Design strategy for microalloyed ultra-ductile magnesium alloys

This article describes a design strategy deployed in developing ultra-ductile Mg alloys based on a microalloying concept, which aims to restrict grain growth considerably during alloy casting and forming. We discuss the efficiency of the design approach, and evaluate the resulting microstructural and mechanical properties. After processing, the so-designed alloys ZQCa3 (Mg–3Zn–0.5Ag–0.25Ca–0.15Mn, in wt.%) and ZKQCa3 (Mg–3Zn–0.5Zr–0.5Ag–0.25Ca–0.15Mn, in wt.%) reveal very fine grains (<10 µm), high ductility (elongation to fracture of up to 30%) at moderate strength or high strength (ultimate tensile strength of up to 350 MPa) at reasonable ductility. These properties are explained based on thermodynamic modelling, microstructure analysis including transmission electron microscopy studies, and microstructural and mechanical testing after annealing, and are compared to a related commercial alloy (ZK31).     

Microstructures and strengthening mechanisms in high-carbon titanium-microalloyed interstitial-free steels

We report the mechanical properties and microstructures of titanium-microalloyed interstitial-free (IF) steels with different carbon contents smelted and rolled in the laboratory. The results show an increase of yield strength and tensile strength with carbon content, which are enhanced with lower coiling temperature (CT). Microstructure analysis reveals that the ferrite grain size varies from 11 to 23?μm, while the average size of TiC as the main second-phase particle ranges from 10 to 35?nm. The sample with 80?ppm carbon and 853?K CT reaches a mechanical strength close to that of high-strength IF steels. The increase of yield strength is mainly attributed to both grain-refining and precipitation strengthening.     

Nanoscale partitioning of Mn between austenite and martensite revealed by Curie temperature variations

The partitioning of Mn between deformation-induced martensite and austenite phases of a Fe?18Mn?2Al?2Si?0.04C high-Mn steel was studied by tracking variations in the Curie temperature of martensite after exposure to successively higher annealing temperatures. The increase in Curie temperature was justified by the Mn depletion of martensite. Assuming a linear relationship between the Curie temperature and Mn concentration, the average Mn concentration of martensite was expressed as a function of annealing temperature. The proposed method, which is applicable in a dilatometer with inductive heating, enables the ready estimation of Mn partitioning in intercritically annealed medium- and high-Mn steels.     

Enhanced uniform elongation by pre-straining with deformation twinning in high-strength β-titanium alloys with an isothermal ω-phase

It is shown that pre-straining with deformation twinning is a novel approach to enhancing the uniform elongation of a high-strength β-type Ti–15Mo alloy (mass%) with isothermal ω-phase precipitation. Pre-existent mechanical {3?3?2}?1?1?3? twins hinder the early onset of plastic instability (necking) after yielding, which is often caused by the presence of the isothermal ω-phase, and enhance the uniform elongation markedly from 0% to 13% at a yield strength level of 900?MPa.     

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.     

Deformation mechanism crossover and mechanical behaviour in nanocrystalline materials

We use molecular dynamics simulations to elucidate the transition with decreasing grain size from a dislocation- to a grain-boundary-based deformation mechanism in nanocrystalline fcc metals. Our simulations reveal that this crossover is accompanied by a pronounced transition in the mechanical behaviour of the material; namely, at the grain size where the crossover occurs (the 'strongest size'), the strain rate under tensile elongation goes through a minimum. This simultaneous transition in both the deformation mechanism and the corresponding mechanical behaviour offers an explanation for the experimentally observed crossover in the yield strength of nanocrystalline materials, from Hall-Petch hardening to 'inverse Hall-Petch' softening.     

Effect of heat treatment on the microstructure and mechanical properties of spray-formed 7055 aluminium alloy

7055 Al alloys samples were prepared by spray forming and hot-extrusion followed by two different aging treatment procedures. Their different distributions of GP zones, and nanoscale precipitates η′ (MgZn) and η (MgZn₂) are extensively investigated by transmission electron microscope (TEM). The mechanical properties, including tensile strength, Vickers hardness and elongation of both aged 7055 Al alloys, have also been measured and analysed. It is found that T6 and T76 aging treatment results in quite different microstructure and mechanical properties. The outstanding performance of the 7055 Al alloys after T6 aging treatment is attributed to nanoscale semi-coherent dispersion precipitates.     

Effect of heat treatment on the NO2-sensing properties of sputter-deposited indium tin oxide thin films

Transparent conducting indium tin oxide (ITO) films were deposited onto glass substrates by radio-frequency magnetron sputtering at 648?K, under an oxygen partial pressure of 1?Pa. The effect of annealing on the electrical properties of the films was studied. Characterization of the coatings revealed an electrical resistivity below 6.5?×?10^??3?Ω?cm. The ITO films deposited at 648?K were amorphous, while the crystallinity improved after annealing at 700?K. The surface morphology examined by scanning electron microscopy appears to be uniform over the entire surface area after annealing. The NO₂-sensing properties of the ITO films were investigated and showed sensitivity at concentrations lower than 50?ppm, at a working temperature of 600?K.     

