On the causes of formation of non-optimal structures in a pressure-treated low-carbon steel

An electron microscopy investigation of structural-phase and stress-strained state of a hot-forged material workpiece is performed. It is found out that the scalar dislocation density in ferrite grains and ferrite interlayers of pearlite of steel from the forged piece fractured after technological processing is one and half times higher than that in the commercial material. The metal in this state has a higher content of sulfides of lamellar morphology. The volume fraction of pearlite in it is 1.5–2 times larger, with the lamellar pearlite prevailing, and the local long-range stresses being comparable with the yield stress. It is found out that the reason for formation of an unfavorable structural-phase state is the elevated carbon content, which resulted in overheating of the metal both during pressure treatment and final thermal treatment. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 8–12, October, 2007.     

Phase transformation behavior of the SMA490BW weathering steel

In order to optimize the heat treatment schedule and the welding procedure of the SMA490BW weathering steel, a precise determination of the continuous cooling transformation and simulated heat affected zone continuous cooling transforming diagrams were carried out. Meanwhile, the hardnesses of the specimens with different cooling rates were measured and the microstructures were observed. The microstructures are composed of pearlite, polygonal ferrite, granular structure, acicular ferrite, and lath bainite depending on the cooling rates and transformation temperature. The experiment results provide a useful guide for the heat treatment and welding of this steel.     

Mechanism of solid-state eutectoid transformation in an aluminium-zinc alloy

The growth rate of pearlite during isothermal transformation of an aluminium-zinc eutectoid has been determined experimentally. Different theoretical models, assuming either volume or boundary diffusion of zinc to be a rate controlling mechanism for the eutectoid transformation, have been applied. With volume diffusion models, the calculated growth rates were lower than the experimental growth rates by a factor of three orders of magnitude. Reasonable agreement between the calculated and experimental growth rates has been obtained on applying the boundary diffusion models. The activation energy for boundary diffusion of zinc in the aluminium-zinc alloy was estimated to be ≅11.6 kcals/mole.     

Electron microscopic study of austenite microtwins and their influence on the crystallographic features of the pearlitic transformation

The crystallographic relationship between the structural components in high-carbon 120G4 steel after a partial isothermal pearlitic transformation has been studied by TEM. It has been found that a high density of microtwins and stacking faults was observed in the retained austenite. A new type of crystallographic relationship has been detected—parallelism of close-packed directions and planes of the structural components of fine-plate pearlite to one of the variants of twinned austenite.     

Phase stresses induced by the γ → α transformation in an iron polycrystal

The evolution of the phase stresses induced in the course of the γ ? α transformation in polycrystalline iron is analyzed in the framework of the elastoplastic model of a spherical inclusion. The isolated regions of the α phase (ferrite) and the γ phase (austenite) are treated as inclusions at the initial and final stages of the transformation, respectively. The stresses are calculated with due regard for the plastic flow in a spherical layer (matrix) around these inclusions. The calculated dependence of the hydrostatic phase stresses on the volume fraction of ferrite suggests that these stresses retard the initial stage and promote the final stage of the transformation.     

The defining role of interface crystallography in corrosion of a two-phase pearlitic steel

ABSTRACT

This study involved thermomechanically processed fine (few hundred nanometers of interlamellar spacing) pearlite wire rods of the different axial alignment of the pearlite colonies, and coarse (several micron interlamellar spacing) pearlite colonies. In the former, appropriate microstructural tailoring, and corresponding axial alignment, reduced the corrosion rate, in chloride solution, by nearly 6.4 times. In the coarse pearlite, on the other hand, dissolution and aqueous corrosion, influenced by microgalvanic coupling, was shown to be restricted to the ferrite side of the ferrite-cementite interface. The orientation relationship between ferrite and cementite determined localised corrosion. In summary, remarkable improvements in the resistance to galvanic corrosion were shown, in coarse two-phase pearlite, by enhancing the population of good-fit interfaces. Though the same observation was not possible, experimentally, in the fine pearlite colonies, the remarkable improvement in the corrosion resistance of aligned pearlite wire rods appears real and extremely reproducible.     

Experimental and theoretical study of the formation and growth of pearlite colonies in eutectoid steels

Optical and electron-microscopy observations of pearlite structures in eutectoid steels are described. These observations suggest that in such steels the processes of sidewise growth of pearlite colonies along grain boundaries may occur even though they were not observed in noneutectoid steels. A simple theoretical model is proposed to study the thermodynamics and kinetics of pearlite transformations. Simulations of the growth of pearlite colonies carried out on this model reveal that, for the usually assumed mechanism of volume diffusion of carbon, such growth is always unstable, and the steady-state growth can only be realized via interfacial carbon diffusion. A model is proposed for the formation of pearlite colonies near the grain boundaries of austenite. This model is based on the assumption that the diffusion of carbon is strongly enhanced near these boundaries, and it can be applied to plastically deformed steels. The results of simulations with this model qualitatively agree with some microstructural features of the formation of pearlite colonies observed in such steels.     

