Formation Of Equilibrium Phases Research Articles

Metastable Phases in Al<sub>80</sub>Fe<sub>10</sub>Ti<sub>5</sub>Ni<sub>5</sub> Alloy Fabricated by Non-Equilibrium Processes

In this study, the metastable phases that can be formed in Al80Fe10Ti5Ni5 alloy by melt spinning and mechanical alloying techniques have been investigated. The produced samples were examined by X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. The results showed that the phases in Al80Fe10Ti5Ni5 alloy during rapid solidification are Al13(Fe,Ni)4 ,A l3Ti and amorphous phases. The formation of equilibrium phases was completely suppressed by mechanical alloying. It is of note that supersaturated solid solution, decagonal, and amorphous phases are three separated phases that can be formed during mechanical alloying of an Al­10%Fe­5%Ni­5%Ti powder mixture. [doi:10.2320/matertrans.M2012061]

Deformation-driven formation of equilibrium phases in the Cu–Ni alloys

The homogeneous coarse-grained (CG) Cu–Ni alloys with nickel concentrations of 9, 26, 42, and 77 wt% were produced from as-cast ingots by homogenization at 850 °C followed by quenching. The subsequent high-pressure torsion (5 torsions at 5 GPa) leads to the grain refinement (grain size about 100 nm) and to the decomposition of the supersaturated solid solution in the alloys containing 42 and 77 wt% Ni. The lattice spacing of the fine Cu-rich regions in the Cu–77 wt% Ni alloy was measured by the X-ray diffraction (XRD). They contain 28 ± 5 wt% Ni. The amount of the fine Ni-rich ferromagnetic regions in the paramagnetic Cu–42 wt% Ni alloy was estimated by comparing its magnetization with that of fully ferromagnetic Cu–77 wt% Ni alloy. According to the lever rule, these Ni-rich ferromagnetic regions contain about 88 wt% Ni. It means that the high-pressure torsion of the supersaturated Cu–Ni solid solutions produces phases which correspond to the equilibrium solubility limit at 200 ± 40 °C (Cu–77 wt% Ni alloy) and 270 ± 20 °C (Cu–42 wt% Ni alloy). To explain this phenomenon, the concept of the effective temperature proposed by Martin (Phys Rev B 30:1424, 1984) for the irradiation-driven decomposition of supersaturated solid solutions was employed. It follows from this concept that the deformation-driven decomposition of supersaturated Cu–Ni solid solutions proceeds at the mean effective temperature T eff = 235 ± 30 °C. The elevated effective temperature for the high-pressure torsion-driven decomposition of a supersaturated solid solution has been observed for the first time. Previously, only the T eff equal to the room temperature was observed in the Al–Zn alloys.

Strengthening phases in the production of Al2024-CNTs composites by a milling process

AbstractCarbon nanotubes (CNTs) synthesized by a chemical vapor deposition (CVD) method and the 2024 aluminum alloy (Al2024) are used in the production of Al2024-CNTs composites. An homogeneous dispersion of the CNTs into the aluminum matrix is achieved by a mechanical milling processing. CNTs keet their morphology after milling and sintering processes. Formation of aluminum carbide as a function of CNTs contents is observed. Formation of equilibrium phases during sintering is observed by electron microscopy. CNTs and aluminum carbide in the composites are characterized by transmission electron microscopy. Hardness results of sintered products show an increment of up to 285% over the unreinforced alloy prepared by the same route.

CO<sub>2</sub>地中貯留に関連した水–岩石相互作用の研究の現状と速度論的アプローチの展望

This paper reviews the present status of studies on water-rock interactions associated with the geological sequestration of carbon dioxide (CO2), and stresses the need for a more realistic kinetic approach. Most studies on CO2-water-rock systems based on laboratory experiments, numerical simulations, and natural analogues suggest the fixation of injected CO2 in formation water as stable minerals. However, these results assume the formation of final equilibrium phases by chemical thermodynamics. The slow geochemical reactions and their reaction pathways should also be considered for a detailed prediction of the time scales needed for CO2 fixation. The slow reaction rates, uncertainty about reactive surface area, and difficulty of controlling surface and environmental conditions are major problems associated with this kinetic approach. Phase-shift interferometry (PSI) , which observes the surface topography of minerals at the nanoscale level, has the potential to overcome some of the above difficulties for determining the precise dissolution rates of minerals with knowledge of surface topography.