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%).     

Effect of cooling rate on size-dependent atomic ordering of CoPt nanoparticles

We report on the effect of cooling rate on the size-dependent atomic ordering of CoPt nanoparticles using aberration corrected high-resolution transmission electron microscopy. It was found that cooling rate plays a crucial role in promoting atomic ordering during the cooling process after annealing. Nanoparticles of ≈3?nm in diameter show the A1-disordered phase after annealing at 873?K for 1?h followed by rapid cooling (110?K/min), while the L1₀-ordered phase is obtained when the cooling rate is slow (1.5?K/min). The disordered phase is also obtained by rapid cooling after annealing at 973?K for 1?h. These results unambiguously indicate that the order–disorder transformation temperature is reduced to a temperature at least lower than 873?K for CoPt nanoparticles smaller than 3?nm in diameter. The slow cooling process promotes the atomic ordering, which resulted in an enhancement of magnetic coercivity as high as 2200?Oe. This study demonstrates that hard magnetic properties of the CoPt nanoparticles can be improved by controlling the cooling rate after heat treatments.     

Recrystallization in a low-density low-alloy Fe–Mn–Al–C duplex-phase alloy

The recrystallization behaviour of a cold-rolled, low-density, low-alloy duplex-phase alloy (Fe–6.57Al–3.34Mn–0.18C, wt.%) has been studied. Temperature-resolved X-ray diffraction and dilatometry showed that the alloy recrystallizes at 850?°C during continuous heating. However, electron back-scattered diffraction investigations using Kernel average misorientation revealed that during annealing ferrite recrystallizes at lower temperatures while austenite remains strained up to 1200?°C. This study underlines the complexity of recrystallization of a microstructure comprising of constituents with high and low stacking fault energy.     

Nanoindentation measurements on deformation-induced α’-martensite in a metastable austenitic high-alloy CrMnNi steel

The hardness of deformation-induced α’- martensite and parent austenitic matrix in high-alloy CrMnNi steel was investigated by nanoindentation measurements inside scanning electron microscope using picoindenter. After the indentation, the microstructure was investigated by electron backscattered diffraction measurements. The hardness values for α’-martensite are only 24% higher than those of austenite. Thus, the increase in strength during the formation of deformation-induced α’-martensite is rather caused by the small grain size of α’-nuclei resulting in a dynamic Hall–Petch effect than by its “intrinsic” hardness.     

New insights on tensile plasticity of ultrafine-grained metallic materials

ABSTRACT

The tensile properties of TiNi_43.5Fe_6.5 alloy samples having different grain sizes (0.16, 0.35, 1.7, 2.3, and 3.9?μm) and fabricated by severe plastic deformation and annealing were investigated. It was observed that both the strength and the elongation of the alloy increase with a decrease in the grain size until the average size reaches 1.7?μm. However, for average grain sizes smaller than 1.7?μm, the elongation decreases continuously with further grain refinement. On the other hand, the strain-hardening rate does not decrease with the decrease in plasticity but instead increases slightly. The poor ductility of the ultrafine-grained TiNi_43.5Fe_6.5 alloy is accompanied by a high degree of strain hardening. This newly observed ductility behaviour of the ultrafine-grained TiNi_43.5Fe_6.5 alloy is elucidated by characterising the intragranular and grain boundaries.     

Strain rate response of a Ni–Ti shape memory alloy after hydrogen charging

In this work, we investigate the susceptibility of Ni–Ti superelastic wires to the strain rates during tensile testing after hydrogen charging. Cathodic hydrogen charging is performed at a current density of 10?A/m² during 2–12?h in 0.9% NaCl solution and aged for 24?h at room temperature. Specimens underwent one cycle of loading-unloading reaching a stress value of 700 MPa. During loading, strain rates from 10^?6 to 5?×?10^?2?^?s^?1 have been achieved. After 8?h of hydrogen charging, an embrittlement has been detected in the tensile strain rate range of 10^?6 to 10^?4?s^?1. In contrast, no embrittlement has been detected for strain rates of 10^?3?s^?1 and higher. However, after 12?h of hydrogen charging and 24?h of annealing at room temperature, the embrittlement occurs in the beginning of the austenite-martensite transformation for all the studied strain rate values. These results show that for a range of critical amounts of diffused hydrogen, the embrittlement of the Ni–Ti superelastic alloy strongly depends on the strain rate during the tensile test. Moreover, it has been shown that this embrittlement occurs for low values of strain rates rather than the higher ones. This behaviour is attributed to the interaction between the diffused hydrogen and growth of the martensitic domain.     

