Microstructure, crystallinity, and properties of low-pressure MOCVD-grown europium oxide films

A study of growth, structure, and properties of Eu₂O₃ thin films were carried out. Films were grown at 500–600 °C temperature range on Si(1 0 0) and fused quartz from the complex of Eu(acac)₃·Phen by low pressure metalorganic chemical vapor deposition technique which has been rarely used for Eu₂O₃ deposition. These films were polycrystalline. Depending on growth conditions and substrates employed, these films had also possessed a parasitic phase. This phase can be removed by post-deposition annealing in oxidizing ambient. Morphology of the films was characterized by well-packed spherical mounds. Optical measurements exhibited that the bandgap of pure Eu₂O₃ phase was 4.4 eV. High frequency 1 MHz capacitance–voltage (C–V) measurements showed that the dielectric constant of pure Eu₂O₃ film was about 12. Possible effects of cation and oxygen deficiency and parasitic phase on the optical and electrical properties of Eu₂O₃ films have been briefly discussed.     

Adsorption of malachite green from aqueous solution onto carbon prepared from Arundo donax root

Arundo donax root carbon (ADRC), a new adsorbent, was prepared from Arundo donax root by carbonization. The surface area of the adsorbent was determined 158 m²/g by N₂ adsorption isotherm. Batch adsorption experiments were carried out for the removal of malachite green (MG) from aqueous solution using ADRC as adsorbent. The effects of various parameters such as solution pH (3–10), carbon dose (0.15–1.0 g/100 ml) and initial MG concentration (10–100 mg/l) on the adsorption system were investigated. The effective pH was 5–7 and the optimum adsorbent dose was found to be 0.6 g/100 ml. Equilibrium experimental data at 293, 303 and 313 K were better represented by Langmuir isotherm than Freundlich isotherm using linear and non-linear methods. Thermodynamic parameters such as ΔG, ΔH and ΔS were also calculated. The negative Gibbs free energy change and the positive enthalpy change indicated the spontaneous and endothermic nature of the adsorption. The adsorption equilibrium time was 180 min. Adsorption kinetics was determined using pseudo-first-order model, pseudo-second-order model and intraparticle diffusion model. The results showed that the adsorption of MG onto ADRC followed pseudo-second-order model.     

Synthesis and microstructure of boron-doped alumina membranes prepared by the sol–gel method

Pure and boron-doped γ-Al₂O₃ membranes have been synthesized by the sol–gel method. The thermal stability of the unsupported alumina membrane was studied by determining the pore structure (including average pore size, pore volume and BET surface area). The average pore size of the pure alumina membrane increased sharply after sintering at temperatures higher than 1000°C. Addition of 16% boron can considerably stabilize the pore structure of the unsupported alumina membrane. The pore diameter for the B-doped membrane was stabilized within 13 nm after sintering at 1200°C for 5 h. The substantial increase in the pore size for the pure alumina membrane at the sintering temperature of 1000–1200°C was accompanied by the phase transformation from γ-Al₂O₃ to -Al₂O₃. The addition of boron can raise the temperature of this phase transformation significantly and, thus, improves the thermal stability of the membranes.     

Effect of ferroic domain pattern changes on the Raman spectra of some ferroic crystals

The influence of changes in the pattern of ferroic domain structure on the Raman spectra of β-LiNH₄SO₄ and (NH₄)₃H(SO₄)₂ single crystals were studied. It was shown that the Raman spectra of β-LiNH₄SO₄ passed from the ferroelastic phase differ from those of “as-grown” crystal and those of the crystal, which was in the paraelectric phase. Significant changes could be observed in the Raman bands related to triply degenerated ν₃ and ν₄ vibrations of the SO₄ tetrahedron. Detailed temperature studies of the Raman spectra of β-LiNH₄SO₄ close to the paraelectric–ferroelectric phase transition, exhibit anomaly of some internal vibrations of SO₄ in the temperature range where a regular large-scale structure is observed. Different types of evolution of the ferroelastic domain structure and temperature behaviour of the donor and acceptor vibrations were shown while heating and cooling the (NH₄)₃H(SO₄)₂ crystal. Different values of temperature hysteresis were found in temperature studies of the ferroelastic domain structure (ΔT_S  3–5 K) and in Raman spectra studies (ΔT_S  12 K). No changes were observed in the pattern of ferroelastic domain structure at the temperature T_II–III  265 K, at which C2/c → P2/n structural phase transition takes place. On the other hand, at T_III–IV  135 K additional domains with W′-type of domain wall orientation were found.     

