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The roles of pre-formed martensite (PM) in below-Ms bainite formation, microstructure, crystallography, strain partitioning and mechanical properties of a low-carbon bainitic steel were investigated using electron-backscattered diffraction, transmission electron microscopy, micro digital image correlation technique and mechanical tests. It is demonstrated that the pre-formation of martensite eliminates the incubation time for bainite transformation at various austempering temperatures below Ms, indicative of its acceleration effect at the early stage of transformation. This effect is mainly attributed to the surfaces or tips of the PM acting as the nuclei of subsequently-formed bainite, with initial bainite tending to form around the PM. However, the finishing time for below-Ms bainite transformation, especially at even lower temperatures, is retarded, owing to the dividing effect of PM on parent austenite grains, the decreasing effect of lowered isothermal temperature on the diffusion rate of carbon atoms and the strengthening effect of lowered isothermal temperature on supercooled austenite. PM and its adjacent bainitic laths have nearly the same crystallographic orientation and belong to the same block. The pre-formation of martensite largely refines the bainitic blocks/laths and retained austenite. The specimens with PM show relatively uniform strain partitioning among various phases, contrasting with the specimens without PM, for which strains are highly concentrated in the bainite region nearby fresh martensite/austenite (M/A) blocks or between adjacent M/A blocks. The impact absorption energies of the specimens with PM, when austempered at 30–60 °C below Ms, are more than twice higher than those of the specimens without PM, at no expense of tensile properties.

In this work, the effects of grain size on the tensile properties, serrated flow, Portevin-Le Chaˆtelier (PLC) band evolution, dislocation density, and microstructure evolution of Fe-22Mn-0.6C steel were investigated using digital image correlation (DIC), X-ray diffraction (XRD), transmission electron microscope (TEM) and monotonic tensile tests. Results show that the yield strength, tensile strength, and work hardening value of Fe-22Mn-0.6C steel increase whereas its elongation decreases with decreasing grain size. The critical strain of serration decreases whereas the amplitude of serration increases with decreasing grain size. Moreover, the plateau of serrations is larger and that the strain concentration within PLC bands is more severe in fine-grained steel than in coarse-grained steel. The dynamic strain aging (DSA) behavior of the steel is enhanced by grain refinement. The speed of PLC band movement decreases faster in fine-grained steel than in coarse-grained steel, which results in a decrease in plasticity. XRD and TEM analyses show that the high dislocation density and unique twin substructure may be the main reasons for the strong DSA phenomenon observed in fine-grained steel.

In this paper, martensite-to-austenite reversion during intercritical annealing was applied to a conventional low-alloy transformation-induced plasticity steel to form reverted austenite, which was followed by austempering to form bainite, and hence the resulting microstructures and mechanical properties were investigated. The obtained microstructure is composed of alternating lamellae of intercritical ferrite and reverted austenite on microscale, with the latter consisting of bainitic ferrite laths and retained austenite films on nanoscale. This unique microstructure exhibits excellent strength–ductility combinations, which surpass those of conventional polygonal blocky-structured counterparts, providing a strategy for developing high-performance structural materials at low production cost.

High tensile strength of ~ 1550 MPa, uniform elongation of ~ 30%, and impact energy of ~ 80 J were achieved in a high-C high-Si bainitic steel, by austempering around the nose-tip temperature of the time–temperature–transformation (TTT) diagram. This bainitic microstructure revealed multiple lengths of nanobainite plates and various sizes and shapes of retained austenite. Its bainite transformation during austempering and microstructure evolution during tensile deformation were discussed in depth. Results demonstrated that large blocks of retained austenite (with inhomogeneous carbon distribution) transformed partly and gradually to multiple martensite grains as strain increases, due to the strong effect of carbon on mechanical stability of retained austenite. Ductile long nanobainite plates (~ 0.13 at% C) suppressed the propagation of microcracks, because of stress relief effect ahead of the current microcracks.

