Papers by Keyword: Orientation Relationship

Abstract: Phase boundary is an important kind of interfaces for the dual-phase metals. Orientation relationship (OR) is a crucial factor affecting the performance of the phase boundaries and the dual-phase metals. Molecular dynamics simulation is performed to examine the structures of the bcc/fcc iron phase boundaries in Nishiyama–Wassermann (N–W) and Kurdjumov–Sachs (K–S) orientation relationships, and the performances of the dual-phase model with these ORs. The structural relaxation shows that the phase boundary with N-W relation has the lower energy than that in K-S relation. Stress-strain curves show that dual-phase model in N-W relation has the higher stiffness and strength than that in K-S relation. Simulation results show that phase boundary in N-W relation has a more exellent performance, and is preferred to be processed in heat treatment.

Abstract: We studied the crystallographic features, especially the orientation relationship with respect to austenite, of martensite in a steel transformed from coarse-grained equiaxed austenite (35 μm), ultrafine-grained equiaxed (2.5 μm) or lamellar (300nm) austenite fabricated by sever plastic deformation. With decreasing the grain size of equiaxed austenite, the orientation relationship changed from Kurdjumov - Sachs relationship to Greninger - Troiano relationship. We inferred that this change of orientation relationship could be attributed to the small size of martensite plate transformed from the ultrafine-grained equiaxed austenite. The martensite transformed from the ultrafine-grained lamellar austenite did not have a definite orientation relationship with austenite. We considered that a high density of dislocations or a high density of low angle boundaries within the ultrafine lamellar austenite grain resulted in the large deviation of orientation relationship.

Abstract: There are four prominent orientation relationships (ORs) between directionally grown precipitates and their parent phases in steel. They are ORs between ferrite precipitate and parent austenite (the Kurdjumov and Sachs OR), between orthorhombic cementite precipitate and parent austenite (the Pitsch OR), between cementite precipitate and parent ferrite (the Bagaryatski OR) and between hexagonal molybdenum carbide precipitate and parent ferrite (the Dyson et al. OR). The directed precipitation occurs at low transformation temperatures. The ORs have been explained by the directed growth model. The solid phase transformation of a metastable phase into a stable phase needs the activation energy. The energy is usually supplied in the form of thermal energy. When the nucleation takes place, the strain energy may develop in the stable nucleus and the metastable matrix. The strain energy can result from a difference in density between the nucleus and matrix and the lattice mismatch along the nucleus:matrix interface. The fundamental concept of the model is that the maximum growth rate of precipitate is along the direction that generates the maximum strain energy and the interface energy is minimized. The four ORs are determined, based on the concept, such that the mismatch along the interface between the minimum shear modulus planes of precipitate and its parent phase that are parallel to the maximum Young’s modulus direction of the precipitate is minimized.

Abstract: The thermal cycling (quenching in liquid nitrogen and reverting back to room temperature: austenite martensite reversible transformation) response of Ni-Ti-Fe shape memory alloys has been investigated. It was clearly noted that residual deformation, estimated in terms of noticeable differences in austenite grain size, depend on the relative clustering of fine grains. During repeated thermal cycling, the residual deformation, in-grain misorientation developments and retained martensite content scaled together: bringing out a clear picture of microstructural irreversibility.

Abstract: Superaustenitic stainless steels exhibit excellent corrosion resistance, at a wide variety of exposure temperatures, especially in chloride containing environments, coupled with desirable mechanical properties. Previous studies have shown that these steels are prone to precipitation of secondary phases, such as sigma phase (σ), chi phase (χ), Laves-phase, carbides, nitrides or secondary austenite, when exposed at elevated temperatures, directly affecting their mechanical properties and corrosion behaviour. A detailed study of the effect of isothermal ageing on the microstructure of S32654 (Fe-24Cr-21Ni-7Mo-0.5N-0.013C) and S31254 (Fe-20Cr-18Ni-6Mo-0.2N-0.012C) superaustenitic stainless steels was carried out. Samples were aged within the temperature range of 650 ^οC to 950 ^οC for times up to 3000 h. Following ageing, precipitation of secondary phases was clearly observed with precipitates varying in volume fraction, size, shape and spatial distribution. Several secondary phases were identified via transmission electron microscopy (TEM) and electron diffraction (ED). The orientation relationships between the austenitic matrix and the secondary phases were identified. Interaction and also phase transformation among different types of precipitates, such as between precipitates and the austenitic matrix were observed and an attempt of understanding these phase transformations was carried out.

