Formation Of Non-equilibrium Phases Research Articles

Nonequilibrium Phases in Rapidly Solidified High-Carbon Fe–Cr–Mo Alloys

The effect of cooling rate and carbon content on the formation of nonequilibrium phases has been investigated in rapidly solidified 10Cr-5Mo and 10Cr-10Mo iron alloys containing very high carbon contents of 2 to 5% in mass. The cooling rate was varied by changing the nozzle size in a single roller method to result in an approximate value of 6.10 4 to 4.10 5 K/s. Nonequilibrium solid solutions of crystalline γ, e and ψ phases as well as an amorphous phase were formed depending on the cooling rate and chemical composition of alloys. The optimum condition for obtaining the e phase in the 10Cr-5Mo alloy and the ψ phase in the 10Cr-10Mo alloy was determined by the combination of cooling rate and carbon content

High Speed Joining of Aluminum Metal Matrix Composites Using Continuous Wave and Pulsed Lasers

AbstractDue to their superior mechanical properties and high strength to weight ratio, metal matrix composite materials are increasingly being used in aerospace, automotive and defense industries. The use of a fusion related conventional joining technique, such as arc welding, leads to the formation of undesirable non-equilibrium phases in the weld zone, where as diffusion and adhesive joining techniques are rather slow. Non contact multi-kilowatt laser joining, though fully developed for high speed autogenous joining of different alloys, is not yet adequate for joining of metal matrix composites. Efforts have been made by several researchers to control the composition and microstructure of laser joined aluminum metal matrix composites by controlling the energy input and by the use of filler wires. In view of the current industry trend to use more light weight structural composite materials, and greater use of high energy density beams for a variety of metalworking applications, the authors have reviewed the current status of high speed laser joining of metal matrix composites. Emphasis is placed on problems and various techniques which can be used to achieve a better control on composition and microstructure during high energy laser joining of aluminum metal matrix composites.

Carbothermal Preparation of Silicon Nitride: Influence of Starting Material and Synthesis Parameters

C and SiO2 of different types were mixed and heat‐treated (1410° to 1550°C), according to a standardized temperature program, in flowing N2 at different pressures. It was found that a starting material combination of C (115 m2/g) and SiO2 (50 m2/g) yielded pure Si3N4 after about 2 h at 1550°C and 1.3 MPa. By adjustment of the pressure in the range 2 to 6 MPa, irreversible evaporation of SiO(g) as well as formation of nonequilibrium phases was suppressed, and the amounts of residual C and O in the sample were controlled. Even a small amount of CO in the N2 was observed to retard the nitridation. Possible explanations are discussed, based on thermodynamic calculations on the Si‐O‐N‐C system.

Excimer Laser Surface Modification: Process and Properties

ABSTRACTSurface modification can improve materials for structural, tribological, and corrosion applications. Excimer laser light has been shown to provide a rapid means of modifying surfaces through heat treating, surface zone refining, and mixing. Laser pulses at modest power levels can easily melt the surfaces of many materials. Mixing within the molten layer or with the gas ambient may occur, if thermodynamically allowed, followed by rapid solidification. The high temperatures allow the system to overcome kinetic barriers found in some ion mixing experiments. Alternatively, surface zone refinement may result from repeated melting-solidification cycles.Ultraviolet laser light couples energy efficiently to the surface of metallic and ceramic materials. The nature of the modification that follows depends on the properties of the surface and substrate materials. Alloying from both gas and pre-deposited layer sources has been observed in metals, semiconductors, and ceramics. Surface enrichment of Cr by zone refinement of stainless steel has also been seen. Rapid solidification after melting often results in the formation of non-equilibrium phases, including amorphous materials. Improved surface properties, including tribology and corrosion resistance, are observed in these materials.

Non-Equilibrium Phase Formation in Fe-Ag-Cu System by Mechanical Alloying

Non-Equilibrium Phase Formation in Fe-Ag-Cu System by Mechanical Alloying

Formation of non-equilibrium solid phases by high-energy electron irradiation

HVEM is one of the most powerful and valuable techniques in the study of non-equilibrium solid phases, the manipulation of which is indispensible for the development of advanced materials. This is owing to the fact that various advantages of HVEM, such as the high penetration power, the clean incident beam and the simple primary defect, can be most effectively used in the in situ study of electron-irradiation-produced non-equilibrium solid phases. Recent HVEM studies to clarify the conditions in which non-equilibrium solid phases can successfully be produced under high-energy (∼ MeV) electron irradiation have been described, with emphasis on the study of electron-irradiation-induced phase decomposition in intermetallic compounds.

Non-equilibrium phase formation

The most important advantage of EM’s is in situ experiments on detailed processes of the same phenomena that occur in bulk materials. In recent years, in situ experiments with HVEM’s, in particular with a 3MV ultra-HVEM , has made it possible to create non-equilibrium phases, which do not exist in nature, or to control and design materials on an atomic scale. Namely, HVEM’s have developed to “Micro-Laboratory”, in which various material-treatments can be done, for natural science from powerful tools for characterization and/or identification of materials.l.The General Rule for Solid Amorphization The author and his cowerkers have succeeded in making amorphous solids of intermetallic compounds by high energy electron irradiation. Using the electron irradiation effect, necessary conditions for the formation of both non-equilibrium phases and extremly supersaturated solid structures[3,4] can be easily and precisely controlled.

Usefulness and Applications of Electron Microscopy to Materials Science

Electron microscopy is the most effective technique to obtain directly and dynamically topographic information in the atomic scale, and results in the sensation of both “seeing is believing” and “the materials are living”. In the present paper, advantages of electron microscopy are discussed from the viewpoint of reliability of the results obtained, and its indispensable applications to materials science are shown in some cases including direct formation of non-equilibrium phases at “Micro-Laboratory”.