Cast pores are known to cause stress/strain concentrations, facilitating the initiation of fatigue cracks and shortening the fatigue lifetime of cast materials; however, it is an open question regarding how the deformation fields generated by different pores interact each other and evolve with fatigue loading. The present study addresses the local deformation behavior in cast-pore containing high manganese steel during fatigue loading investigated using single- and multi-pore finite-element models. These models take into account the actual size, shape and distribution of pores based on X-ray computed tomography images and consider the actual material properties of the steel, i.e. the high strain-hardening rate, cyclic hardening law and Bauschinger effect using a viscoplastic constitutive model within the framework of Chaboche model. The results demonstrate that the local stress, strain and hysteresis loop energy around an actual pore evolve progressively with fatigue loading, and reach as large as 2 to 4, 2.5 to 8.5 and 6 to 30 times their corresponding far-field values at high fatigue cycles. More importantly, two different effects, i.e. "umbrella-like stress shielding" and "overlapping stress amplifying", are identified in the presence of multiple pores, due to the deformation fields interactions between pores. And, which of the two effects takes place or predominates depends on the specific location of concern and the spatial distribution of pores. These findings may be extended to provide some new insight into the fatigue damage micromechanisms in various cast-pore containing materials and to help understand the scatters of their fatigue lifetime in terms of the local deformation fields around pores and their interactions.

We report observations of gamma-ray emissions with energies in the 100 TeV energy region from the Cygnus region in our Galaxy. Two sources are significantly detected in the directions of the Cygnus OB1 and OB2 associations. Based on their positional coincidences, we associate one with a pulsar PSR J2032+4127 and the other mainly with a pulsar wind nebula PWN G75.2+0.1 with the pulsar moving away from its original birthplace situated around the centroid of the observed gamma-ray emission. This work would stimulate further studies of particle acceleration mechanisms at these gamma-ray sources., Accepted for publication in the Physical Review Letters

The isothermal transformation kinetics, microstructure and mechanical properties of a low-carbon bainitic steel, subjected to below-Ms/above-Ms austempering with or without prior ausforming, have been investigated via dilatometric measurements, microstructural characterization and mechanical tests. The results show that for all austempered samples, prior ausforming largely refines the bainitic laths and enhances the mechanical stability of supercooled austenite, the latter leaving more austenite untransformed at the finish of isothermal treatment. Nevertheless, divergent consequences on the final microstructure arise: for the above-Ms austempered samples, prior ausforming increases the amount of large, unwanted brittle martensite/austenite blocks and decreases the volume fraction of retained austenite at room temperature; however, for the below-Ms austempered samples, prior ausforming decreases the size and amount of martensite/austenite blocks and increases the volume fraction of retained austenite. Accordingly, the below-Ms austempered samples with prior ausforming exhibit a product of strength and ductility of ∼ 43 GPa% and impact toughness of ∼180 J/cm2, in sharp contrast with those of the above-Ms austempered samples with prior ausforming, ∼33 GPa% and ∼80 J/cm2, respectively. The present results clearly demonstrate a favorable effect of the combined process of ausforming and below-Ms austempering against the adverse combining effect of ausforming and above-Ms austempering. Keywords: Mechanical properties, Carbide-free bainite, Transformation kinetics, Isothermal treatment, Ausforming

In this study, a plastic deformation process consisting of hot compression at 350 °C and heat treatment at 400 °C was performed to bond AZ31 magnesium alloy. Microstructural evolution around the bonding interface was systematically characterized to investigate the bonding process and clarify the bonding mechanism. When the plastic deformation strain reached 0.6, the bonding zone was full of fine dynamic recrystallized grains and the initial interface was eliminated. The post-heating treatments were conducted to achieve a sound interface bonding. The tensile tests and the corresponding fracture morphologies analysis indicated that the optimum holding time of heat treatment was 8 h. The interfacial bonding strength of the specimens holding for 8 h reached 164.7 MPa, an enhancement of about 9% compared with that of the specimens holding for 1 h. The microstructure analysis indicated that the bonding quality was affected by migration of the interfacial grain boundary (GB), the development of recrystallized grains and the evolution of interfacial oxides around the bonding area.

The excellent properties of transformation-induced plasticity (TRIP) steels with a lamellar structure are generally claimed to mechanically stabilize retained austenite (RA) grains, enabling the TRIP effect to persist up to higher strains. Nevertheless, the present work finds that nearly no RA grains are transformed into martensite at the strain range of 0.10–0.14 in lamellar-structured specimens. Furthermore, the strain-hardening exponent remains nearly unchanged. To explain the underlying mechanisms of this strain-hardening behavior, the block/packet/prior austenite grain boundaries of the hierarchically lamellar-structured TRIP steel were reconstructed. Moreover, the evolutions of full-field strain distribution and back stress hardening with straining were examined through the in situ micro digital image correlation technique and cyclic loading–unloading–reloading tensile tests. Results show that the block structure controls the microstructural deformation behavior of lamellar-structured specimens. Within a block, ferrite and bainite layers could deform synergistically. However, next to block boundaries, ferrite layers are susceptible to a large strain gradient due to the different deformation tendencies of adjacent blocks. Compared to conventional polygonal-structured specimens, the more refined ferrite grains of lamellar-structure specimens reduce the strength difference between ferrite and bainite layers. The strains are more uniformly partitioned among various constituents, thus delaying the occurrence of tensile necking. Back stress hardening dominates the strain-hardening in conventional polygonal-structured specimens, facilitating the increase of flow stress but decreasing the toughness. In lamellar-structured specimens, combined moderate back stress and dislocation hardening should be responsible for the persistent strain-hardening exponent after the strains exceed 0.10.