Solid-solid phase transformation via virtual melting significantly below the melting temperature

A new phenomenon is theoretically predicted, namely, that solid-solid transformation with a relatively large transformation strain can occur through virtual melting along the interface at temperatures significantly (more than 100 K) below the melting temperature. The energy of elastic stresses, induced by transformation strain, increases the driving force for melting and reduces the melting temperature. Immediately after melting, the stresses relax and the unstable melt solidifies. Fast solidification in a thin layer leads to nanoscale cracking, which does not affect the thermodynamics and kinetics of solid-solid transformation. Seven theoretical predictions are in quantitative agreement with experiments conducted on the beta-->delta transformation in the HMX energetic crystal.     

On the molecular mechanism of the crystal transformation (tetragonal-hexagonal) in polybutene-1

The phase transformation from the tetragonal to the hexagonal crystal modification in highly oriented lamellae of poly-butene-1 has been followed by transmission electron microscopy (TEM). It is found that the reaction-controlling step is the nucleation process. No lattice orientation relationship (besides the [001]-direction, which is parallel in both crystal modifications) exists between non-transformed and transformed crystals. The nucleation is strongly enhanced by thermal or external stresses. Crystal growth, nucleated by external stresses, was observed at temperatures as low as — 150°C. The molecular mechanisms of the transformation are discussed.     

不同应力场中马氏体形核的择优取向

系统地模拟计算了不同方位四方相ZrO₂(T)椭球片分别在拉、压、剪应力诱导下发生ZrO₂(T)→ZrO₂(M)马氏体相变时系统应变能的变化.并用能量极小准则确定了有利的形核方位.结果表明相变中伴随较大的剪切变形将使马氏体形核呈现明显的择优取向;不同应力场诱导相变的临界应力相差很大. 关键词：     

Magnetic-field-induced microstructural features in a high carbon steel during diffusional phase transformation

In this work, a high purity, high carbon steel was heat treated without and with a 12-T magnetic field. The microstructural features induced by magnetic field during its diffusion-controlled austenite decomposition were investigated by means of optical microscopy and SEM/EBSD. It is found that the magnetic field increases the amount of the abnormal structure, which is composed of proeutectoid cementite along the prior austenite boundaries and ferrite around it, because magnetic field increases the austenite grain size and promotes the transformation of carbon-depleted austenite to ferrite. No specific orientation relationship between abnormal ferrite and cementite has been found in the non-field- or the field-treated specimens. Magnetic field evidently promotes the spheroidization of pearlite, due to its effect of enhancing carbon diffusion through raising the transformation temperature and its effect of increasing the relative ferrite/cementite interface energy. As magnetic field favors the nucleation of the high magnetization phase-pearlitic ferrite, the occurrence of the P-P2 OR that corresponds to the situation that ferrite nucleates prior to cementite during pearlitic transformation is enhanced by the magnetic field.     

Effects of N and B on continuous cooling transformation diagrams of Mo–V–Ti micro-alloyed steels

Effects of the single addition of nitrogen (N) and boron (B) and the combined addition of N and B on continuous cooling transformation (CCT) diagrams and properties of the three Mo–V–Ti micro-alloyed steels were investigated by means of a combined method of dilatometry and metallography. Microstructures observed in continuous cooled specimens were composed of pearlite (P), quasi-polygonal ferrite (QPF), granular bainite (GB), acicular ferrite (AF), lath-like bainite (LB) and martensite (M) depending on the cooling rates and transformation temperatures. Single addition of 12?ppm B effectively reduced the formation of QPF and broadened the cooling rate region for LB and M. Added N makes the action of B invalid and the QPF region was prominently broadened, and even though the cooling rate is higher than 50°C?s^?1, it cannot obtain full bainite.     

Generation and relaxation of reactive stresses in Cu-Al-Ni shape-memory alloy

Reactive stresses in Cu-Al-Ni shape-memory single crystals are experimentally determined on constrained samples heated at a constant rate in the temperature range 293–800 K. At temperatures up to 600 K, the stresses increase with temperature. At higher temperatures, they begin to decrease as a result of the decomposition of the β-phase and vanish at 800 K. The theory of diffuse martensitic transformation is used to calculate the reactive stresses, including the case when the volume fraction of the β-phase decreases, at temperatures above 600 K.     

Alternate stresses and temperature variation as factors of influence of ultrasonic vibration on mechanical and functional properties of shape memory alloys

It is known that the main factors in a variation in the shape memory alloy properties under insonation are heating of the material and alternate stresses action. In the present work the experimental study of the mechanical behaviour and functional properties of shape memory alloy under the action of alternate stresses and varying temperature was carried out. The data obtained had demonstrated that an increase in temperature of the sample resulted in a decrease or increase in deformation stress depending on the structural state of the TiNi sample. It was shown that in the case of the alloy in the martensitic state, a decrease in stress was observed, and on the other hand, in the austenitic state an increase in stress took place. It was found that action of alternate stresses led to appearance of strain jumps on the strain–temperature curves during cooling and heating the sample through the temperature range of martensitic transformation under the constant stress. The value of the strain jumps depended on the amplitude of alternate stresses and the completeness of martensitic transformation. It was shown that the heat action of ultrasonic vibration to the mechanical behaviour of shape memory alloys was due to the non-monotonic dependence of yield stress on the temperature. The force action of ultrasonic vibration to the functional properties was caused by formation of additional oriented martensite.     

