Kinds Of Phase Transformation Research Articles

Microstructure evolution, crack initiation and early growth of high-strength martensitic steels subjected to fatigue loading

Fatigue loading induced microstructure evolution featured by Fine Granular Area (FGA) in high-strength martensitic steels have close ties to the fatigue crack initiation and early growth, which are worthy of being investigated. On this issue, we conducted Very High Cycle Fatigue (VHCF), High Cycle Fatigue (HCF) and Low Cycle Fatigue (LCF) tests on three different types of specimens. Combined with post-mortem/quasi in-situ Scanning Electron Microscope (SEM) and Electron Back-Scattered Diffraction (EBSD) observations, as well as further Transmission Kikuchi Diffraction (TKD) and Transmission Electron Microscope (TEM) analyses, we captured two different types of microstructure evolution behaviors during the fatigue loading. One is the “dynamic precipitation”, a kind of phase transformation from martensitic matrix (BCC) to Nb- and Mo-rich small carbides (MC, M7C3) accelerated by repeated slight plastic deformation, whose formation does not rely on the stress concentration effect and also has no ties to the plastic strain localization and fatigue crack initiation. Another is the “local grain refinement” around those stress singularities (crack tip, or interior inclusion) generating substantial amounts of fine sub-grains in VHCF, HCF and LCF regimes, which can then promote fatigue crack initiation and early growth along sub-grain boundaries, fine/coarse grain boundaries and martensitic packet boundaries.

Thermo-kinetic connectivity by integrating thermo-kinetic correlation and generalized stability

• A negative driving force integrating strain energy, interface energy and any kind of energy that inhibits phase transformations was proposed. • Unified expressions of thermo-kinetic correlation and generalized stability were derived for different types of phase transformation. • A thermo-kinetic connectivity was achieved by similar thermo-kinetic correlation and generalized stability between phase transformations and plastic deformations. • A criterion of high driving force-high generalized stability was discussed to achieve structural materials with excellent strength-plasticity combinations. Designing structured materials with optimized mechanical properties generally focuses on engineering microstructures, which are closely determined by the processing routes, such as phase transformations (PTs) and plastic deformations (PDs). Both PTs and PDs follow inherent trade-off relation between thermodynamic driving force Δ G and kinetic energy barrier Q , i.e., so-called thermo-kinetic correlation. By analyzing nucleation and growth and proposing a conception of negative driving force integrating strain energy, interface energy and any kind of energy that equivalently inhibits the PT itself, Δ G S , unified expressions for the thermo-kinetic correlation and generalized stability (GS) were derived for three kinds of PTs, i.e., diffusive PTs with simultaneously decreased Δ G and increased Q , diffusive PTs with simultaneously increased Δ G and decreased Q , and displacive PTs with simultaneously increased Δ G and decreased Q . This leads to so-called thermo-kinetic connectivity by integrating the thermo-kinetic correlation and the GS, where, by application in typical PTs, it was clearly shown, a criterion of high Δ G -high GS can be predicted by modulating chemical driving force, negative driving force and kinetic energy barrier for diffusion or nucleation. Following thermo-kinetic connectivity, analogous procedure for dislocation evolution upon PDs was performed, and materials design in terms of the high Δ G -high GS criterion was discussed and prospected.

Study of Cold Rolling on the Transformation Mechanism, Microstructure, and Properties of 304 Austenitic Stainless Steel

The work hardening mechanism and microstructure properties of 304 austenitic stainless steel (304 ASS) during cold rolling are studied by means of electron backscatter diffraction (EBSD), a transmission electron microscope (TEM), X‐ray diffraction (XRD), hardness test, and permeability measurement. Meanwhile, the mechanical properties and permeability properties of 304 ASS with different cold rolling reductions are investigated. The results show that when the cold rolling reduction is in the range of 5−20%, there are two kinds of phase transformations (γ → ϵ → α′ and γ → α′) in the 304 ASS. With increase in cold rolling reduction (cold rolling reduction ≥ 23%), γ → α′ became the main phase transformation. It is known that α′‐martensite significantly affects the permeability of 304 ASS. To obtain the weak magnetic property that meets the needs of end users, the cold rolling reduction should be controlled at 5−10%.

Selective Carbon Material Engineering for Improved MEMS and NEMS.