The low-cycle fatigue properties, microstructural evolution and crack growth behaviors of a low-carbon carbide-free bainitic steel subjected to austempering at 300, 320 and 350 °C have been investigated. The results indicated that the fatigue lifetimes of the specimens increase with decreasing the austempering temperature at lower total strain amplitudes, but a comparable fatigue lifetime or even an opposite result is observed at higher total strain amplitudes. During fatigue loading, the blocky retained austenite with inhomogeneous carbon distribution partially transforms into martensite, and the transformation amount increases with the total strain amplitude, especially for the specimens austempered at high temperatures. The transformed martensite and the untransformed austenite satisfy a Kurdjumov-Sachs (K–S) orientation relationship, which can increase the cooperative deformation capacity between the both phases. However, with continuous cycling, more martensite is formed, and these newly formed martensite grains are more likely to form micro-voids/cracks due to their brittleness, which reduces the crack initiation lifetime, and become the fast path for crack propagation, which deteriorates the crack propagation lifetime. Alternatively, more crack branching, crack deflection and severely twisted bainitic ferrite laths are observed for the specimens austempered at high temperatures, which helps to delay/hinder crack growth and increase the crack propagation lifetime, especially at high total strain amplitudes. This suggests that the fatigue lifetime can be closely associated with the interactions of the crack initiation and propagation caused by mechanically-induced martensite from retained austenite.

Based on the density functional theory, the interfacial structures, the interfacial ideal work of adhesion and the interfacial bonding of VN(100)/VC(100), VN(110)/VC(110), and VN(111)/VC(111) interfaces were investigated. The results reveal that the (100) surface is more stable than (110) for both VN and VC. The surface energy of VC (111) or VN (111), as nonstoichiometric surface, is not constant that varies with the different chemical potentials of V (or C/N). The V-terminated structure is more stable than the C-terminated structure for VC (111) surface. For VN (111) surface, the V-terminated structure is more stable than the N-terminated one in the N-rich field, in contrast the N-terminated structure is more stable than the V-terminated one in the V-rich field. The adhesion energy of the most stable (111) interface is larger than that of (100) interface and (110) interface. Interfacial termination structure plays an important role in the interface stability, and the interface with the V C or V N bonds forming is more stable than other interfaces with V V and C N bonds. The stacking sequence also influences the interfacial stability, and those building types keeping the continuity of VN (VC) structure have high stability. These theoretical analyses will provide some suggestions on the orientation control during the multilayered film preparation.

The effects of Cr/Mo alloying on the tensile deformation behavior of Fe-Mn-C twinning-induced plasticity steel were investigated. The results showed that Cr/Mo alloying increases both the strength and ductility significantly and concurrently. As compared with the premature saturation of deformation twinning in the Cr/Mo-free steel, a unique twinning behavior occurs in the Cr/Mo-alloyed steel, i.e., retarded but more persistent formation of thinner twins with straining. This unique twinning behavior gives rise to a sustained high strain-hardening rate up to higher strains, which delays the plastic instability and increases both strength and ductility simultaneously.

The local cyclic stress/strain responses around an actual, irregular pore in cast Hadfield steel under fatigue loading are investigated numerically, and compared with those around a spherical and an ellipsoidal pore. The actual pore-containing model takes into account the real shape of the pore imaged via high-resolution synchrotron X-ray computed tomography and combines both isotropic hardening and Bauschinger effects by using the Chaboche's material model, which enables to realistically simulate the cyclic deformation behaviors around actual pore. The results show that the stress and strain energy density concentration factors ( K σ and K E ) around either an actual irregular pore or an idealized pore increase while the strain concentration factor ( K e ) decreases slightly with increasing the number of fatigue cycles. However, all the three parameters, K σ , K e and K E , around an actual pore are always several times larger than those around an idealized pore, whatever the number of fatigue cycles. It is suggested that the fatigue properties of cast pore-containing materials cannot be realistically evaluated with any idealized pore models. The feasibility of the methodology presented highlights the potential of its application in the micromechanical understanding of fatigue damage phenomena in cast pore-containing materials.

