current understanding regarding the origin of the PCR behaviour in engineering alloys is limited to the phenomenological observations from the changes in dislocation structures whereas a good mechanistic understanding of the PCR behaviour is crucial for developing robust plasticity-creep predictive models. In this study, we investigated the micromechanical origin of PCR behaviour in type 316H stainless steel at 650 °C using high-temperature mechanical testing and neutron diffraction. A cyclic creep experiment was conducted in-situ at a neutron diffraction beamline, during which various degrees of unloading and reverse loading were applied to the specimen, followed by creep deformation under a load above the material's yield strength. Partial PCR was observed after reverse plastic loading for all the creep dwells, which is contrary to current high-temperature lifetime assessment's procedures advice which is to account for full recovery of primary creep after any reverse plastic loading. The extent of PCR is observed to be proportional to the magnitude of reverse plastic strain up to a level of 0.5% reverse plastic strain. From the measured neutron diffraction data, a strong correlation was observed between the changes in magnitude of the accumulated intergranular residual lattice strains and the macroscopic primary creep strain. Moreover, the increases of lattice strain to saturation and transition from primary creep to secondary regime occur at the same time. Based on these correlations it can be postulated that the PCR behaviour observed due to cyclic loading in type 316H stainless steel at elevated temperature originates from the accumulation of intergranular residual lattice strains during the reverse plastic loading and those time-dependent changes during the creep dwells. [ABSTRACT FROM AUTHOR]

We successfully prepared an Fe 22 Co 20 Ni 19 Cr 20 Mn 12 Al 7 alloy consisting of a face-center cubic (fcc) phase and body-center cubic (bcc) phase that exhibits an outstanding combination of true strength of 1430 MPa and ductility of 19.9% at room temperature (RT). The micromechanical behavior at RT and 77 K for the studied alloy during tensile deformation was investigated using in situ time-of-flight (TOF) neutron diffraction in combination with synchrotron-based high-energy X-ray diffraction (HE-XRD). Here, the striking finding of a large elastic strain of 7.0% and 5.6% is reported for the {200} bcc crystal plane, which was achieved at RT and 77 K, respectively. Such a large lattice distortion observed in the bcc phase was attributed to a new type of stress-induced confined martensitic transformation. We attributed the physical origin of this specific martensitic transformation to the intrinsic microstructural feature of a nano-scale continuous distribution of an ordered-to-disordered crystal structure within the bcc phase, i.e., the disordered A2-structure was distributed continuously in the ordered B2-structure matrix. The stress-induced martensitic transformation from the metastable nano-sized disordered A2-phase was confined by the stable B2-ordered matrix. The new findings in this study provide additional understanding of the deformation mechanisms of high-entropy alloys and insights into alloy design for further enhancement of the mechanical properties of high-performance structural materials used at cryogenic temperatures. [ABSTRACT FROM AUTHOR]