Twinning behaviour of TWIP steel studied by Taylor factor analysis

The twinning behaviour of Twinning-induced plasticity (TWIP) steel has been studied by analysing the grain orientation and the Taylor factor, based on microstructural and electron backscatter diffraction device observations. It is demonstrated that the Taylor factor can give an important guideline for determining the deformation mode of TWIP steel. The higher the Taylor factor, the easier the formation of twins and thus a tendency for the deformation mode to be mechanical twinning, while a low Taylor factor corresponds to a slip deformation mode. When the loading temperature is relatively low, the high Taylor factor regions increase and thus deformation twinning becomes easier while slip becomes more difficult, leading to increased tensile stress and decreased elongation.     

Liquid zinc embrittlement of a high-manganese-content TWIP steel

The cracking resistance of a twinning induced plasticity (TWIP) steel with high-manganese content has been studied in relation to the liquid metal embrittlement phenomenon. This phenomenon was investigated by hot tensile tests carried out on electrogalvanised specimens using a Gleeble® simulator at temperatures ranging from 600°C to 1000°C. The results show that this steel can be embrittled by liquid zinc within a limited range of temperature depending on strain rate.     

Crystallographic texture formation during recrystallization of cold-rolled Fe-3%Si single crystal under high DC magnetic fields

In a flat single crystal of Fe–3%Si alloy with (1 1 0)[0 0 1] orientation, regions with (0 0 1)[1.–1.0] orientation (deformation twins) were created by rolling at a low temperature. After cold rolling at 80% at an angle of 45º to the [0 0 1] direction, a (1 1 2) [1.–1.0] deformation texture was formed. In the recrystallization process during annealing of such a sample, a polycrystalline structure having a nearly cubic texture (0 0 1) [1–1.0] is formed. It is demonstrated that annealing under a high DC magnetic field enhances the cubic texture sharpness as compared to annealing carried out under similar conditions without a magnetic field.     

High-pressure torsion of metastable austenitic stainless steel at moderate temperatures

We subjected samples of a 304 metastable austenitic stainless steel to high-pressure torsion (HPT) in the temperature range of 303–573 K, (i.e. at different austenite stabilities), to examine their microstructures and mechanical properties. HPT processing at room temperature led to the formation of a lamellar microstructure with austenitic and martensitic phases, of which sizes were characterised by prior austenite grains, whereas HPT processing at moderate temperatures produced nanostructured austenite grains through mechanical twinning. The nanostructured 304 steel with an average grain size of ~70 nm exhibited a fine balance between tensile strength (~1.7 GPa) and reduction of area (~55%).     

On diffusion and interfacial enrichment of boron in the Laves phase: an in situ TEM-heat-treatment study on newly developed 3Co–3W–9Cr steel

Using in situ transmission-electron-microscopy (TEM) heating and subsequent energy-filtered TEM (EF-TEM) and high-resolution-TEM (HR-TEM) analysis on a newly developed 3Co–3W–9Cr steel, enrichment of boron at the Fe₂W and matrix interface has been studied for the first time. The heat treatment has been carried out at a temperature of 770°C. From the EF-TEM analysis, it is confirmed that a stacking-fault-assisted diffusion mechanism is responsible for enrichment of boron at the interface. From the HR-TEM study, it is ascertained that the diffusion behaviour of boron in the Laves phase and the extent of the enrichment at the interface is dependent on the amount of stacking faults in the Laves phase.     

Extraordinary plastic behaviour of the γ′ precipitate in a directionally solidified nickel-based superalloy

The deformation behaviour of the γ′ precipitate in a directionally solidified nickel-based superalloy is investigated using microscopic observations after tensile testing at room temperature. It is found that coarse γ′ precipitates (604 nm) are sheared by strongly coupled dislocations, and some γ′ precipitates are elongated to approximately 3–6 times of their original lengths. It reveals that, at room temperature, the γ′ precipitate within the experimental superalloy has a significant plastic deformation capacity in comparison with Ni₃Al bulk alloys. Based on the experimental observations, the extraordinary plastic behaviour of the γ′ precipitate is analysed.     

Tensile behaviour of submicrocrystalline ferritic steel processed by large-strain deformation

Mechanical tests have been carried out on Fe–15%Cr ferritic stainless steel with various microstructures. Ultrafine-grained microstructures with grain sizes of 0.2–0.3 µm were developed by large-strain cold-working and light annealing. The effects of severe deformation on the mechanical behaviour of as-processed and recovered steel were evaluated with reference to the same material having conventional work-hardened and recrystallised microstructures. Despite the low dislocation density in the fine grain interiors in the as-processed state, the samples with strain-induced submicrocrystalline structure were characterised by high internal stresses that resulted in a higher strength than could be expected from simple grain-size strengthening. These internal stresses were associated with a non-equilibrium state of strain-induced grain boundaries after severe deformation.     