A self-lubricating plasma electrolytic oxidation&ndash, polytetrafluoroethylene (PEO&ndash, PTFE) composite coating was successfully fabricated on the surface of commercially pure titanium by a multiple-step method of plasma electrolytic oxidation, dipping and sintering treatment. The microstructure and tribological properties of the PEO&ndash, PTFE composite coating were investigated and compared with the PEO TiO2 coating and the PTFE coating on titanium. Results show that most of the micro-pores of the PEO TiO2 coating were filled by PTFE and the surface roughness of PEO&ndash, PTFE composite coating was lower than that of the PEO TiO2 coating. Furthermore, the PEO&ndash, PTFE composite coating shows excellent tribological properties with low friction coefficient and low wear rate. This study provides an insight for guiding the design of self-lubricating and wear-resistant PEO composite coatings.

In this study, low-carbon martensite steel was obtained by rapid quenching and was then subjected to cold rolling and electroplastic rolling with different deformation amounts. The mechanical properties and microstructures of the steel after rolling deformation were compared and analyzed. The results showed that with the increase in the deformation amount, the material strength and hardness of the steel subjected to cold rolling and electroplastic rolling increased, while the elongation decreased. Under the same rolling thickness-reduction amount, the strength (hardness) of the electroplastic-rolled material was slightly lower than that of the cold-rolled specimen, and the elongation was significantly improved. Owing to the various angles between the lath direction and rolling direction, the deformed laths featured different characteristics. Regarding the lath morphology, there were no significant differences between the electroplastic rolling deformation and cold rolling deformation. During the plastic deformation process, the texture exhibited significant rotation; the texture transformed to a relatively more stable {112} , and the steel showed strong ɑ texture and weak γ texture. The {111} texture was the strongest in the γ direction; it is stronger than the {111} texture. The {111} plane of the electroplastic-rolled specimen was significantly higher than that of the cold-rolled specimen. The positive relationship between texture and plasticity was obtained.

Recently, many steel researchers have been attempting to develop superfine low-carbon bainitic microstructure; however, the results are not satisfactory. In this paper, we applied a new thermo-mechanical process for achieving low-carbon superfine bainitic structure. By applying the new process, i.e. ausforming at temperatures above Ms followed by austempering at 355 °C, which is below Ms , we successfully obtained a superfine, ladder-like bainitic structure, with bainitic laths as thin as ∼100 nm and Vickers hardness as high as ∼500 HV, being of the same level of medium-carbon bainitic steels. This new process shows a potential for wide industrial applications.

A novel heat-treatment process, below- Ms austempering, was performed on two low-C high-Si/Al steels, and accordingly the resultant microstructures and mechanical properties were investigated. The results showed that bainitic transformation occurs at isothermal temperatures below Ms and unexpectedly, the transformation process is accelerated significantly by the prior formation of martensite. Furthermore, as compared with the conventional above- Ms austempering process, the below- Ms austempering refines effectively the bainitic plates and reduces the size and volume fraction of blocky martensite/austenite phases, giving rise to much enhanced impact toughness and improved tensile properties.

The twinning behavior, strain hardening and tensile properties of coarse- and fine-grained Fe-13/18/22Mn-1.0C twinning-induced plasticity (TWIP) steels were investigated by microstructural observations and mechanical tests. These steels have stacking fault energy ranging from ~27 to ~37 mJ/m2 with increasing Mn content, and thus deformation twinning is the dominant mechanism. Tensile test results show that, with increasing Mn content, the yield and tensile strengths decrease slightly for both coarse- and fine-grained steels while the ductility increases significantly, especially for coarse-grained steels. Microstructural observations indicate that, at lower strains, an increase in Mn content suppresses the formation of twins, resulting in a lower twinning rate; however, at intermediate and higher strains, even higher twinning rate is observed in higher-Mn steels. In addition, increasing Mn content enables deformation twins to form in a wider range of strain. Such a varying twinning behavior with Mn content is even more apparent for coarse-grained steels. Furthermore, the sizes of twins vary with Mn content and also with grain size: both the thickness and spacing of twins or twin bundlers increase with increasing Mn content but decrease with refining grain sizes. Moreover, increasing Mn content decreases strain-hardening rate, but allows persistent strain hardening to higher strains. These strain-hardening behaviors and tensile properties observed in the twinning-dominated Fe–Mn–C steels with different Mn contents are correlated well their microstructural evolutions.