Liquid-Solid Equilibrium and Intermediate Phase Formation during Solidification in Mg-1.3 at%Zn-1.7 at%Y Alloy

Mg-Y-Zn alloys have attracted much attention in recent years due to their excellent mechanical properties, which are said to be the effect of the intermediate phase (X phase) in the Mg solid solution (α phase). The formation behavior of the X phase has been reported to be a eutectic type reaction in the Mg-Y-Zn pseudo-binary system. However, the phase diagram of this system still requires clarification. In the present study, the 97.0Mg-1.3Zn-1.7Y (at%) alloy was prepared to clarify the liquidus and solidus in the liquid-solid coexistence region, and the solubility limit in the α phase. The chemical compositions of the alloy were analyzed by electron probe X-ray microanalysis after isothermal heat treatment. The results indicate that the Mg-Y-Zn ternary system can be represented as a pseudo-binary system between the α and X phases. In addition, the solidus line for the α phase has been clarified. The solubility limit for the α phase is not much different from that of the previous report, whereas the formation behavior of the X phase is not in the manner of a eutectic reaction, but rather that of a peritectic one.

Effect of large cold deformation on characteristics of age-strengthening of 2024 aluminum alloys

Effect of large cold deformation on the age-hardening characteristics of 2024 aluminum alloys was investigated. The results reveal: 1) the aging response is accelerated after large cold deformation, and the peak strength is attained after aging for 40 min; 2) double aging peaks can be found in the age-hardening curves, and the first peak appears when aged for 40 min. The corresponding peak tensile strength (σ b) and elongation are up to 580 MPa and 9.2% respectively, the second peak appears when aged for 120 min, but the peak tensile strength (520 MPa) is lower than the first one; 3) in early stage of aging (<40 min), elongation slightly increases from 8% with prolonging aging time of the alloy. Elongation markedly decreases to 2% after aging for 60 min, and shows a plateau with the prolonging of aging time on the age-elongation curve. It is indicated that the high density of dislocation introduced by large deformation accelerates the precipitation of GP zones and the aging response of the alloy. The first aging peak is due to the formation of GP zones and the deformation strengthening caused by the high density of dislocation. And the second peak present in the aging curve is attributed to the nucleation and growth of S' phase. The offset between dislocation density decreases and precipitation S'-phase finally results in the phenomenon of double aging peaks when aged at 190 °C. Moreover, it is suggested that the formation of PFZ and coarse equilibrium phase accompanied by the precipitation of S' phase decrease the elongation.

A differential scanning calorimetry and Vickers microhardness study of phase transformations in an Al–Zn–Mg alloy

Precipitate phase formation following different thermal cycles has been investigated on an age-hardenable Al–4.8 wt% Zn–1.3 wt% Mg alloy to obtain the best hardness conditions. Differential scanning calorimetry and microhardness measurements have been performed. It has been evidenced that a distribution of Guinier–Preston zones and their partial transformation into an intermediate phase lead to the highest hardness. The formation of equilibrium phases acts as an obstacle to an increase in the hardness in the various cycles examined. The different stabilities of the coherent structures formed in the course of thermal treatments is discussed and a multistage procedure is claimed to be the best condition for obtaining high hardness within a reasonable heat treatment time.

Phase changes induced in hexagonal boron nitride by high energy mechanical milling

Six crystallographic forms of BN have been reported: hexagonal (h-BN), cubic (c-BN), rhombohedral, wurtzite, orthorhombic, and monoclinic. Only the first two of these have engineering applications. Cubic BN is prepared by a high-pressure, high-temperature process and is valued as an ultrahard material for cutting and grinding tools. Hexagonal BN is the most common form of the material and serves as a soft refractory material with high lubricity and high electrical resistivity. BN has also been prepared in an amorphous state (a-BN) [1, 2]. Bulk a-BN prepared by a process similar to that used to produce c-BN has been reported to be exceptionally hard [3]. The equilibrium phase diagram for BN indicates that c-BN is the thermodynamically stable phase under ambient conditions [4]; however, the transformation of h-BN to c-BN is difficult to achieve. This transformation is hindered by the sluggish kinetics of the process; commercial c-BN production uses high temperatures, high pressures, and catalysts to transform h-BN to cBN [5, 6]. Mechanical milling of powders can lead to formation of new equilibrium or metastable phases [7–9]. Previous work has shown that ball milling h-BN can transform it to c-BN [1, 2]. In one study, h-BN milled in a planetary mill transformed completely to c-BN at 1170 K without a catalyst; in other studies milled h-BN transformed to nano-crystalline h-BN, turbostratic BN, and a-BN [10]. In yet another investigation, an Ar ion beam created such high defect densities in an h-BN film that it transformed to c-BN [11, 12] without the high pressures and temperatures needed in commercial c-BN production. Although the h-BN to c-BN transformation has received considerable attention, less study has been devoted to formation and characterization of the a-BN phase. This project was designed to produce a-BN by high energy milling, measure its properties, and study its stability at elevated temperature. These experiments milled −325 mesh h-BN powder in Spex 8000 mills with hardened steel vials and balls. All Spex milling was done with a 5:1 ball-to-charge mass ratio. The powders were maintained in a dry, inert gas atmosphere throughout powder loading and milling. The Spex-milled powder specimens were analyzed by XRD to determine the degree of amorphization induced by the milling. XRD alone cannot provide a