The relationship between dynamic strain aging and serrated flow behaviour in magnesium alloy

The relationship between dynamic strain ageing (DSA) and serrated flow has been investigated via alternately switching strain rates at various temperatures in a Mg–3Nd–1Zn alloy. The results reveal that serrated flow is enhanced with decreasing strain rate and increasing temperature and tends to vanish, while the DSA continually intensifies as revealed by a higher flow stress even after the serration flow disappears. A mechanism is proposed, which could explain some abnormal deformation behaviour, such as a negative strain rate sensitivity and thermal hardening.     

Deformation of nanograined Ni–60Co alloy with low stacking fault energy

The evolution of deformation texture in a Ni–60Co alloy with low stacking fault energy and a grain size in the nanometre range has been investigated. The analyses of texture and microstructure suggest different mechanisms of deformation in nanocrystalline as compared to microcrystalline Ni–60Co alloy. In nanocrystalline material, the mechanism responsible for texture formation has been identified as partial slip, whereas in microcrystalline material, a characteristic texture forms due to twinning and shear banding.     

Influence of heat treatment on the microstructural evolution of Al–3 wt.% Cu during high-pressure torsion

Solution-treated, peak-aged and overaged samples of the model alloy Al–3?wt.% Cu, obtained by selective heat treatments of the pre-material, have been subjected to high-pressure torsion at room temperature and at 200?°C. The mechanical behaviour of the samples was investigated with torque measurements during deformation and microhardness measurements after deformation. Irrespective of the initial material condition, in the saturation regime a comparable equilibrium microstructure was found consisting of ultrafine aluminium grains stabilized by precipitates formed at grain boundaries.     

Mantle region accommodating two-dimensional grain boundary sliding in ODS ferritic steel

Two-dimensional grain-boundary sliding (GBS) was achieved microscopically in an oxide-dispersion-strengthened ferritic steel with an elongated and aligned grain structure, which was deformed perpendicular to the long axis. At the border between superplastic regions II and III, microscopic deformation was observed using sub-micron grids drawn on the material surface using a focused ion beam. GBS was accommodated by intragranular deformations in narrow areas around grain boundaries, which has been predicted by earlier researchers as characteristics of the core–mantle model. These observations suggest that dislocations slip only in the mantle regions around wavy boundaries to relax the stress concentration caused by GBS during superplasticity.     

Neutron and X-ray diffraction studies and cohesive interface model of the fatigue crack deformation behavior

The crack-tip deformation behavior during a single overload, fatigue test of ferritic stainless steel, and Ni-based HAYNES 230 superalloy is studied at different structural levels using (1) neutron-diffraction, from which both the elastic-lattice strain and volume-averaged total dislocation densities are obtained, (2) polychromatic X-ray microdiffraction to probe the geometrically necessary dislocations and boundaries distribution, and (3) an irreversible and hysteretic cohesive interface model which has been implemented into a finite element framework to simulate the stress/strain evolution near the fatigue crack tip. Neutron strain measurements and finite element simulations are in qualitative agreement on the macroscopic length scale. Large plastic deformation induced by the overload and the resulting compressive residual strains are observed in front of the crack tip after the overload, and are the principal reason for the fatigue-crack-growth retardation. Strong strain gradients surrounding the crack propagation result in the formation of a high density of geometrically necessary dislocations near the fractured surface and cause local lattice rotations on the submicron level.     

Iron in solution with aluminum matrix after non-equilibrium processing: an atom probe tomography study

In this paper, we report on the influence of rapid solidification and severe plastic deformation on the solid solubility of Fe in Al. Atom probe tomography, for the first time, was performed on fine (3–4 μm diameter) and coarse (~100 μm) as-atomised Al-5 at.% Fe powder and cryomilled Al-5 at.% Fe powder. The atomised powders exhibited negligible Fe in solution with Al, whereas the cryomilled powder contained ~2 at.% Fe in solution. Moreover, our results suggest that severe plastic deformation is preferable to atomisation/rapid solidification for increasing the non-equilibrium solid solubility of Fe in Al.     

Static and dynamical ageing processes at room temperature in a Fe25Ni0.4C virgin martensite: effect of C redistribution at the nanoscale

The work-hardening behaviour of virgin martensitic steel has been investigated in a strictly un-aged state and after various ageing conditions. At room temperature (RT), the un-aged alloy shows astonishing tensile performances (ultimate tensile stress?=?1600?MPa/uniform elongation?=?15%) but unexpected serrations. These serrations can be suppressed by static ageing (at RT or higher) while maintaining the initial work-hardening rate (ageing at RT). Parallel investigations using atom probe tomography reveal that the distribution of carbon at the atomic scale evolves from purely homogeneous for virgin martensite to partly segregated at a very fine scale (5–10?nm) after static ageing. This particular mechanical behaviour can therefore be associated with a very local decrease in available carbon in solid solution due to redistribution and segregations on defects (nanotwins) that occurs rapidly, even after few days at RT.     