Phase formation in co-deposited metallic alloy thin films

Phase formation in co-deposited thin films of Pd-Rh, Cu-Ag and Cu-Sn has been examined. Depending on the substrate temperature, either equilibrium or non-equilibrium phases are formed. It is postulated that the formation of multiphase structures in co-deposited alloy thin films is controlled by the diffusional breakdown of fully intermixed depositing atoms, so that three kinetic regimes are observed: (i) at low substrate temperatures the surface mobility is insufficient for the decomposition of the fully intermixed depositing atoms and the films contain non-equilibrium single-phase structures; (ii) with increasing substrate temperature decomposition to non-equilibrium two phase structures is observed; and finally (iii) with a further increase in substrate temperature the atomic mobility at the surface is sufficient to allow the full atomic rearrangements necessary for the formation of equilibrium phases. By relating the distance an atom can move on the surface during deposition to substrate temperature and deposition rate, it hau proved possible to account for the temperature ranges of the above deposition regimes. A hypothesis is put forward that relates the non-equilibrium phases formed in the first kinetic regime to free energy against composition diagrams at the temperature of the substrate, and this provides the basis for understanding the complex non-equilibrium phase formation found in the Cu-Sn system.

Microstructure of rapidly solidified materials

The basic features of rapidly solidified microstructures are described and differences arising from alternative processing strategies are discussed. The possibility of achieving substantial undercooling prior to solidification in processes such as quench atomization and chill block melt spinning can give rise to striking microstructural transitions even when external heat extraction is nominally newtonian. The increased opportunity in laser and electron beam surface melting for epitaxial growth on the parent solid at an accelerating rate, however, does not exclude the formation of non-equilibrium phases since the required undercooling can be locally attained at the solidification front which is itself advancing at a sufficiently high velocity. The effects of fluid flow indicated particularly in melt spinning and surface melting are additional to the transformational and heat flow considerations that form the present basis for interpretation of such microstructural effects.

ESR study of the peculiarities of formation of low temperature nonequilibrium phases with anomalous cation distribution in Mo−Ti oxide catalysts

ESR spectroscopy is applied to study the structural species in the formation of Mo−Ti oxide catalysts using Mo5+ ions as spin probes. Formation of a defect crystal structure of rutile with a predominant stabilization of Mo5+ ions in the interstitial positions of lowtemperature catalysts is proposed.

Equation for the multi-plateau isotherm of hydrogen + transition metal systems

A model for multi-plateau isotherms is developed with emphasis on the role of the lattice expansion which the parent metal always undergoes with hydrogen absorption. The contribution of the allotropic order–order phase transition of the parent metal itself or the formation of non-equilibrium phases is not considered in the model. Unlike Rees' equation, the equation, derived on the basis of statistical thermodynamics with the aid of continuum mechanics, is obtained as a combination of the component isotherms with their cross-effects. With the help of a computer the validity of the model is tested by comparison with the isotherm of the hydrogen + zirconium system as an example. The state parameters contained in the equation are determined by numerical comparison. The model is sufficient to explain the characteristic phase-pattern which the system displays in the phase diagram. There are two conditions under each of which the equation provided for double-plateau isotherms gives single-plateau isotherm curves. One of the conditions is required by the principle of thermodynamic stability and the other is concerned with the physical situation of the system.

Phase diagram relationships in cryobiology

The reactions which occur during freezing in biological systems employing DMSO as a cryoprotective agent may well involve information given by a near equilibrium ternary H 2O-DMSO-NaCl phase diagram. The initial freezing point depressions for solutions with three different DMSO-NaCl initial ratios ( R) have been determined over the onefold surface of ice saturation. DMSO has been shown to be more effective in reducing NaCl concentration in the residual liquid than had been previously predicted. The temperature and the fraction solid which must be reached for the occurrence of second phase coprecipitation with ice have been shown to be a strong function of initial R value. Ternary invariant reactions have been identified at −35 °C, and tentatively identified at −115 and −105 °C for solutions having DMSO/NaCl ratios of R = 9, 5, and 1, respectively. Metastable nonequilibrium phase formation has been observed for slow cooling of a solution with R = 1. This metastable condition results in different phase relationships upon thawing than upon the initial freezing. By quenching the system after partial rewarming, it has been demonstrated that this metastable condition can be eliminated.

Cu-30%Ni-Be-Si合金の時効性について

The aging characteristics of Cu-30%Ni-Be-Si alloys containing both Be(0.4∼0.5%) and Si(0.3∼0.6%) were investigated by means of hardness tests, optical microscopic observations and micro-beam X-ray diffraction. The results obtained were as follows:(1) These alloys showed superior mechanical properties (hardness and Kb value) than Cu-30%Ni-Be alloy, and the maximum hardness values of these alloys increased with the addition of silicon.(2) It is shown that the age-hardening of these alloys is due to the precipitation of NiBe (γ phase) at an earlier stage of aging and due to the precipitation of Ni3Si (β1 phase) at a later stage of aging.(3) By addition of silicon, G.P.zone was stabilized for a long period, the formation of non-equilibrium phase γ′ (or γ″) was retarded and the grain boundary reaction was supressed.(4) The qualitative relationship was observed between the width of the nodule (the discontinuous precipitates) at grain boundaries and the orientation difference angle of the neighbouring crystals. The width of the nodule became wider with the increasing orientation difference angle of the neighbouring crystals.