In this letter, the fracture absorption energies of a carbide-free bainitic steel at different loading rates were investigated using three-point bending test. The results show that the crack initiation, propagation and total absorption energies of U-notched samples of the steel increase significantly with increasing the loading rate. In comparison, for pre-cracked samples, although the crack initiation energy almost does not change with loading rate, the propagation and total absorption energies remain to increase with raising the loading rate. These observations are analyzed based on the loading-rate sensitivity of strain-induced martensitic transformation and also its twofold roles in fracturing of the steel.

This paper deals with the cyclic deformation behavior and microstructural evolution in a low-carbon carbide-free bainitic steel with two different microstructures. Low-cycle fatigue tests were performed at room temperature at various strain amplitudes under total strain control. The variations of the amount of retained austenite and the substructures versus the number of fatigue cycles were evaluated by the X-ray diffraction technique and electron microscopy. Fatigue test results demonstrate that the two microstructures exhibit very similar cyclic stress responses, i.e. initial cyclic hardening followed by cyclic softening or by cyclic saturation and softening till failure, depending on the strain amplitude applied. Parametric studies of the microstructure–property relationship indicate that the major cause for the initial cyclic hardening is neither martensitic transformation nor increased dislocation density. Based on these results and considering the initial high density of dislocations, which are pre-existent and mobile in the starting microstructure and which are entangled, rearranged or annihilated with cycling, the mechanisms responsible for the initial cyclic hardening followed by softening are analyzed. Keywords: Low-cycle fatigue, Cyclic deformation, Carbide-free bainitic steel, Transformation-induced plasticity

The influence of Al on fatigue crack growth (FCG) behavior of the high-Mn austenitic twinning-induced plasticity (TWIP) steel was investigated by conducting FCG tests on Fe–22Mn–0Al–0.6C and Fe–22Mn–3Al–0.6C TWIP steels (hereafter, referred to as 0Al and 3Al TWIP steel, respectively). The FCG tests were performed at stress ratio of 0.1 under the control of stress intensity factor range using three-point bending specimens. Excepting that the traditional two-dimensional (2D) observation methods (optical, scanning and transmission electron microscopes) were used to observe the crack paths, fracture surfaces and microstructure features, a high-resolution synchrotron X-ray computed tomography was also applied to observe the three-dimensional (3-D) crack morphology. The results indicate that the FCG resistance of the 0Al TWIP steel is superior to that of 3Al TWIP steel in the near threshold regime. Observed from the 2D crack paths and 3D crack morphologies, it can be found that the crack surface roughness and crack deflection of the 0Al steel are greater than those of 3Al steel. It is suggested that the degree of roughness-induced crack closure decreases with the addition of Al. And the 0Al steel shows much larger plastic zone sizes ahead of the crack tip than the 3Al steel, suggesting that plasticity-induced crack closure may also play an important role in decreasing the FCG rate in the 0Al steel. By excluding the crack closure effects, the 0Al steel still exhibits a higher effective crack growth threshold value than the 3Al steel; this is considered to be due to the higher planarity of slip in the 0Al steel than in the 3Al steel, and the mechanical twins generated in the 0Al steel reduce the stress concentration at crack tip.

The microstructure and the mechanical properties of a low-carbon Al/Si-alloyed carbide-free bainitic steel austempered at temperatures between 300 and 350 °C have been investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, x-ray diffraction analysis, and mechanical property tests. The results show that an excellent combination of tensile strength, ductility, and impact toughness is obtained at the austempering temperature of 320 °C—but neither at the lowest temperature of 300 °C nor at the highest temperature of 350 °C. These results are correlated with the features of the constituent phases of the microstructure, especially the amount and size of retained austenite, which are largely dependent on the austempering temperature. The observations are also due to the inconsistent effects of strain-induced martensitic transformation on strength, ductility, and toughness. The results of the present study suggest that an optimized isothermal temperature may also exist for any other bainitic steel, at which, if austempering treatment is carried out, an improved combination of tensile and impact properties can be obtained.