Metastable and equilibrium phase formation in sputter-deposited Ti/Al multilayer thin films

The sequence and kinetics of metastable and equilibrium phase formation in sputter deposited multilayer thin films was investigated by combining in situ synchrotron x-ray diffraction (XRD) with ex situ electron diffraction and differential scanning calorimetry (DSC). The sequence included both cubic and tetragonal modifications of the equilibrium TiAl3 crystal structure. Values for the formation activation energies of the various phases in the sequence were determined using the XRD and DSC data obtained here, as well as activation energy data reported in the literature.

Microstructural studies on phase evolution sequence in rapidly solidified Zr 3Al based binary and ternary alloys

Transformation sequences in the alloys Zr 3Al, Zr 3Al–3Nb and Zr 3Al–10Nb are described and rationalized in terms of some basic tendencies such as phase separation and chemical ordering in the β phase and displacive omega and β→α (h.c.p.) transformations. These alloys were melt-spun and aged at different temperatures to study the evolution of different phases. The rapidly solidified Zr 3Al alloy showed the presence of an amorphous phase or of a spinodally decomposed microstructure depending upon the severity of quenching, whereas the microstructure of the Zr 3Al–3Nb and the Zr 3Al–10Nb alloys exhibited a distribution of cuboidal particles (D8 8 phase) in the β matrix. Ageing of these alloys showed the formation of several metastable phases prior to the formation of equilibrium phase. The transformation sequence was found to involve a gradual filling of the open structure into a close packed configuration without reconstruction of the lattice.

Ternary phase diagram studies in Ti–Zr–Ni alloys

Equilibrium phase formation is reported for ternary Ti–Zr–Ni alloys near the i-phase-forming composition, for temperature slices between 500 and 700°C. Selected microstructural results in as-cast and annealed samples are discussed. Dominant equilibrium phases identified are the C14-like Laves phase, a Ti2Ni-like phase, α(Ti/Zr), the b.c.c. 1/1 W-phase and the i-phase. The i-phase forms over a small compositional range from a high-temperature equilibrium phase mixture of the Laves and α(Ti/Zr) solid solution phases. Lower annealing treatments have not been found to transform the i-phase, suggesting that in this alloy, the quasicrystal phase might be the ground state phase. Additions of small amounts of Pb and Pd, 1–2 at.%, are demonstrated to substantially effect the equilibrium phase formation for the i-phase and the W-phase. The addition of 1 at.% of Pb significantly orders the i-phase structure.

Solid state reactions in Ni–Al–Ti–C system by mechanical alloying

Several Ni–Al–Ti–C compositions from different areas of the Ni–Al phase diagram containing various amounts of Ti and C were prepared by mechanical alloying. Synthesized samples were investigated by X-ray diffraction and differential scanning calorimetry methods. Solid state reactions in Ni–Al–Ti–C powder mixtures with an equiatomic Ni/Al ratio lead to the formation of NiAl and TiC equilibrium phases. Metastable solid solution of Ni(Al,Ti,C) and non-stoichiometric TiC x are formed in the mixtures with C Ni/C Al ratio of 1.7–3. The grain size of metastable intermetallic phases is less than 5 nm, if Ni and Al content in the mixtures corresponds to the dual-phase and Ni 3Al-areas of the Ni–Al phase diagram.