Abstract: The excellent mechanical properties of austempered ductile iron are due to the unique matrix microstructure called “ausferrite”. Such microstructure is obtained from the first stage reaction during isothermal transformation of austenite into ferrite and high carbon austenite. The second stage reaction is the decomposition of high carbon austenite into ferrite and carbide. In this study, the microstructure of Cu-alloyed ductile iron treated by two-step austempering was investigated using SEM and TEM. The two-step austempering consists of 1) quenching from 900°C to 300°C and hold for 6 minutes in a salt bath, and 2) increasing the temperature of the salt bath by 30°C at the rate of 3°C per minute and holding until the total time was 120 minutes. The SEM samples were prepared by grinding to 1200 grit, polishing with 0.3 micron alumina powder and etching using 2% nital. Thin foil discs of 3 mm diameter for TEM observation were prepared by mechanical thinning to 80-100 microns and were then dimpled to 20 microns. Final thinning was carried out using Gatan precision ion polishing system (PIPS^TM). The thin foils were then examined using Jeol-JEM2010 operating at 200 kV. TEM results show that the matrix microstructure is ferrite subunits interspersed with retained austenite films. The subunits and austenite films exhibit either Kudjamov-Sachs (K-S) or Nishiyama-Wasserman (N-W) orientation relationships. Occasionally, fine carbide precipitate was observed.

Abstract: Low dimensional nanostructures, e.g. nanowires, self-assembled through heteroepitaxy, present a variety of crystallographic features that do not always follow conventional V-W or S-K growth mode. Applying Δg parallelism rules and edge-to-edge matching (E2EM) model in β-DySi₂/Si and CoSi₂/Si systems provides a better understanding of the natural preference of the interface orientation and the orientation relationship (OR) during heteroepitaxial growth. This may help improving the quality of nanowires through optimizing the substrate orientation.

Abstract: The motion of steps during the growth of hydride precipitates has been observed by in situ transmission electron microscopy. Precipitates in different orientation relationships (OR) are shown to obey to the rules of three-dimensional edge-to-edge matching. They form clusters in order to realize a more isotropic distribution of the volume expansion, and to decrease their total elastic energy.

Abstract: The nucleation of bcc ferrite precipitates at austenite grain corners in a Co-15Fe alloy was studied by serial sectioning coupled with electron backscatter diffraction (EBSD) analysis. Grain corners were identified by recombination of triple points and triangular annihilation, whereas quite a few precipitates were surrounded by more than four matrix grains when twins were counted as individual grains. More than 40% of corners composed all of high angle grain boundaries were vacant at an undercooling of ~60°C from the g/(a+g) phase boundary. All the precipitates had K-S or N-W orientation relationship with at least one grain and a larger proportion of them had the OR with two and three grains. For half of vacant corners a hypothetical precipitate could have the OR with more than one grain. It is likely that not only the misorientations among the matrix grains, but also the orientations of the grain boundary planes have a major influence on nucleation potency even at grain corners.

Abstract: The preference of the habit planes (HPs) developed from precipitation in the fcc/bcc system has been investigated. The interfacial energy of different interface orientations has been examined with variation of the orientation relationships (OR) and lattice parameters by a classical molecular dynamics method. The results show that interface has the lowest interfacial energy when it contains parallel Burgers vectors and a set of dislocations. The local minimum of interfacial energy may not associated with a maximum of dislocation spacing. It is also found that the near Kurdjumov-Sachs OR is more preferable than the near Nishiyama-Wasserman OR. Contrary to the previous interfacial energy calculations, which usually limit to rational ORs, the present work allows ORs to be irrational, which agrees with the observations.