Importance of crack-propagation-induced ε-martensite in strain-controlled low-cycle fatigue of high-Mn austenitic steel

We investigated the roles of deformation-induced ε-martensitic transformation on strain-controlled low-cycle fatigue (LCF) through crack-propagation analysis involving a notching technique that used a focused ion beam (FIB) setup on Fe–30Mn–4Si–2Al austenitic steel. Using the FIB notch, we separated the microstructure evolution into macroscopic cyclic deformation-induced and crack-propagation-induced microstructures. Following this, we clarified the fatigue crack-propagation-induced ε-martensitic transformation to decelerate crack propagation at a total strain range of 2%, obtaining an extraordinary LCF life of 1.1 × 10⁴ cycles.     

In situ characterization of the martensitic transformation temperature of NiTi shape memory alloys via instrumented microindentation

A novel, instrumented microindentation technique was successfully used to measure the temperature associated with the martensitic transformation leading to the recovery of plastic strain in a Nickel–Titanium (NiTi) shape memory alloy. Following a standard indentation cycle, the indenter was partially unloaded such that a good contact was maintained between indenter and specimen surface. The onset and finish temperature of the martensitic transformation, the associated volume contraction, and the amount of the recovered plastic deformation were determined by quantifying the indenter displacement as a function of temperature. These experiments were compared to conventional measurements of the transformation temperature by differential scanning calorimetry and compression testing.     

Strengthening of a thin austenitic stainless steel coil by cold wavy rolling with no magnetic and dimensional changes

A comparison of the effects of wavy rolling and cold rolling on microstructure variation, phase evolution, tensile and magnetic properties of a thin coil of Fe-18.47Cr-8.10Ni-0.94Mn austenitic stainless steel was made at room temperature. Wavy rolling led to strengthening with no change in magnetic property and thickness, unlike the conventional cold rolling that changed all these properties by deformation induced martensitic transformation, in addition to substructure evolution. The yield strength of 413 MPa and magnetic saturation 3.7 emu/g under mill-annealed condition increased, respectively, to 1208 MPa and 11.8 emu/g, upon four cycles of wavy rolling. While the maximum yield strength of 1790 MPa could be achieved by combining this stage of four cycles of wavy rolling with subsequent 50% conventional cold rolling, the magnetic saturation increased to 73.3 emu/g by deformation induced martensitic transformation caused by the latter.     

A combination of Al diffusion and surface nanocrystallization of carbon steel for enhanced corrosion resistance

Surface nanocrystallization is beneficial to the corrosion resistance of passive alloys, but generally has a negative effect on the corrosion behavior of non-passive alloys due to the enhanced surface reactivity. In this study, a combination of Al diffusion treatment and surface nanocrystallization was applied to carbon steel with the aim of exploring an alternative approach to improve the corrosion resistance of non-passive carbon steel. The surface nanocrystallization was achieved by sandblasting and subsequent recovery treatment. The former resulted in severe plastic deformation, while the latter turned high-density dislocation cells into nano-sized grains. The present study demonstrates that the combined Al diffusion and nanocrystallization generated a nanocrystalline Al-containing surface layer on the carbon steel with its surface grain diameter in the range of 10–300 nm. The corrosion resistance of the treated steel was evaluated. It is demonstrated that treated specimens possess increased resistance to corrosion with higher surface electron stability. Surface microstructure of the treated specimens was examined using SEM, AFM, and EDS in order to elucidate the mechanism responsible for the improved corrosion resistance.     

Commensurate-incommensurate phase transition in muthmannite,AuAgTe2: first evidence of a modulated structure at low temperature

To study the temperature-dependent structural changes and to analyze the crystal chemical behavior of silver as a function of temperature, a crystal of muthmannite, AuAgTe₂, has been investigated by X-ray single-crystal diffraction methods at 300 K and 110 K. At room temperature, muthmannite was confirmed as belonging to the space group P2/m, while at low temperature (110 K) it undergoes a reversible commensurate–incommensurate phase transition with a modulation wave vector q = 0.215(1)a* + 0.379(2)c*. Muthmannite reconverts to the commensurate type upon returning to room temperature, thus indicating that the phase transition is completely reversible in character. The average structure of the low-temperature muthmannite remains monoclinic, space group P2/m, and shows only normal thermal compression over the entire temperature range investigated. Crystal-chemical characteristics are compared with published data on the other members of the system Au–Ag–Te. Speculations on the possible origin of the modulated structure at low temperature are also given.     

TEM in situ straining experiments in Fe at low temperature

In situ transmission electron microscopy straining experiments were carried out in pure Fe to investigate the origin of the discontinuity observed at 250 K in the temperature variation of the deformation activation parameters. The results show that the motion of screw dislocations is steady at 300 K, in agreement with a kink-pair mechanism, but jerky at 110 K. This change has been attributed to a transition from a kink-pair mechanism to a locking–unlocking mechanism, similar to that observed previously in Ti.