On the coverage and structure of the [formula omitted]-R30 surface phase

The study addresses the nature and coverage of the (√3×√3) surface phase of the Ag Si(1 1 1) interface that has remained unresolved in literature. Ag is adsorbed in ultra-high vacuum on to the (7 × 7) reconstructed Si(1 1 1) surface with a deposition rate of 3.3 × 10 12 atoms cm −2s −1. The initial stages of growth and intermediate equilibrium phase formation are determined by using Low Energy Electron Diffraction (LEED), Auger Electron Spectroscopy (AES) and Electron Energy Loss Spectroscopy (EELS). For depositions carried out at higher temperatures (> RT), several plateaus in the Auger uptake curve at 0.33, 0.66 and 1.0 ML coverages of Ag with the (√3×√3)-R-30° LEED structures are formed. It is observed that a minimum coverage (θ) of 0.33 ML is required for the formation of the (√3×√3) phase. The (√3×√3) phase obtained at higher substrate temperatures causes the reappearance of the p-electron related states, that were quenched by 1.0 ML adsorption at RT. The EELS obtained at these (√3×√3) plateaus are observed to be different, indicating different local structural surface phases for the same long range order.

Amorphous alloy formation and thickness dependent growth of Gd–silicides in solid phase thin film reaction

The formation of amorphous and equilibrium phases was investigated during the solid-phase reaction of Gd thin film with (111) and (100) oriented Si substrate as a function of thickness and annealing by X-ray diffraction, Rutherford backscattering and transmission electron microscopy. For Gd films thinner than 30 nm, the phase formation was affected by the substrate orientation. At low temperature (320°C), amorphous phase developed on Si(100). At higher temperatures epitaxial hexagonal GdSi 1.7 was found on Si(111), while on Si(100), epitaxial orthorhombic GdSi 2 was formed. For thicker gadolinium films on Si(111), a conventional diffusion–reaction process appeared. The hexagonal GdSi 1.7 phase formed first and then transformed to the second phase (orthorhombic GdSi 2). The ratio of these phases could be described by a model. On Si(100) substrate at each thickness and annealing, only orthorhombic GdSi 2 phase was formed. The phase formation depended on the time and temperature of the annealing and even on the initial Gd film thickness and substrate orientation.

The epitaxial growth of Ag on Si(111)-(7 x 7) surface and its (√3x√3)-R30 surface phase transformation

Ag is adsorbed in ultra-high vacuum on to the (7x7) reconstructed Si(111) surface with submonolayer coverage control with a deposition rate of 3-3 x 1012 atoms/cm2/sec. The initial stages of growth and intermediate equilibrium phase formation are determined by using low energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD) for structural information, and auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS) for composition and interaction analyses. Room temperature (RT) adsorption results in the nearly epitaxial (1 x 1) surface phase growth in the simultaneous multi-layer growth mode. The quenching of the dangling bond states during adsorption is observed by monitoring thep-character of the Si LVV auger peak. For depositions carried out at high temperatures (HT), several plateaus in the auger uptake curve with the (√3 x √3)-R30° LEED structures are formed. It is observed that a minimum coverage of 0–33 monolayer (ML) is required for the formation of the (√3 x √3) phase and this phase causes the reappearance of thep-electron-related states that were quenched by 1.0 ML adsorption at RT. However the (√3 x √3) is observed for higher coverages (0.66 and 1.0 ML) also. The polar angle anisotropy of Si(2p) emission in XPD indicates the rearrangement of substrate Si atoms for the formation of the (√3 x √3) phase. The EELS data also shows relevant changes due to adsorption of Ag at RT and upon annealing. The results suggest the importance of controlled deposition parameters, the lack of which may have kept the determination of the nature and coverage of the (√3 x √3) surface phase unresolved in literature.

On the Nanocrystalline to Amorphous Phase Transition in Al Based Alloys during Ball Milling

We have found that mechanical grinding of Al enriched alloys (Al > 80 at.%) containing late transition metals as solute components such as Ni and Co in the composition ranges of 6 to 9 at.% Ni and 3 to 5 at.% Co (with an atomic ratio Ni/Co equal to 2) leads to the formation of nanostructured equilibrium phases: fcc-Al solid solution and Al 9 (Co,Ni) 2 compound for Al-Ni-Co systems. Significant differences towards the amorphization reaction were observed in these alloys as result of addition of a few atomic percent of Zr or Ti. Upon disordering (Al 0.88 Ni 0.08 Co 0.04 ) 100-x (Zr/Ti) x for x = 1 to 5 at.% prealloyed powders, partial to complete amorphization occurs by the following sequence: fcc-Al + Al 9 (Co,Ni) 2 + Al 3 Zr / or Al 3 Ti --> fcc-Al + amorphous I --> amorphous II. While mechanical grinding is shown to amorphize Al 3 Zr which enters in equilibrium with the fcc-Al solid solution, amorphization of Al 3 Ti compound is not evident. However, increasing Zr or Ti content, the thermal stability of the amorphous phase increases without appreciable variation of the melting temperature of the quaternary alloys. Moreover, the amorphization reaction rate was found tow times larger for Ti than for Zr substitution. Presumably due to an increase of the reduction rate of the fcc-Al particles allowing a faster dissolution. The melting temperature, the crystallization temperature and enthalpy of the ternary and quaternary prealloyed powders were determined.