A study of thermal vacancies and dislocation structure in Fe–40 at.% Al

The densities of thermal vacancies and residual dislocations in bulk specimens of Fe–40?at.%?Al have been investigated using differential dilatometry and X-ray diffraction. A large quenched-in vacancy concentration at temperatures above about 873?K was apparent from the decrease in average lattice parameter. This was correlated to a lowering of the effective enthalpy of vacancy formation from about 91 to 42?kJ mol^?1, possibly caused by the presence of different types of point defect in lower- and higher- temperature regimes. The residual dislocations were found to have a major concentration on {100} planes at any given temperature. An increase in the dislocation density and a concurrent fall in the vacancy concentration was observed with a lowering of the quenching temperature from 1223 to 1073?K, indicating the possibility of vacancy annihilation at 1073?K.     

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.     

Tensile properties of precracked tempered martensitic steel specimens tested at ultralow strain rates in high-pressure hydrogen atmosphere

Tensile tests were performed on precracked Cr–Mo martensitic steel (C: 0.38, Si: 0.22, Mn: 0.84, P: 0.024, S: 0.021, Ni: 0.08, Cr: 1.11, Mo: 0.15, Cu: 0.12, Fe: bal. (wt%)) specimens at various strain rates (ranging from 6.5 × 10^?8 s^?1 to 1.0 × 10^?4 s^?1) in high-pressure (95 MPa) hydrogen and helium atmospheres. Irrespective of the strain rate, the tensile strength in the helium atmosphere was 1400 MPa. In the hydrogen atmosphere, the tensile strength decreased to less than 600 MPa at a strain rate of 2.0 × 10^?5 s^?1. However, the tensile strength increased to 900 MPa when the strain rate was decreased to 6.5 × 10^?8 s^?1. This recovery of the tensile strength was because of the decrease in the local stress in the vicinity of the precrack because of hydrogen.     

Attainment of an exceptionally high strength in low-carbon steel along with modest ductility through a novel heat treatment route

ABSTRACT

Over the last few decades, the use of steel (the most significant structural engineering material) is facing a significant challenge due to its replacement by other materials (such as composites) possessing higher strength-to-weight ratio/specific strength. This necessitates further enhancement in the strength of steel. In particular, low-carbon steel, in the annealed condition, suffers from inherent problems of poor strength and discontinuous yielding. In this research work a novel heat treatment route of incomplete austenitisation-based cyclic ice-brine quenching has been adopted on low-carbon steel (AISI 1010 steel containing 0.1 wt.% C) without considering any costly alloying or thermo-mechanical treatment. Accordingly, exceptionally high strength (UTS?=?1.7?GPa) and specific strength (226?MPa?g^?1cm³) are achieved after three cycles along with a modest ductility (% Elongation?=?9). This is the highest strength reported so far for low-carbon steel containing 0.1 wt.% C. Yield point phenomenon is also eliminated. This is attributed to a novel microstructure consisting of highly sub-structured fine plate martensite crystals containing internal twin and dislocation tangles along with dispersion of nano-sized cementite particles and clusters of cementite particles.     

Tensile deformation mechanisms of the hierarchical structure consisting of both twin-free grains and nanotwinned grains

A series of large-scale molecular dynamics simulations have been performed to investigate the tensile properties and atomistic deformation mechanisms for the nanostructured Cu with three typical microstructures: the hierarchical structure consisting of both twin-free grains (d?=?70?nm) and grains with bundles of smaller nanotwins (d?=?70?nm, λ?=?10?nm), the fully nanograined structure and the fully nanotwinned structure. The average flow stress of the hierarchically structure is found to be higher than that calculated by rule of mixture. As compared with that of fully nanograined structure, the strength for the twin-free grains in the hierarchical structure is promoted and gives extra hardening due to the increased dislocation density and dislocation behaviours. It is also found that the nanotwin bundles are more deformable than the twin-free grains in the hierarchical structure according to the deviatoric strain invariant contour. This indicates that the fully nanograined structure cannot only be strengthened to a higher level, but also obtain better ductility by embedded with stronger bundles of smaller nanotwins. Thus, a superior strength–ductility synergy could be obtained in this kind of hierarchical structures, and this novel strategy has also been implemented in bulk austenitic steels or copper by recent experiments.     

Study on the deformation of nanocrystalline cobalt

A study was carried out on the deformation of nanocrystalline cobalt. For cold-rolled nanocrystalline cobalt, the X-ray diffraction peaks narrowed instead of broadened on deformation. The results of transmission electron microscopy showed that grain size (17.8?nm on average) was not increased. The results can be explained in terms of the production and activity of vacancies and vacancy clusters.