Size and temperature dependent stability and phase transformation in single-crystal zirconium nanowire

A novel size dependent FCC (face-centered-cubic) → HCP (hexagonally-closed-pack) phase transformation and stability of an initial FCC zirconium nanowire are studied. FCC zirconium nanowires with cross-sectional dimensions <20 Å are found unstable in nature, and they undergo a FCC → HCP phase transformation, which is driven by tensile surface stress induced high internal compressive stresses. FCC nanowire with cross-sectional dimensions >20 Å, in which surface stresses are not enough to drive the phase transformation, show meta-stability. In such a case, an external kinetic energy in the form of thermal heating is required to overcome the energy barrier and achieve FCC → HCP phase transformation. The FCC-HCP transition pathway is also studied using Nudged Elastic Band (NEB) method, to further confirm the size dependent stability/metastability of Zr nanowires. We also show size dependent critical temperature, which is required for complete phase transformation of a metastable-FCC nanowire.     

Nature of the structural degradation of rail surfaces during operation

Patterns in the transformation of the structural and phase states and the defect substructure of rail surface layers up to 10 mm thick during long-term operation (gross transit tonnages of 500 and 1000 mln t) are found via optical, scanning, and transmission electron microscopy. According to the nature of the fracture and the degree of defectiveness, three layers can be distinguished: a surface layer, a transition layer, and the base metal. It is shown that the operation of steel rails is accompanied by full fractures in surface layers up to 15 μm thick with lamellar pearlite grains and the formation of ferrite–carbide mixtures with nanosized particles. The deformation of steel increases the densities of scalar and excess dislocations, the curvature–torsion values of the crystal lattice, and the amplitudes of internal stress fields. Structural elements that can act as stress concentrators are identified.     

Cross-sectional geometry dependence of spontaneous phase transformation of copper nanowires

Molecular dynamics simulations have been performed to investigate the spontaneous phase transformation of copper nanowires. It is found that the spontaneous phase transformation exhibits distinct dependence on the cross-sectional geometry of the nanowires and can lead to the reconstruction of atoms into different atomistic configurations, e.g., pure hexagonal-close-packed crystals, fivefold deformation twins, and core/shell structures. For single-crystal copper nanowires, the critical cross-sectional size, above which no spontaneous phase transformation can occur, is determined. The physical mechanisms underlying the complicated transformation behavior are analyzed from the viewpoints of energy and stresses.     

Mechanism of the phase transformation of the pyrochlore phase into the perovskite phase in lead zirconate titanate films on silicon substrates

The phase transformation from the pyrochlore phase into the perovskite phase in ferroelectric films of lead zirconate titanate on silicon substrates due to annealing of samples has been investigated experimentally and theoretically. It has been proved that this transformation is a typical first-order phase transition, which is accompanied by a change in the density of the phases and the release of the latent heat of the phase transition. The quantitative evaluations have demonstrated that the difference in the densities of two phases, namely, the perovskite phase and the original parent pyrochlore phase, leads to the generation of elastic stresses in the original parent phase. In turn, these stresses bring about the nucleation of micropores in the bulk of the lead zirconate titanate film. The thermodynamic conditions providing the formation of micropores have been established and the critical size of the micropores has been calculated. A characteristic relationship between the critical size of nuclei of the perovskite phase and the radius of micropores at which the perovskite phase is separated from the parent pyrochlore phase has been derived. This relationship has been verified experimentally. The sizes of the micropores have been determined using scanning electron microscopy, and the changes in the phase composition during the phase transformation have been found using an electron probe X-ray microanalysis. It has been demonstrated theoretically and experimentally that the relaxation of elastic stresses in the lead zirconate titanate thin films during the phase transition occurs through the nucleation and growth of micropores at the interface between the new and parent phases.     

Phase transition of ZrN under pressure

A first principles constant pressure approach is carried out to probe the high-pressure behaviour of the rocksalt (RS) structured zirconium nitride (ZrN). The existence of first order reconstructive phase transition from the RS crystal to a CsCl-type crystal is, for the first time, established throughout the simulations. Upon decompression, the CsCl type phase converts back to the original RS structure by following the same transformation mechanism, suggesting a reversible phase transformation in ZrN. The RS-to-CsCl phase change is additionally considered through the thermodynamic theorem and projected to take place at around 225?GPa in experiments. The structural parameters and mechanical properties computed are found to be comparable with some of the previous findings. Additionally, we investigate the response of ZrN to uniaxial compression and tension stresses. The uniaxial stresses initially lead to a tetragonal modification of the simulation box having an I4/mmm symmetry and subsequently structural failure that is expected to occurs at about ?10 and 15?GPa in experiments.