Crystallization behaviour of calcium aluminate glass fibres

Crystallization behaviour of as-spun and fully-nucleated calcium aluminate (CA) glass fibres produced via inviscid melt spinning (IMS), was studied. Differential thermal analysis (DTA) scans on the as-spun and fully-nucleated CA fibres were performed at different heating rates. By applying the Kissinger method to the DTA scan data the activation energy values for crystallization were determined to be 569 and 546 kJ mol−1, respectively for the as-spun and fully-nucleated CA fibres. The Ozawa analysis on the DTA scan data gave the Avrami parameters at 2.7 and 2.4, respectively, for the as-spun and fully-nucleated CA fibres, which indicates high tendency of bulk crystallization mode. The formation of equilibrium phases of Ca12Al14O33 and CaAl2O4 in the crystallized CA fibres was identified by using X-ray diffraction (XRD).

Impurity effects on heterogeneous nucleation

Heterogeneous nucleation is important in controlling the undercooling during solidification and therefore the formation and microstructure of equilibrium and metastable phases. This paper describes recent experimental results concerned with the effect of impurities on heterogeneous nucleation during solidification, using the entrained droplet and sedimentation techniques. The materials discussed include: Al-(Pb,Cd)-(Ge,Si); Al-Si-(P,Na,Sr); Al-(Si,Fe)-(Ti,V,Zr); and X-(TiAl 3,ZrAl 3). (For a system referred to as A-B-C, a catalyst A nucleates solidification of B mediated by the presence of impurity C; and X refers to an unknown impurity). Analysis of recent results demonstrates that heterogeneous nucleation is highly sensitive to the presence of ppm levels of potent impurities, often causing dramatic changes in undercooling and solidified microstructure.

Aluminium Diffusion Coatings on Medium Carbon Steel

In the present study, medium carbon steel has been hot dip aluminized (HDA) typically at 973 K/1-2 m, as well as calorized (GAL) by a cementation process, typically 1173-1223 K/2-4 h in a controlled atmosphere. It is found that in HDA. coatings, the outermost pure Al layer is followed by predominantly hard intermetallic phases such as FeAl3 and Fe2Al5, the latter forming a serrated interface with the base. In CAL coatings, depending upon the process parameters (such as pack composition, temperature and time of calorizing), phases such as FeAl and Fe3Al form predominantly and the other Al-rich phases are presented to a lesser extent. Since the HDA presents a quasi-equilibrium situation with regard to phases formed between the base metal and the Al-coating (in view of the very short time of treatment), an attempt has been made to study the phase changes accompanying subsequent isothermal holding (773-933 K/10 m). This leads to the formation of stable equilibrium phases, possibly Fe3Al and solid solution next to the base, resulting in improved endurance to thermal shock (1123 K/WQ) and bending. In the CAL process, parameters have been optimised to produce a mixture of favourable phases, as defined by Pilling-Bedworth parameter

Formation of metastable and equilibrium phases during mechanical alloying of Al and Mg powders

Formation of metastable and equilibrium phases during mechanical alloying by high-energy ball milling of mixtures of Al and Mg powders was studied using X-ray diffractometry (XRD) and differential scanning calorimetry (DSC). Different phases were formed, depending on the overall composition of the starting mixture. A powder mixture of nominal composition 40 at. pct Mg gradually becomes converted into a metastable supersaturated Al(Mg) face-centered cubic (fcc) solid solution having approximately 23 at. pct Mg in solution. Powder mixtures of nominal composition of 60 or 80 at. pct Mg gradually transform into the equilibrium γ phase during mechanical alloying, but for the composition of 80 at. pct Mg, some unalloyed Mg is left. Mechanical alloying is comparable to rapid solidification in producing metastable phases in the Al-Mg system, except that mechanical alloying is likely to leave some residual unalloyed elements. There is no indication of the formation of the other equilibrium phase, the β phase, present in the phase diagram. The reason why the β phase does not form is thought to be related to the complex structure and a very large unit cell associated with this phase. However, the β phase is obtained if the mechanical alloyed powders are heat-treated at higher temperatures.