Chemical vapour deposition of praseodymium oxide films on silicon: influence of temperature and oxygen pressure

Metal-organic chemical vapour deposition (MOCVD) of various phases in PrO_x system has been studied in relation with deposition temperature (450–750 °C) and oxygen partial pressure (0.027–100 Pa or 0.2–750 mTorr). Depositions were carried out by pulsed liquid injection MOCVD using Pr(thd)₃ (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) precursor dissolved in toluene or monoglyme. By varying deposition temperature and oxygen partial pressure amorphous films or various crystalline PrO_x phases (Pr₂O₃, Pr₇O₁₂, Pr₆O₁₁) and their mixtures can be grown. The pure crystalline Pr₂O₃ phase grows only in a narrow range of partial oxygen pressure and temperature, while high oxygen pressure (40–100 Pa) always leads to the most stable Pr₆O₁₁ phase. The influence of annealing under vacuum at 750 °C on film phase composition was also studied. Near 90% step coverage conformity was achieved for PrO_x films on structured silicon substrates with aspect ratio 1:10. In air degradation of Pr₂O₃ films with transformation to Pr(OH)₃ was observed in contrast to Pr₆O₁₁ films.     

On the optical, structural, and morphological properties of ZrO2 and TiO2 dip-coated thin films supported on glass substrates

This article reports the optical and morphological properties of dip-coated TiO₂ and ZrO₂ thin films on soda-lime glass substrates by metal-organic decomposition (MOD) of titanium^IV and zirconium^IV acetylacetonates respectively. Thermogravimetric and differential thermal analysis (DTA–TG) were performed on the precursor powders, indicating pure TiO₂ anatase and tetragonal ZrO₂ phase formation. Phase crystallization processes took place in the range of 300–500 °C for anatase and of 410–500 °C for ZrO₂. Fourier Transform Infrared Spectroscopy (FT-IR) was used to confirm precursor bidentate ligand formation with keno-enolic equilibrium character. Deposited films were heated at different temperatures, and their structural, optical and morphological properties were studied by grazing-incidence X-ray Diffraction (GIXRD) and X-Ray Photoelectron Spectroscopy (XPS), Ultraviolet Visible Spectroscopy (UV-Vis), and Atomic Force Microscopy (AFM) respectively. Film thinning and crystalline phase formation were enhanced with increasing temperature upon chelate decomposition. The optimum annealing temperature for both pure anatase TiO₂ and tetragonal ZrO₂ thin films was found to be 500 °C since solid volume fraction increased with temperature and film refractive index values approached those of pure anatase and tetragonal zirconia. Conditions for clean stoichiometric film formation with an average roughness value of 2 nm are discussed in terms of material binding energies indicated by XPS analyses, refractive index and solid volume fraction obtained indirectly by UV-Vis spectra, and crystalline peak identification provided by GIXRD.     

Characterization and microstructural evolution of SiC(OAl) fibers to SiC(Al) fibers derived from aluminum-containing polycarbosilane

The SiC(OAl) fibers and the SiC(Al) fibers were fabricated by the use of aluminum-containing polycarbosilane (Al–PCS) precursor. The two types of fibers have been characterized. Chemical element analysis, AES, SEM, XRD, RMS and NMR have been employed. The chemical formula of SiC(OAl) fibers is SiC_1.31O_0.25Al_0.018 with C and O rich on the surface. The microstructure of SiC(OAl) fibers is a mixture of β-SiC nanocrystals, free carbon, and an amorphous silicon oxycarbide (Si–C–O phase), which have been confirmed by an amount of SiC₂O₂, SiCO₃, SiO₄ and SiC₃O units in the ²⁹Si MAS NMR spectrum. A small quantity of aluminum is embedded uniformly in the Si–C–O amorphous continuous phase. For SiC(Al) fibers, nearly stoichiometric composition was confirmed as chemical composition of SiC_1.03O_0.013Al_0.024. The fiber is composed of a large number of β-SiC crystallites, a small amount of -SiC crystalline and SiC amorphous phase. The aluminum in the SiC(Al) fibers mainly exists in two manners: Al–C bonds connected with the surfaces of the β-SiC grains and Al–O bonds, or Al₂O₃, to the amorphous phase.     

