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Carbon steel materials are widely used in mechanical transmission. Under different temperature and pressure service conditions, the microscopic changes of stress and strain that are difficult to detect and analyze by experimental means will lead to failure deformation, thus affecting their operational stability and life. In this study, the molecular dynamics method is used to simulate the heating–cooling phase transition process of common carbon steel materials. Austenite transformation temperatures of 980 K (0.2 wt.%) and 1095 K (0.5 wt.%) are acquired which is determined by the volume hysteresis before and after transformation, which is consistent with the results of JMatPro phase diagram analysis. The internal stress state of the material varies between compressive stress and tensile stress due to the change of phase structure, and the dislocation characteristics during the phase transition period are observed to change significantly. Then, an α / γ two-phase interface model is constructed to study the migration of the phase interface and the change of the phase structure by applying a continuously changing external load. At the same time, the transition pressure of α → ϵ is obtained with a value of 37 GPa under three different initial loads showing the independence of the initial load and the historical path. Based on the molecular dynamics simulation and the phase diagram calculation of the carbon steel, the analysis method for the microstructure transformation and the stress–strain behavior of the phase interface under the external load can provide a reference for the design of microstructure and mechanical properties of alloy steel in the future. [ABSTRACT FROM AUTHOR]

The martensitic transformation in pure Fe and its alloys has been studied over many decades. Several theoretical models have been proposed to describe the atomic motion that leads to the fcc-to-bcc martensitic transformation. However, such models do not account for the effect of pre-existing planar defects such as twin boundaries and stacking faults, present in the high-temperature austenite phase prior to the transformation process. This work systematically studies the role of nano-spaced planar faults with different inter-spacing on the martensitic transformation using molecular dynamics simulations. Research shows that the investigated planar defects affect the nucleation and growth mechanisms during martensite formation, the morphology of the resulting microstructure, the specific atomic path leading to the phase transformation, and the martensite start temperatures. Martensite variants were identified by the analysis of the atomic shears and slip systems during the transformation process. A crystallographic analysis is done to explain the existence of different shear mechanisms of martensite transformation at different locations in the fcc austenite. The present investigation provides fundamental insights into the martensitic transformation process in presence of pre-existing planar defects and can be applied to other material systems, e.g., Fe alloys. [ABSTRACT FROM AUTHOR]

In recent years, several studies have validated the use of piezoelectric materials for in situ biological stimulation, opening new interesting insights for bio-electric therapies. In this work, we investigate the morphological properties of polyvinylidene fluoride (PVDF) in the form of microstructured films after temperature-driven phase transition. The work aims to investigate the correlations between morphology at micrometric (i.e., spherulite size) and sub-micrometric (i.e., phase crystallinity) scale and in vitro cell response to validate their use as bio-functional interfaces for cellular studies. Morphological analyses (SEM, AFM) enabled evidence of the peculiar spherulite-like structure and the dependence of surface properties (i.e., intra-/interdomain roughness) upon process conditions (i.e., temperature). Meanwhile, chemical (i.e., FTIR) and thermal (i.e., DSC) analyses highlighted an influence of casting temperature and polymer solution on apolar to polar phases transition, thus affecting in vitro cell response. Accordingly, in vitro tests confirmed the relationship between micro/sub-microstructural properties and hMSC response in terms of adhesion and viability, thus suggesting a promising use of PVDF films to model, in perspective, in vitro functionalities of cells under electrical stimuli upon mechanical solicitation. [ABSTRACT FROM AUTHOR]

In a previous study, it was shown that the (111)fcc, (110)fcc and (111)bcc free surfaces do not assist the phase transitions as nucleation sites upon heating/cooling in iron (Fe) thin slabs. In the present work, the three surfaces are denoted as "inactive" free surfaces. The phase transitions in Fe thin films with these "inactive" free surfaces have been studied using a classical molecular dynamics simulation and the Meyer–Entel potential. Our results show that shear deformation helps to activate the free surface as nucleation sites. The transition mechanisms are different in dependence on the surface orientation. In film with the (111)fcc free surface, two body-centered cubic (bcc) phases with different crystalline orientations nucleate at the free surface. In film with the (110)fcc surface, the nucleation sites are the intersections between the surfaces and stacking faults. In film with the (111)bcc surface, both heterogeneous nucleation at the free surface and homogeneous nucleation in the bulk material are observed. In addition, the transition pathways are analyzed. In all cases studied, the unstrained system is stable and no phase transition takes place. This work may be helpful to understand the mechanism of phase transition in nanoscale systems under external deformation. [ABSTRACT FROM AUTHOR]