The fatigue crack growth behavior of a coarse-grained (CG) and a fine-grained (FG) high manganese austenitic twin-induced plasticity (TWIP) steel has been investigated at room temperature. Crack growth tests were performed at stress ratios of 0.1 and 0.6 under the control of stress intensity factor range using three-point bending specimens. The results indicate that at the two stress ratios, the CG steel exhibits a higher fatigue crack growth resistance than the FG steel in both the near threshold and Paris regimes. Furthermore, a decreased stress ratio and an increased grain size both lead to an increased fatigue crack growth threshold. Microstructural observations reveal that cracks propagate more tortuously in the CG steel than in the FG steel, accompanied by rougher fracture surfaces, which tends to generate more roughness-induced crack closure and thus a higher fatigue threshold value. Additionally, the CG steel shows much larger plastic zone sizes ahead of the crack tip than the FG steel, suggesting that plasticity-induced crack closure may also play an important role in decreasing the fatigue crack growth rate in the CG steel. By excluding the crack closure effects, the CG steel still demonstrates a higher effective crack growth threshold than the FG steel; this is considered to be due to the increased planarity of slip in the CG steel, as compared with that in the FG steel.

The monotonic tensile properties and deformation mechanisms of Fe–Mn–C twinning-induced plasticity (TWIP) steels have been extensively studied; however, the low-cycle fatigue (LCF) properties of this series of advanced steels have not been well understood. The present paper addresses the cyclic deformation behavior and the deformed microstructure of an as-annealed TWIP steel. Fully reversed push–pull LCF tests were performed at room temperature under total strain amplitude control with a strain rate of 0.006 s −1 and strain amplitudes ranging from 0.002 to 0.01. The results show initial rapid cyclic hardening within the initial 10% of the fatigue life at all strain amplitudes, and demonstrate an obviously enhanced cyclic yield strength. Different types of cyclic stress responses were revealed, which are featured by initial cyclic hardening followed by cyclic saturation, or followed by cyclic softening and saturation, or followed by cyclic softening without saturation till the final fracture, depending on the strain amplitude applied. The microstructure prior to and after fatiguing were examined by means of optical and transmission electron microscopy. The typical optical microstructure of fatigued samples is characterized by increases in slip band density with increasing strain amplitude or number of cycles at a given strain amplitude applied. The substructures of the deformed samples are featured by the formation of stacking faults and vein/labyrinth dislocation structures, while fine twins and cell or wall dislocation structures, besides those generated at lower strain amplitudes, are formed at high strain amplitudes.

The tensile and impact properties of two low-carbon bainitic transformation-induced plasticity (TRIP) steels, one being Al-free and the other being Al-containing, have been investigated. The two steels are featured by the existence of different sizes and shapes of retained austenite showing different mechanical stabilities. Compared with the Al-containing steel, the Al-free steel shows significantly improved tensile strength, uniform strain and total elongation, but exhibits much decreased impact toughness. These results demonstrate conflicting effects of strain-induced martensitic transformation of retained austenite on tensile ductility and impact toughness of TRIP steels. It is indicated that a wide distribution of size and shape of retained austenite is favorable for sustained high strain hardening rate and increased uniform tensile strain; however, thin films of retained austenite, which is mechanically more stable, tends to enhance impact toughness.

La(Fe, Si)13-based compounds are well-known magnetocaloric materials, which show a pronounced negative thermal expansion (NTE) around the Curie temperature but have not been considered as NTE materials for industrial applications. The NaZn13-type LaFe13-xSix and LaFe11.5-xCoxSi1.5 compounds were synthesized, and their linear NTE properties were investigated. By optimizing the chemical composition, the sharp volume change in La(Fe, Si)13-based compounds was successfully modified into continuous expansion. By increasing the amount of Co dopant in LaFe11.5-xCoxSi1.5, the NTE shifts toward a higher temperature region, and also the NTE operation-temperature window becomes broader. Typically, the linear NTE coefficient identified in the LaFe10.5Co1.0Si1.5 compound reaches as much as -26.1 × 10(-6) K(-1), with an operation-temperature window of 110 K from 240 to 350 K, which includes room temperature. Such control of the specific composition and the NTE properties of La(Fe, Si)13-based compounds suggests their potential application as NTE materials.