The development of micro and nano electromechanical systems and achievement of higher performances with increased quality and life time is confronted to searching and mastering of material with superior properties and quality. Those can affect many aspects of the MEMS, NEMS and MOMS design including geometric tolerances and reproducibility of many specific solid-state structures and properties. Among those: Mechanical, adhesion, thermal and chemical stability, electrical and heat conductance, optical, optoelectronic and semiconducting properties, porosity, bulk and surface properties. They can be affected by different kinds of phase transformations and degrading, which greatly depends on the conditions of use and the way the materials have been selected, elaborated, modified and assembled. Distribution of these properties cover several orders of magnitude and depend on the design, actually achieved structure, type and number of defects. It is then essential to be well aware about all these, and to distinguish and characterize all features that are able to affect the results. For this achievement, we point out and discuss the necessity to take into account several recently revisited fundamentals on carbon atomic rearrangement and revised carbon Raman spectroscopy characterizing in addition to several other aspects we will briefly describe. Correctly selected and implemented, these carbon materials can then open new routes for many new and more performing microsystems including improved energy generation, storage and conversion, 2D superconductivity, light switches, light pipes and quantum devices and with new improved sensor and mechanical functions and biomedical applications.

Selective electron beam melting of TiAl alloy: Microstructure evolution, phase transformation and microhardness

In this research, a Ti-47Al-2Cr-2Nb alloy was fabricated by selective electron beam melting (SEBM). The microstructures at different heights of SEBM-produced TiAl alloy sample and their contribution to microhardness were investigated. The results revealed that the last deposited layers displayed near lamellar structure with fine α2 lamellae. Lamellar colonies degraded at the height of 19 mm of the as-built sample, accompanied with the α2 lamellae decomposition and B2 precipitation along primary α2 lamellae. Meanwhile, equiaxed γ and blocky B2 phase dispersed on the colony boundaries. Lamellar colonies further degraded and colony boundaries further coarsen at the heights of 15 mm and 5 mm due to the in-situ heat treatment during SEBM process. Three kinds of phase transformations could be observed during SEBM process, which were α2 → γ, α2 → B2 and α2 + γ → B2. The microhardness exhibited slight fluctuation as the building height increased from 3 mm to 19 mm, and the microhardness rapidly increased with the building height increasing from 19 mm to 20 mm. Eventually, the mechanism of microstructure evolution during SEBM process was discussed in detail.

Phase transformation behavior of a Mn containing β-solidifying γ-TiAl alloy during continuous cooling

The research was mainly focused on the phase transformation of Ti-42Al-5Mn (at%) alloy produced by vacuum induction melting (VIM) and vacuum arc remelting (VAR) followed by wrought process. The phase transformation kinetics of the alloy during continuous cooling under the cooling rates of 0.1 °C/s, 0.5 °C/s, 2 °C/s, 10 °C/s, 50 °C/s, and 200 °C/s were identified by the Gleeble-3800 system, which is finally used to construct a schematic continuous cooling transformation (CCT) diagram of this alloy. The related phase composition, microstructure and property of the alloy were studied using X-ray diffraction (XRD), electron probe micro analyzer (EPMA), transmission electron microscope (TEM) and Vickers hardness (HV). It shows that six kinds of phase transformation are existed during continuous cooling from 1300 °C with the cooling rates of 0.1 °C/s, 0.5 °C/s, 2 °C/s, 10 °C/s, 50 °C/s, 200 °C/s and WQ, including β→α2′, β→α, β→γ, β→βo/α→α2, α2→α2/γ and α2/γ→βo,sec. It reveals that partial transformations would be inhibited when the cooling rate is relatively high. The martensitic structures (α2′) obtained by β→α2′ were formed under the WQ condition after holding at 1300 °C for 5min, and it will be completely restrained below the cooling rate of WQ. For the β→γ, it is restrained if the cooling rates are higher than 50 °C/s. The α2→α2/γ could be also suppressed when cooling with 200 °C/s, and the γ lath within the supersaturated α2-grains would be formed at lower cooling rate below 50 °C/s. For 0.5 °C/s and 0.1 °C/s, a small amount of dotted βo (βo,sec) phases can be formed in the lamellar interfaces, which is proved the reaction products of α2/γ→βo,sec. Furthermore, the effect of cooling rates on the microstructure, including interlamellar spacing (λ) and γ grains are also identified and discussed.

Investigation on performance characteristics and metallographic transformation on drilling aluminium/titanium sandwich

Materials sandwiches of aluminium/titanium are widely used in structural aerospace, especially in the surface coating and skeletons of aircrafts. Due to their inhomogeneous behavior and poor machinability, drilling aluminium/titanium sandwich in one-shot operation has brought great challenges to the modern manufacturing community. In this paper, a series of experiments were conducted to understand and characterize the process of drilling aluminium/titanium sandwich. Experimental results have shown that the serrated chip is not common in drilling. Only the outer edge of the spiral chip has saw teeth because of the variable cutting speed along the cutting edge. In addition, a better understanding of the phase transformation on surface and subsurface of drilled hole has been provided. Two kinds of phase transformations on the surface of drilled hole have been observed: one is the allotropic transformation from α to β phase and the other is that from β to minor phases α′ (hexagonal martensites). All these are mainly due to the interaction of mechanical force and high temperature.

