Microstructures and tensile properties of 304 steel with dual nanocrystalline and microcrystalline austenite content prepared by aluminothermic reaction casting

The microstructures of 304 stainless steels with different amounts of nanocrystalline and microcrystalline austenite prepared by an aluminothermic reaction casting, without and with annealing at 1073?K for 8?h, have been investigated by X-ray diffraction, an electron probe micro-analyser, a transmission electron microscope and a scanning electron microscope. The steels, both without and with annealing, consisted of different dual nanocrystalline and microcrystalline austenite combinations and a little nanocrystalline δ ferrite, while the average grain size of the nanocrystalline austenite increased from 19 to 26?nm and volume fraction of the microcrystalline austenite increased from 17 to 30% after annealing. The tensile strength of the steel was dramatically increased from 500 to 1000?MPa and the tensile elongation ratio increased from 8 to 12% after annealing. However, the tensile strength was decreased to 600?MPa and the tensile elongation ratio increased from 12 to 22% after an annealing at 1273?K. The combination of dual nanocrystalline and microcrystalline austenite obtained after the annealing at 1073?K results in the best tensile properties.     

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.     

Low-temperature mechanical properties of Ce68Al10Cu20Co2 bulk metallic glass

Although it is well known that traditional metals and alloys will become brittle at low temperatures, the effect of low temperature on the mechanical properties of bulk metallic glasses (BMGs) is yet to be fully understood. In this research, the mechanical properties of Ce₆₈Al₁₀Cu₂₀Co₂ BMG are investigated at low temperatures. It is found that the yield strength of the Ce-based BMG increases significantly with the decrease of temperature. The elastic moduli of the BMG also increase monotonically with the drop of temperature, indicating the continuous stiffening of the BMG, while both Poisson's ratio and global plasticity decline at low temperatures. It is considered that the stiffer atomic bonds of the Ce-based BMG at low temperatures result in the increase of strength, and the higher energy required for nucleation of shear bands also leads to the increase in yield strength.     

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.     

Tension and compression of bulk Al-7.5 wt% Mg alloy

The mechanical behaviour of bulk ultrafine-grained Al-7.5 wt% Mg alloy consolidated from cryomilled powders has been investigated. The experimental data show that the alloy exhibits high strength, low strain hardening, serrated flow and relatively high ductility. In addition, the data indicate that the yield strength in tension is essentially equal to that in compression. The yield and flow strengths of the alloy are discussed in terms of strengthening processes that are related to grain size, the Orowan mechanism and solid-solution hardening. The serrations in the stress-strain curve are discussed in terms of dynamic strain ageing and deformation twinning.     

Nanoindentation-induced elastic–plastic transition and size effect in α-Al2O3(0001)

The mechanical properties of α-Al₂O₃(0001) have been investigated using the technique of nanoindentation with a Berkovich indenter. Coupled with the Hertzian contact theory, a theoretical shear strength of 28?±?2?GPa was determined from the onset of pop-in events on load–displacement curves during loading, and the intrinsic hardness 30?±?3?GPa was obtained by analysis of the so-called indentation size effect, based on the concept of geometrically necessary dislocations. The predicted values of the shear strength, hardness and elastic modulus are in good agreement with available experimental data. The importance of experimentally calibrating the area function over the contact depth range prior to nanoindentation tests is emphasized.     

Anisotropic grain growth kinetics in nanocrystalline nickel

ABSTRACT

Intriguing properties exhibited by nanocrystalline metals, including a high level of mechanical strength, arise from their nanometer-scale grain sizes. It is critical to determine the evolution of grain size of nanocrystalline materials at elevated temperature, as this process can drastically change the mechanical properties. In this work, a nanocrystalline Ni foil with grain size ～ 25?nm was annealed in situ in an X-ray diffractometer. X-ray diffraction peaks were analysed to determine the grain growth kinetics. The grain growth exponents obtained were ～ 2–4 depending upon the crystallographic direction, indicating the anisotropic nature of the grain growth kinetics.     