The damage of a Hadfield steel crossing in a railway turnout due to wheel rolling impact passages is investigated numerically. A three dimensional elastic–plastic finite element model has been developed by taking into account non-linear material properties, dynamic wheel impact and wheel–crossing contact. The effects of wheel velocity on the damage and deformation of the crossing is analysed. In the present study, four rolling velocities of the wheel, i.e., 50 km/h, 100 km/h, 150 km/h, and 200 km/h were chosen. The wheel exhibits elastic material behaviour, whereas the crossing exhibits elastic–plastic material behaviour. To represent the damage and deformation behaviours of the crossing, non-linear dynamic work hardening is further included in the calculation. Dynamic stress and strain in the crossing were obtained by the explicit finite element method, including the maximum von Mises stress, the maximum strain, the maximum horizontal stress, perpendicular stress and parallel stress. The snake-like motion of the wheel rolling on the crossing was verified by the simulation; in the meantime, there is a significant impact load on the nose rail, which caused a big von Mises stress in the nose rail. The big von Mises stress is the main reason for the nose rail fatigue spalling. The maximum von Mises stress and maximum strain in the nose rail increase with the train speed increasing. The study provides a method for the further research on the fatigue and the wear behaviours of railway crossings.

The serrated flow behavior of a Fe–Mn–C twin-induced plasticity steel was investigated at room temperature. Tensile tests were performed at various strain rates under extensometer-measured strain control, as compared to conventional crosshead displacement-controlled tests. Different types of serrations were observed with type C ones identified for the first time.

The purpose of the paper is to clarify the serrated flow behavior and to explore the grain-size and strain-rate effects on serrations in coarse- and fine-grained FeMnC twin-induced plasticity (TWIP) steels at room temperature. Tensile tests were performed under extensometer-measured strain control, rather than under conventional cross-head displacement control, at strain rates ranging from 6 × 10−6 to 6 × 10−3 s−1. The results indicate that both the coarse- and fine-grained steels show different types of serrations, which depend on strain rate, and demonstrate a nonmonotonic strain-rate sensitivity of the critical strain for the onset of serrations, i.e., a positive strain-rate sensitivity of the critical strain in the high strain-rate region typified by type A serrations, and a negative strain-rate sensitivity in the low strain-rate region typified by type C ones. As compared to the coarse-grained steel, the fine-grained steel increases the critical strains, showing an inverse grain-size effect. The findings suggest that the fine-grained FeMnC TWIP steel suppresses the serrated flow, possibly due to the enhanced dynamic recovery associated with the decreased planar slip length in the fine-grained low stacking-fault-energy steel.

A low carbon, low alloy steel has been investigated for producing low carbon carbide-free bainitic microstructure by co-addition of alloying elements of aluminum and silicon. The influence of heat treatment process on microstructure, impact toughness as well as tensile properties was investigated by light optical microscopy, transmission electron microscopy, X-ray diffraction and mechanical property tests. The results demonstrate that the co-addition of aluminum and silicon in the investigated steel plays an effective role in suppressing the precipitation of cementite. A desired microstructure consisting of mainly fine-scale carbide-free bainitic ferrite and thin film-like retained austenite located between the ferrite laths was obtained and accordingly an excellent combination of toughness, ductility and strength was achieved by optimized heat treatments, i.e. by isothermal treatment at 320 °C for ∼84 min or more. The microstructure-mechanical property relationships are discussed.

Hydrogen embrittlement and secondary cracks of bainitic steels were studied by means of the slow strain rate test (SSRT), in situ tension in transmission electron microscopy (TEM) analysis and scanning electron microscopy (SEM). The results showed that the microstructure of the bainitic steels became finer, the phase interfaces as irreversible hydrogen traps significantly were increased, the nano-scale carbides were precipitated from retained austenite, and hydrogen embrittlement was decreased greatly with Al addition. Lots of secondary cracks were formed with Al addition. The stress concentration was relaxed and the hydrogen embrittlement was reduced significantly because of the presence of secondary cracks. Due to hydrogen, the dislocation emission and motion were enhanced and the formation of secondary cracks was reduced.

Samples of Hadfield steel, high manganese austenite steel with 13 wt% manganese and 1.2 wt% carbon, were solidified under a pressure of 6 GPa. The microstructures of the samples were analyzed by metallography and X-ray diffraction. The results indicate that the solidification microstructure of the Hadfield steel was remarkably refined under high pressure. Additionally, the carbide of M23C6 was obtained in the Hadfield steel solidified under high pressure was different from the carbide of M3C obtained by solidification under normal pressure. Furthermore, high pressure promoted the formation of orientational solidified microstructure of the Hadfield steel.