Using molecular dynamics (MD) simulation, the austenitic and martensitic phase transitions in pure iron (Fe) thin films containing coherent twin boundaries (TBs) have been studied. Twelve thin films with various crystalline structures, thicknesses and TB fractions were investigated to study the roles of the free surface and TB in the phase transition. In the austenitic phase transition, the new phase nucleates mainly at the (112)bcc TB in the thicker films. The ( 11 1 ¯ )bcc free surface only attends to the nucleation, when the film is extremely thin. The austenitic transition temperature shows weak dependence on the film thickness in thicker films, while an obvious transition temperature decrease is found in a thinner film. TB fraction has only slight influence on the austenitic temperature. In the martensitic phase transition, both the ( 1 ¯ 10) fcc free surface and (111)fcc TB attribute to the new body-center-cubic (bcc) phase nucleation. The martensitic transition temperature increases with decreased film thickness and TB fraction does not influent the transition temperature. In addition, the transition pathways were analyzed. The austenitic transition obeys the Burgers pathway while both the Kurdjumov–Sachs (K–S) and Nishiyama–Wassermann (N–W) relationship are observed in the martensitic phase transition. This work may help to understand the mechanism of phase transition in the Fe nanoscaled system containing a pre-existing defect. [ABSTRACT FROM AUTHOR]

Using molecular dynamics (MD) simulation, the strain-induced phase transitions in pure body-centered-cubic (bcc) iron (Fe) thin films containing twin boundaries (TBs) with different TB fractions and orientations are studied. Two groups of bcc thin films with different TB-surface orientation relationships are designed. In film group 1, the (112) [ 11 1 ¯ ] TBs are perpendicular to the ( 11 1 ¯ ) free surfaces, while the (112) [ 11 1 ¯ ] TBs are parallel to the free surfaces in film group 2. We vary the TB numbers inserted into the films to study the effect of TB fraction on the phase transition. Biaxial strains are applied to the films to induce the bcc to close packed (cp) phase transition. The critical strain, at which the first phase transition takes place, decreases with the TB fraction increase in film group 1 with a perpendicular TB-surface orientation, while such a relationship is not observed in film group 2 with parallel TB-surface orientation. We focus on the free surface and TB as the nucleation positions of the new phase and the afterward growth. In addition, the dynamics of the phase transition is discussed. This work may help to understand the mechanism of phase transition in nanoscale or surface-dominant systems with pre-existing defects. [ABSTRACT FROM AUTHOR]

Small concentrations of alloying elements can modify the α - γ phase transition temperature T c of Fe. We study this effect using an atomistic model based on a set of many-body interaction potentials for iron and several alloying elements. Free-energy calculations based on perturbation theory allow us to determine the change in T c introduced by the alloying element. The resulting changes are in semi-quantitative agreement with experiment. The effect is traced back to the shape of the pair potential describing the interaction between the Fe and the alloying atom. [ABSTRACT FROM AUTHOR]

Using molecular dynamics simulation, we studied the influence of pre-existing dislocations on the austenitic and the martensitic phase transformations in pure iron. The simulations were performed in a thin-film geometry with (100) surfaces. We found that dislocations alleviate the transformation by lowering the austenitic transformation temperature and increasing the martensitic transformation temperature. In all cases, the new phase nucleates at the dislocations. The orientation relationships governing the nucleation process are dominated by the Burgers, Kurdjumov–Sachs, and Nishiyama–Wassermann pathways. However, upon growth and coalescence of the transformed material, the final microstructure consists of only few twinned variants separated by twin boundaries; this simple structure is dictated by the free surfaces which tend to form conserved planes under the transformation. After transformation, the material also contains abundant dislocations. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations have been used to study the effects of different orientation relationships between fcc and bcc phases on the bcc/fcc interfacial propagation in pure iron systems at 300 K. Three semi-coherent bcc/fcc interfaces have been investigated. In all the cases, results show that growth of the bcc phase starts in the areas of low potential energy and progresses into the areas of high potential energy at the original bcc/fcc interfaces. The phase transformation in areas of low potential energy is of a martensitic nature while that in the high potential energy areas involves occasional diffusional jumps of atoms. [ABSTRACT FROM AUTHOR]

Using molecular-dynamics simulation, we study the austenitic and martensitic phase transformation in Fe–C nanowires with C contents up to 1.2 at%. The transformation temperatures decrease with C content. The martensite temperature decreases with wire diameter towards the bulk value. During the transformation, the bcc and fcc phases obey the Kurdjumov–Sachs orientation relationship. For ultrathin wires (diameter D ⩽ 2.8 nm), we observe wire buckling as well as shape-memory effects. Under axial tensile stress the martensite transformation is partially suppressed, leading to strong plastic deformation. Under the highest loads, the austenite only partially back-transforms while the crystalline phases in the wire re-orient giving the multi-phase mixture a high tensile strength. [ABSTRACT FROM AUTHOR]