ChemInform Abstract: Theory and Practice of SiC Growth on Si and Its Applications to Wide‐Gap Semiconductor Films.

The recent advances in epitaxial SiC films' growth on Si are overviewed. The basic classical methods currently used for SiC films' growth are discussed and their advantages and disadvantages are explored. The basic idea and the theoretical background for a new method of the synthesis of epitaxial SiC films on Si are given. It will be shown that the new method is significantly different from the classical techniques of thin-film growth where the evaporation of the atoms onto the substrate surface is exploited. The new method is based on the substitution of some atoms in the silicon matrix by the carbon atoms to form the molecules of silicon carbide. It will be shown that the following process of SiC nucleation happens gradually without destroying the crystalline structure of the silicon matrix, and the orientation of a grown film is imposed by the original crystalline structure of the silicon matrix (not only by the substrate surface as in conventional methods of film growth). A comparison of the new method with other epitaxy techniques will be given.The new method of solid-phase epitaxy based on the substitution of atoms and on the creation of dilatation dipoles solves one of the major problems in heteroepitaxy. It provides the synthesis of low-defective unstrained epitaxial films with a large difference between the lattice parameters of the film and the substrate without using any additional buffer layers. This method has another unique feature distinguishing it from the classical techniques of SiC films' growth—it allows the growing of SiC films of hexagonal polytypes. A new kind of phase transformation in solids owing to the chemical transformation of one substance into another will be described theoretically and revealed experimentally. This type of phase transformation, and the mechanism of a broad class of heterogeneous chemical reactions between gas and solid phases, will be illustrated by an example of the growth of SiC epitaxial layers due to the chemical interaction of CO gas with the monocrystalline silicon matrix. The discovery of this mechanism yields a new kind of template: namely, substrates with buffer transition layers for wide-gap semiconductor growth on silicon. The properties of a variety of heteroepitaxial films of wide-gap semiconductors (SiC, AlN, GaN and AlGaN) grown on a SiC/Si substrate by solid-phase epitaxy will be reported. Grown films contain no cracks and have a quality sufficient to manufacture micro- and opto-electronic devices. Also, the new abilities in the synthesis of large (150 mm diameter) low-defective SiC films on Si substrates will be demonstrated.

Microstructural Changes during Thermal Fatigue in a Nickel-Base Directionally Solidified Superalloy

Microstructural evolution of a nickel-base directionally solidified superalloy during thermal fatigue was investigated. Three kinds of phase transformation have been observed: μ phase precipitated due to the relatively increasing of the concentration of forming μ phase elements as a result of the depletion of Al and Cr; Script-like carbides and γ phase dissolved into the matrix. Also, thermal fatigue was strongly influenced by oxidation.

Method for arbitrary phase transformation by a slab based on transformation optics and the principle of equal optical path

The optical path lengths travelled by rays across a wavefront essentially determine the resulting phase front irrespective of the shape of a medium according to the principle of equal optical path. Thereupon we propose a method for the transformation between two arbitrary wavefronts by a slab, i.e. the profile of the spatial separation between the two wavefronts is taken to be transformed to a plane surface. Interestingly, for the mutual conversion between planar and curved wavefronts, the method reduce to an inverse transformation method in which it is the reversed shape of the desired wavefront that is converted to a planar one. As an application, three kinds of phase transformation are realized and it is found that the transformation on phase is able to realize some important properties such as phase reversal or compensation, focusing, and expanding or compressing beams, which are further confirmed by numerical simulations. The slab can be applied to realizing compact electromagnetic devices for which the values of the refractive index or the permittivity and permeability can be high or low, positive or negative, or near zero, depending on the choice of coordinate transformations.

A study on graphitisation acceleration during annealing of martensitic hypereutectoid steel

In this experimental study, the influence of primary martensitic structure on acceleration of graphitisation process in hypereutectoid steels during subsequent heat treatment cycle was investigated by using optical microscopy and scanning electron microscopy. Then, the results have been compared with prior pearlitic and spheroidized structures. Dilatometric experimental results demonstrated the following graphitisation process from Johnson–Mehl–Avrami equation for steels with a prior martensitic structure. The Avrami exponent was determined about 1.4 that indicates a diffusion-controlled nucleation and growth mechanism in this kind of phase transformation.