Synthesis of some ferromagnetic composite resins and their metal removal characteristics in aqueous solutions

In this study, a procedure for synthesis of new organic-inorganic magnetic composite resins was established. The procedure was based upon immobilization of magnetite (Mag) as a ferromagnetic material within the polymer poly(acrylic acid acrylonitrile) P(AA-AN) and the ion exchange resin (Amberlite IR120). The produced magnetic resins, IR120-PAN-Mag (R1) and P(AA-AN)-Mag (R2) were assessed as sorbents for Cr(VI). Various factors influencing the sorption of Cr(VI), e.g., pH, equilibrium time, initial concentration and temperature were studied. The sorption process was very fast initially and maximum sorption was achieved within 3 h and pH 5.1. The kinetic of the system has been evaluated with pseudo first order model, second order model, Elovich model, intra-particle diffusion model and liquid film diffusion model. Chromium interaction with composite particles followed second-order kinetics with a correlation coefficient extremely high and closer to unity and rate constant (k_s) has the values 1.68 × 10⁻⁴ and 1.9 × 10⁻⁴ g (mg⁻¹ min⁻¹) for R1 and R2, respectively. The values of equilibrium sorption capacity (q_e) are consistent with the modeled data and attain the range 893–951 mg g⁻¹. Kinetically, both pore diffusion and film diffusion are participating in ruling the diffusion of Cr(VI) ions. The sorption data gave good fits with Temkin and Flory–Huggins isotherm models. The isotherm parameters related to the heat of sorption are in the range 8–16 kJ mol⁻¹ which is the range of bonding energy for ion exchange interactions and so suggest an ion exchange mechanism for removal of Cr(VI) by the composite sorbents. The adsorption process was exothermic with ΔH in the range of −73 to −97 kJ mol⁻¹. The negative values of Gibbs free energy confirm the feasibility and the spontaneous nature of Cr(VI) removal with these novel composites.     

Growth of lithium doped silver niobate single crystals and their piezoelectric properties

Lithium doped silver niobate (Ag_1−xLi_xNbO₃, 0 < x < 0.1) is one of the candidate materials for lead-free piezoelectric materials. In this study, Ag_1−xLi_xNbO₃ single crystals were successfully grown by a slow cooling method. Crystal structure was assigned to perovskite-type orthorhombic (monoclinic) phase. Dielectric properties were measured as a function of temperature. As a result, with increasing lithium contents, the phase transition at around 60 °C was shifted to lower temperature while the phase transition at around 400 °C was shifted to higher temperature. On the basis of these peak shifts, the lithium contents in Ag_1−xLi_xNbO₃ single crystals were determined. Moreover, P–E hysteresis measurement revealed that pure silver niobate crystal was weak ferroelectrics with P_r of 0.095 μC/cm² while Ag_0.9Li_0.1NbO₃ (ALN10) crystal was normal ferroelectrics with P_r of 10.68 μC/cm². About this ALN10 crystal, polling treatment was performed and finally piezoelectric properties were measured. As a result, high electromechanical coupling coefficient k₃₁ over 70% was observed.     

(Ca,Fe)共掺铈酸镧陶瓷的制备及红外辐射性能研究

以La₂O₃、CeO₂、CaO和Fe₂O₃为起始原料, 采用高温固相烧结工艺制备得到(Ca,Fe)共掺铈酸镧(La_1.9Ca_0.1Ce_1.9Fe_0.1O₇)红外辐射陶瓷材料。采用X射线衍射仪、扫描电子显微镜、能谱仪、紫外-可见-近红外(UV-Vis-NIR)分光光度计分别对样品的物相组成、显微结构、化学成份、UV-Vis-NIR反射率及UV-Vis吸收进行了表征, 同时采用IR-2双波段红外发射率测量仪测试了样品3~5 μm波段的红外发射率。结果表明: 掺杂元素(Ca, Fe)均固溶进入铈酸镧晶格, 共掺后样品的晶体结构也为萤石结构; (Ca,Fe)共掺样品内部形成杂质能级, 强化了自由载流子吸收与杂质能级吸收, 使样品红外短波段(0.75~5 μm)性能由高反射转变为高吸收(辐射), 其中在0.75~2.5 μm波段的平均红外吸收率为0.88, 3~5 μm波段的红外发射率为0.752, 与未掺杂样品相比分别提高了1660%与60%。     