Influence of test temperature on the size effect in molybdenum small-scale compression pillars

Previous research has shown that body-centred cubic (bcc) metals exhibit a smaller size dependence of strength than what is commonly observed in face-centred cubic (fcc) metals. This work investigates compression testing of focused ion beam-manufactured molybdenum pillars ranging in size from 300?nm to 5?μm, both above and below its critical temperature at 300 and 500?K. At 500?K the size effect is found to be consistent with what is observed in fcc metals, owing to the increased mobility of screw dislocations.     

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.     

Frank–Read sources and the yield of anisotropic cubic crystals

Frank–Read sources are among the most important mechanisms of dislocation multiplication, and their operation signals the onset of yield in crystals. We show that the critical stress required to initiate dislocation production falls dramatically at high-elastic anisotropy, irrespective of the mean shear modulus. We hence predict the yield stress of crystals to fall dramatically as their anisotropy increases. This behaviour is consistent with the severe plastic softening observed in α-iron and ferritic steels as the α ? γ martensitic phase transition is approached, a temperature regime of crucial importance for structural steels designed for use in future nuclear applications.     

Comparison of progressive prompt delay with and without instructive feedback

We examined the effectiveness and efficiency of 2 instructional arrangements using progressive prompt delay (PPD) with 3 young children with autism and 1 child with developmental delays. Specifically, we compared PPD with instructive feedback (IF) to PPD without IF in an adapted alternating treatment design. The results suggested that (a) children with autism and developmental delays can learn when PPD is used with IF, (b) IF can be an effective method of instruction for young children with autism and developmental delays, and (c) the combination of PPD and IF can increase the efficiency of instruction. Data collected 8 to 9 weeks after instruction ended showed that participants maintained mastery of 58% to 92% of the acquired behaviors. We discuss these results within the constraints and limitations of the data and recommend areas for future research.     

Le JCR facteur d’impact (IF) et le SCImago Journal Rank Indicator (SJR) des revues françaises : une étude comparative

One of the major functions of academic journals is to contribute to the evaluation of research activities and scientists. Invented more than 50 years ago, the ISI impact factor (IF) became the most important indicator of the quality of journals, in spite of well-known problems and critics such as the over-representation of English-language journals. This is a specific problem for French publishers and scientists; publishing in French is not valorising. Since 2007, the new SCImago Journal Rank Indicator (SJR) offers an alternative to the IF. SJR applies the Google algorithm (PageRank) to the journals of the SCOPUS bibliographic database that indexes more journals than ISI Web of Science. The goal of our study is to compare the two indicators for French academic journals, with three questions: Which is the coverage of French journals by ISI and SCOPUS (title number, scientific disciplines)? Which are the differences of the two indicators IF and SJR for the ranking of French journals? How do they cover the French academic journal publishing market (representativity)? The results of our study of 368 French journals with IF and/or SJR are in favour of the usage of the new indicator, at least as a complement to the IF. (1) Coverage: 166 journals are indexed by ISI (45%), 345 journals are indexed by SCOPUS (94%), 143 journals are indexed by both (39%). 82% of the journals are from STM, 18% are from SS&H. In particular, SCOPUS covers much better the medical and pharmacological sciences. (2) Ranking: The correlation of IF and SJR for the 143 journals with both indicators is high (0.76). The IF better differentiates the journals than the SJR indicator (155 vs. 89 rankings). On the other side, because of the larger source database, more French titles become visible on an international level through SJR than through IF. (3) Representativity: The SJR is more interesting and representative of the French academic journal publishing market than the IF (19% vs. 9%), especially for STM titles (38% vs. 19%), much less for SS&H titles (6% vs. 2%). Nevertheless, ISI (Web of Science) and SCOPUS index journals from only a small part of the French academic publishers (10%–20%). Again, SCOPUS is more representative than the ISI dababase (17% of the publishers vs. 10%). Methodological problems and perspective of a multidimensional evaluation are discussed. Our study compares the ISI impact factor (IF) with the new SJR for 368 French academic journals with IF and/or SJR. The results: The SJR coverage is better than of the IF (94% vs. 45%), especially in medical sciences. The correlation of IF and SJR for journals with both indicators is high (0.76). The IF better differentiates the journals than the SJR indicator (155 vs. 89 rankings). The SJR coverage is more representative of the French academic journal publishing market than ISI/IF (19% vs. 9%), especially for STM titles (38% vs. 19%), less for SS&H titles (6% vs. 2%).     