Formation of supersaturated solid solutions in aluminum alloys with Mo, Ti, Zr, and Cr

Rapidly hardened alloys of the Al – Cr – Ti, Al – Mo – Ti, and Al – Mo – Zr systems are studied with the use of methods of physicochemical analysis. The regular features of variation of the phase composition of aluminum-base alloys are determined in extra-rapid cooling as a function of the content of alloying additives. The kinetics of decomposition of metastable phases, including supersaturated solid solutions, and the kinds of phase transformation in rapidly hardened alloys are studied in the temperature range of 200 – 400°C.

Hydrothermal degradation of cubic zirconia

Tetragonal ZrO 2 undergoes a tetragonal to monoclinic transformation with the presence of water or water vapor when annealed at relatively low temperatures (65–400 °C); such a transformation has been shown to be due to the annihilation of oxygen vacancies. Nevertheless, cubic ZrO 2 has always been presumed to be stable under similar conditions. However, after annealing at 250 °C under a vapor pressure of 2.6×10 −2 atm for 2 years, the 8 mol% Y 2O 3-doped cubic ZrO 2 ceramic samples cracked. Even though the sample phase was still predominantly cubic, subsequent X-ray diffraction studies indicated that a kind of phase transformation actually occurred, while TEM investigations and electron diffractions demonstrated the precipitation of monoclinic ZrO 2. After annealing, the electrical conductivity of the sample was found to increase by about 30%, suggesting proton conduction. This is the first observation of the hydrothermal degradation in cubic ZrO 2.

Creep-induced phase transformations in a Ti-Al alloy

Abstract The alloy Ti-46.5 at. % Al-2 at. % Cr-3 at. % Nb-0.2 at. % W with a fully lamellar microstructure was subjected to creep deformation at 1073K and 270 MPa. After creep deformation, fine lamellae consisting of γ and α2 laths and β precipitates were observed, which do not exist in the undeformed material, and these observations are explained by α2 → β and α2 → β-phase transformations. To understand the mechanisms of phase transformations, an overall investigation has been made by combining transmission electron microscopy, high-resolution electron microscopy, energy-dispersive spectroscopy and the fast Fourier transform technique. It was found that the two kinds of phase transformation have a close relationship but proceed by different mechanisms. The α2 → γ-phase transformation is a kind of stress-induced phase transformation, while the α2 β/-phase transformation is due to the segregation of the elements Cr and W.

Creep‐induced phase transformations in a Ti-Al alloy

Abstract The alloy Ti-46.5 at. % Al-2 at. % Cr-3 at. % Nb-0.2 at. % W with a fully lamellar microstructure was subjected to creep deformation at 1073K and 270 MPa. After creep deformation, fine lamellae consisting of γ and α2 laths and β precipitates were observed, which do not exist in the undeformed material, and these observations are explained by α2 → β and α2 → β-phase transformations. To understand the mechanisms of phase transformations, an overall investigation has been made by combining transmission electron microscopy, high-resolution electron microscopy, energy-dispersive spectroscopy and the fast Fourier transform technique. It was found that the two kinds of phase transformation have a close relationship but proceed by different mechanisms. The α2 → γ-phase transformation is a kind of stress-induced phase transformation, while the α2 β/-phase transformation is due to the segregation of the elements Cr and W.

Effects of Heat-Treatment Processes on the Electroless Ni-B Coating and its Natural Aging Mechanism

SUMMARYA Ni-B coating was prepared with EN using potassium borohydride reducing agent. The as-plated micro-structure of the coating was confirmed from XRD to be a mixture of amorphous and supersaturated solid solution. Three kinds of phase transformation were observed from the DSC curve. Different from the previous works, the formation of Ni4B3 and Ni2B was found during some transformation processes. The key factors which influence the variation of micro-hardness and micro-structure in deposits are the formation, the size and amount of Ni3B, Ni4B3 and Ni2B. Aging of the deposits treated under some heat treatment conditions occurred at room temperature. Changes of micro-hardness indicated aging phenomena evidently. The natural aging phenomena are concerned with various kinds of decomposition of borides, especially with Ni4B3 phase. The extent of natural aging depends on the formation and the quantity of Ni4B3 and Ni2B.