Stress/strain analysis is the key to understanding and predicting wear and fatigue behavior of a crossing. By taking into account non-linear material properties, a three dimensional elastic–plastic finite element model, which is composed of wheel, crossing and ties, is presented for the simulation of stress/deformation in a railway crossing. The influences of dynamic wheel load and wheel–crossing contact are examined. Stress, plastic strain and vertical displacement of the simulated crossing under dynamic wheel load at different wheel–crossing contact positions are investigated. The maximum vertical displacement occurs at the wheel–crossing contact region, and decreases gradually from the wheel–crossing contact region to the toe-end and heel-end of the crossing. The maximum von Mises stress and maximum equivalent plastic strain in the crossing increase remarkably with the increase of the train speed. The maximum vertical displacement in the crossing increases obviously and varies linear approximately with the train speed. The maximum von Mises stress, maximum vertical displacement and maximum equivalent plastic strain in the crossing are very sensitive to the axle load and are linear approximately with the axle load.

Three expanded graphites, EG1, EG2 and EG3 that were prepared by rapid heating expandable graphite to 600, 800 and 1000 °C, respectively, were ball-milled in a high-energy mill (planetary-type) in an air atmosphere with 16.7% iron powder addition. The X-ray diffraction patterns of the products show that the amorphization process of the powder EG-Fe mixtures during ball-milling is accelerated with increasing the heating temperature of expandable graphite. In addition, amorphous EG-Fe system and Fe3C phase are formed in the ball-milled EG3-Fe. Compared with the amorphization process of EGs without iron addition during ball-milling, the presence of iron restrains the process of EG1 in rate, but it promotes the process of EG3.

A process primarily based on carburization and successive low-temperature austempering, was proposed for the generation of low-temperature bainitic microstructure in the surface layer (∼2.5 mm in thickness) of low-carbon steel. For a potential use in the manufacture of heavy-duty gears, wear properties of this type of microstructure were studied. Comparisons were made with carburized commercial steel of 20CrMnTi subjected to quenching followed by tempering, which is widely used in the area of gear. The result showed that the experimental steel and the control steel present different wear mechanisms under the same sliding wear process. The low-temperature bainitic steel exhibits an excellent wear resistance as a consequence of the following three main factors: (i) high strength plus high toughness, (ii) carbon enriched austenite, and (iii) extremely fine α-phase microstructure. The low-temperature bainitic microstructure exhibits high strength and high toughness due to the refinement of the microstructure. Secondly, the film-like carbon enriched austenite (∼30 nm in thickness) uniformly distributed between the carbide-free bainitic ferrite plates can retard the crack propagation during sliding friction and play an advantageous role in wear resistance. Lastly, an extremely fine single α-phase microstructure was promptly induced in the dry sliding friction of the surface layer of the low-temperature bainitic steel; such fine refined microstructure lead to an increase in hardness, which can improve wear resistance.

A novel surface modifying technique, high speed pounding (HSP), has been developed to prepare a prehardened layer. By optimizing the HSP parameters an ideal prehardened layer 16 mm in depth, with a maximum surface hardness of ∼50 HRC and a gradual decrease in hardness from the surface to the matrix was successfully fabricated on Hadfield steel. Practical service tests demonstrated that the service lifetime of Hadfield steel railway crossings prehardened by HSP is more than triple that without prehardened treatment.

Phase boundaries play significant roles in the crack initiation and propagation behavior of duplex phase materials such as nanoscale bainite–austenite ( α / γ ) microstructures. Simulation of α / γ -iron phase boundary interacting with crack propagation was carried out using molecular dynamics method. Bi-phase systems consisting of α -iron with body-centered cubic structure and γ -iron with face-centered cubic structure, were constructed. Phase boundaries were created using Nishiyama–Wasserman and Kurdjumov–Saches (K–S) orientation relationships, respectively. Eight orientations of cracks in α - or γ -phase were analyzed to illustrate the orientation dependence of crack growth. Finnis–Sinclair potential for Fe fit by Mendelev was used to describe interatomic potentials. Tensile load was applied normal to the crack surface. Simulation results reveal that in most cases phase boundary provides an obstacle to slip and dislocation motion during crack propagation, except in two orientations with K–S relation. New-cracks always nucleate phase boundary as a result of slip blocking and dislocation pileups. Bcc/fcc iron phase boundary is therefore demonstrated to be a great contribution to the strengthening of nanoscale duplex steel.