Improvement in mechanical properties of commercially pure titanium through reverse rolling

Commercially pure (cp) titanium plates were subjected to 90% reduction in thickness through UDR (unidirectional rolling) and RR (reverse rolling) in the cold-rolling route. These rolled plates were then annealed at 600 °C for 20 min. The plates processed through UDR and RR exhibited similar values of ductility, but the plates subjected to RR exhibiting a significant increase in their strength compared to those rolled through UDR. However, on annealing the yield strength of the RR plate was reduced slightly while its ductility improved. Relative growths of dislocation density in the samples appeared to be responsible for such differences in the mechanical properties of the samples.     

Unusual room-temperature compressive plasticity in nanocrystal-toughened bulk copper-zirconium glass

Cast Cu₅₀Zr₅₀ alloy rods with a diameter of 1?mm have been found to consist of a glassy phase containing fine crystalline particles with a size of about 5?nm. They have a glass transition temperature T _g of 675?K, and a large supercooled-liquid region extending 57?K above T _g. The rods exhibit a high yield strength of 1860?MPa and a Young's modulus of 104?GPa. Because they contain a dispersion of embedded nanocrystals, the as-cast bulk metallic glass rods can sustain a compressive plastic strain at room temperature of more than 50%, an exceptional value which is explicable by compensation of any shear softening by nanocrystal coalescence and pinning of shear bands.     

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.     

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.     

Dynamic and quasi-static mechanical properties of fibre-reinforced metallic glass at different temperatures

We have investigated the dynamic and quasi-static mechanical properties of a tungsten fibre-reinforced Zr_41.25Ti_13.75Ni₁₀Cu_12.5Be_22.5 bulk metallic glass composite (W/BMG) at temperatures ranging from 77 to 473?K. It was found that the yield stress and elastic modulus of the composite increase while strain-to-failure decreases with decreasing temperature. Tests under uniaxial dynamic compression showed that the W/BMG has a higher yield stress than under quasi-static compression. It was shown that yielding of the composite material is dominated by a shear banding process in the glass matrix, which is essentially a viscous flow as a consequence of local adiabatic heating. The interval between the nominal testing temperature and the temperature at which shear bands initiate is a key factor affecting the shear banding process, rendering the properties of the W/BMG very sensitive to temperature.     

Yielding and strain hardening in aluminium single-crystal foils subjected to tension and bending

A series of tensile and microbend tests were conducted on aluminium single-crystal foil specimens with different thicknesses ranging from 20 to 90?µm. Two different crystal orientations relative to the tensile direction were considered in the tests: one is an orientation that was excepted to activate at least four slip systems on four different slip planes, and the other is an orientation that was expected to activate only a single-slip system. In the tensile tests, typical size dependence of initial yield strength was observed. The microbend test method was extended to include a reversal of bending direction. Using the curves of bending moment normalized by the square of foil thickness versus surface strain, which were obtained from the reversed bending tests, the total amounts of strain hardening were divided into isotropic and kinematic hardening components. In the microbend tests, a pronounced size-dependent kinematic hardening behaviour was clearly observed; meanwhile, the amounts of isotropic hardening were very small, particularly for the single-slip orientation.     

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.     

Emergent control for play responses following instructive feedback for children with autism spectrum disorder

Play provides children learning opportunities in the natural environment to acquire communication skills. While working to establish skills that may lead to additional leisure and social opportunities of children with autism spectrum disorder (ASD), procedures need to be both effective and efficient. One way to increase efficiency of teaching is through the use of instructive feedback (IF). Therefore, the purpose of the current study is to systematically replicate and extend previous research by examining if play behaviors emerge when using IF to teach conceptually related primary (i.e., tacts) and secondary targets (i.e., play skills). The results of the present study extend previous research by demonstrating the effectiveness of teaching tacts to individuals with ASD, and that using IF during tact training can help promote the acquisition of play skills.