Effect of Ag or Cu doping on erasable phase-change Sb-Te thin films

The crystallization process of the Ag or Cu doped Sb/sub 2/Te/sub 3/ amorphous film was investigated by isothermal treatment for one hour at 100/spl deg/C, 200/spl deg/C and 300/spl deg/C respectively. The crystallization temperatures of Ag/sub 8.3/Sb/sub 34.9/Te/sub 56.8/ and Cu/sub 8.4/Sb/sub 35.1/Te/sub 56.5/ films determined by DSC analysis were more than 125/spl deg/C and 120/spl deg/C respectively, while that of the undoped Sb/sub 2/Te/sub 3/ film was 97/spl deg/C. It indicates that doping Ag or Cu can improve the room temperature stability of Sb/sub 2/Te/sub 3/ amorphous films. For pure and some Ag, Cu doped Sb/sub 2/Te/sub 3/ films, phase transformations from lower temperature metastable phases to higher temperature thermodynamic stable phases were found. This kind of phase transformation would cause the appearance of ambiguous exothermic peak in DSC curve. When the Ag content is increased to about 6.6 at%, the melting temperature of the film is enhanced from 415/spl deg/C for undoped film to higher than 450/spl deg/C. For the Cu doped film, same tendency can be achieved as Cu content is increased to 8.4 at%. The contrast ratios between as-deposited and 300/spl deg/C annealed films of Ag/sub 3.2/Sb/sub 35.8/Te/sub 61.0/ and Cu/sub 41/Sb/sub 37.4/Te/sub 58.5/ films were more than 42% and 37% respectively.

THERMODYNAMICS OF A SUPERRADIANCE WITH A CLUSTER EXPANSION

N-atom single-mode radiation field interaction system is regarded as an interacting quasi-boson-photon system and is treated as a grand canonical ensemble.The explicit expression for the thermodynamical potential of the system is obtained using cluster expansion method.Some thermodynamic quantities are derived in terms of the general thermodynamic formulas.The numerical calculation and diagrams exhibit the thermodynamic behavior of the system.The singularities appearing in the critical line and other places furnish evidences of existence of the second kind of phase transformation in this collective system.

Ordered Omega Derivatives in (Zr3Al)-Nb Alloys

ABSTRACTVarious kinds of phase transformations, viz., spinodal decomposition, omega transformation, precipitation reactions and martensitic transformation can be induced in ternary (Zr3Al) -Nb alloys in conditions far removed from equilibrium. Transformation sequences in alloys containing 3% niobium are described and rationalized in terms of some basic tendencies such as phase separation and chemical ordering in the β (bcc) phase and displacive omega and β to α (hcp) transformations. Microstructures of rapidly solidified alloy showed a distribution of cuboidal (D88 phase) particles in the β matrix. The periodic arrangement of these particles along the <100>β directions was indicative of a spinodal transformation which preceded their formation. The β → D88 transformation could be accomplished by the superimposition of three processes, namely, chemical ordering, lattice collapse akin to ω transformation and vacancy ordering. During isothermal aging the D88 phase transformed into the B82 phase. The observed lattice correspondence and transformation morphology suggested that the D88 to B82 structural change involved the replacement of structural vacancies in the former by zirconium atoms without any reconstitution of the lattice. The evolution of the equilibrium Zr3Al (L12 structure) phase during prolonged aging were also studied.

In-(13−<I>X</I>) at%Pb-<I>X</I> at%Sn擬2元系合金のマルテンサイト変態

Changes in both crystal structure and surface relief during the phase transformation in Indium-(13−X) at%Pb-X at%Sn pseudobinary alloys have been studied by means of metallurgical techniques. Lattice parameters at each temperature during the phase transformation show that the alloys undergo two kinds of phase transformation: fct(c⁄a<1)↔fco phase transformation in the alloys containing (0-9) at%Sn and fct(c⁄a<1)↔fct(c⁄a>1) one in the alloys containing (11-13) at%Sn. The characteristic feature of phase transformation becomes more recognizable first-order gradually with increasing tin content. The appearance of a banded surface relief in a low-temperature phase concludes that these phase transformations are martensitic ones taking place by the cooperative movement of atoms. The change in surface relief is reversible in the alloys containing (0-7) at%Sn, but not in others. Corresponding to this, the amount of shape recovery in the former alloys is nearly constant, and the latter alloys show a decrease in shape recovery with increasing tin content. The characteristic features of shape recovery associated with phase transformation on heating is discussed in terms of the crystallographic reversibility in each phase transformation. Furthermore, the fct(c⁄a<1)↔fco phase transformation is analyzed phenomenologically on the basis of the Landau theory, and some physical quantities such as the change in entropy and the elastic constant C′ are calculated. The temperature dependence of the stress-strain relation is also discussed in terms of ferro- and super-elasticity. The analyzed results suggest that the characteristic behavior of the fct(c⁄a<1)↔fco phase transformation depends upon the average atomic size as well as the valence electron concentration.