Synthesis and sintering of hydroxyapatite–zirconia composites

Hydroxyapatite (HA) has been synthesised in presence of 10–30 wt.% of m-ZrO₂ by solid state reaction between tricalcium phosphate (TCP) and Ca(OH)₂ at 1000 °C for 8 h. The m-ZrO₂ was partly converted into t-ZrO₂ by partial consumption of CaO which in turn resulted in a mixture of β-TCP and HA. On sintering these HA–β-TCP–ZrO₂ composite powders at 1100–1400 °C for 2 h, the HA is further decomposed into β-TCP and CaO. The CaO so produced reacts further with m-ZrO₂/t-ZrO₂ generating a mixture of t-ZrO₂ and CaZrO₃ in different proportions. These various phases formed interfere with the sinterability of the composites due to their differential shrinkages leading to a overall reduced density as compared to that of pure HA. The composites show a T-onset of decomposition at around 1150 °C and a 40% HA yield was obtained at the highest sintering temperature of 1400 °C. The products were subjected to XRD for phase analysis and the microstructural features were studied by SEM.     

Preparation and characterization of chemically deposited PbSnS3 thin films

High-quality and well-reproducible PbSnS₃ thin films have been prepared by a simple and inexpensive chemical-bath deposition method from an aqueous medium, using thioacetamide as a sulphide ion source. X-ray diffraction analysis of the deposited films revealed that the as-deposited films were amorphous, however, an amorphous-to-crystalline phase transition was observed as the result of thermal annealing at 425 K for 1 h. The X-ray structure analysis of the collected powder from the bath annealed at 425 K for 1.5 h revealed an orthorhombic phase.

Analysis of the optical absorption data of crystalline PbSnS₃ films revealed that both direct and indirect optical transitions exist in the photon energy range 1.24–2.48 eV with optical band gaps of 1.68 and 1.42 eV, respectively. However, a forbidden direct optical transition with a band gap value of 1.038 eV dominates at low energy (<1.24 eV). The refractive index changes from 3.38 to 2.16 in the range 500–1300 nm. The high frequency dielectric constant and the carrier concentration to the effective mass ratio calculated from the refractive index analysis were found to be 4.79 and 2.3×10²⁰ cm⁻³, respectively. The temperature dependence of the electrical resistivity of the deposited films follows the semiconductor behaviour with extrinsic and intrinsic conduction. The determined activation energies range are 0.35–0.42 and 0.76–85 eV, respectively.     

Mathematical model for the plateau region of P–C-isotherms of hydrogen-absorbing alloys using hydrogen reaction kinetics

A theoretical treatment is presented to account for the two-phase (–β) region of pressure-composition (P–C) isotherms of hydrogen-absorbing alloys, i.e., the dependence of equilibrium pressure on the hydrogen concentration in hydrogen-absorbing alloys at different temperatures. The model is based upon a consideration of the hydrogen–metal reaction kinetics (i.e., static equilibrium condition between adsorbed and absorbed hydrogen at the particle interface) and the hydrogen–hydrogen (H–H) interaction kinetics. The relationship between the slope, the pressure level and the length of plateau region of the P–C isotherms can be derived from this expression and this then enables us to understand the hydrogen reaction mechanism in the alloy powders. The model is found to fit very well with experimental data obtained on LaNi_4.7Al_0.3. It can be used to analyse the enthalpies for the hydride decomposition and hydride formation processes, and the slope changes for the two-phase ( and β) coexistence region. The average enthalpies (H/M=0.02 to 0.9) for hydride decomposition and hydride formation of the LaNi_4.7Al_0.3 alloy are ΔH_d=34.5 kJ mol H₂⁻¹ and ΔH_a=−31.8 kJ mol H₂⁻¹. The effect of lateral H–H interactions on the values of equilibrium pressure and variation of slope with temperature are discussed. H–H interactions contribute to the slope, and the slope of P–C isotherms will become lower when the degree of H–H interaction is high in the LaNi_4.7Al_0.3 alloy. The H–H interactions do not contribute to the hysteresis of P–C isotherms for hydrogen-absorbing alloys. It is proposed that hysteresis originates from energy differences rather than from entropy differences.     

Infrared Transmission Curves of Pb0.1 Ca0.9La2S4 Pellets Densified by Pressureless Sintering

Highly sinterable submicron Pb_0.l Ca_0.9La₂S₄(PCLS) powders were prepared by sulfidizing calcium and lanthanum alkoxides al 500°C under CS, atmosphere for 8 hours and then in pure H₂S atmosphere at 600-800°C for 8 hours. After sintering the pellets were used as infrared transmitting window material of 8-14 μm wavelength. The CdS was added from 3 to 7 wt.% lo improve the sinterability by forming liquid phase during sintering. For sulfidization of lanthanum alkoxide, sulfide powder with LaS₂ phase was formed at 500°C, and a pure Th3P4 phase formed follow by 700°C heat treatment. A powder with β-La₂S₃phase formed at 800°C, and a pure Th₃ P₄phase formed follow by 900°C heat treatment. The powder with β-La₂S₃ phase was sintered to full density at 1350°C by adding 3 wt.% CdS. The PCLS powder with Th3P4 phase sintered to full density at 1400°C by also adding 3 wt.% CdS. The pellet exhibited 45% transmittanceat 13 μm when sintered from the powder with p-La₂S₃phase. The transmittance at 2.5 μm for the pellet sintered from the PCLS powder with Th₃P₄ type structure was 3 times higher than that from the p-La₂S₃ powder.     

Enhanced ferroelectric properties of multilayer SrBi2Ta2O9/SrBi2Nb2O9 thin films for NVRAM applications

Ferroelectric SrBi₂Ta₂O₉/SrBi₂Nb₂O₉ (SBT/SBN) multilayer thin films with various stacking periodicity were deposited on Pt/TiO₂/SiO₂/Si substrate by pulsed laser deposition technique. The X-ray diffraction patterns indicated that the perovskite phase was fully formed with polycrystalline structure in all the films. The Raman spectra showed the frequency of the O–Ta–O stretching mode for multilayer and single layer SrBi₂(Ta_0.5Nb_0.5)₂O₉ (SBNT) samples was 827–829 cm⁻¹, which was in between the stretching mode frequency in SBT (813 cm⁻¹) and SBN (834 cm⁻¹) thin films. The dielectric constant was increased from 300 (SBT) to 373 at 100 kHz in the double layer SBT/SBN sample with thickness of each layer being 200 nm. The remanent polarization (2P_r) for this film was obtained 41.7 μC/cm², which is much higher, compared to pure SBT film (19.2 μC/cm²). The coercive field of this double layer film (67 kV/cm) was found to be lower than SBN film (98 kV/cm).     

Microstructure and mechanical properties of two-phase Cr–Cr2Nb, Cr–Cr2Zr and Cr–Cr2(Nb,Zr) alloys

The microstructures and mechanical properties of binary and ternary Cr-based alloys containing Nb, Zr, or both Nb and Zr, have been studied in both the as-cast and annealed conditions. The level of alloying in each instance was targeted to lie below, or approximately at, the maximum solubility in chromium. The as-cast microstructures of these alloys consisted of Cr-rich solid solution surrounded by small amounts of interdendritic Cr–Cr₂X eutectic structure. Annealing at 1473 K resulted in solid-state precipitation of the Cr₂X Laves phase in the Cr–Nb and Cr–Nb–Zr alloys, but not in the Cr–Zr alloys. The binary Cr₂Nb phase consisted of an extensively twinned ({111}<112> twins) C15 structure whereas the presence of Zr modifies its appearance substantially; the twinned C15 structure persists. Oxides were occasionally present and their compositions were qualitatively determined. Vickers hardness primarily depended upon the volume fraction of the Cr₂X Laves phase present. Age hardening due to solid-state precipitation of Cr₂X Laves phase within the Cr-rich matrix was observed in the Nb-containing alloys. The room temperature bend strength of the alloys was strongly affected by the presence of grain-boundary Cr₂X phase. It is considered that porosity as well as oxides in the alloys also lowers their bend strength.     

Effect of sputtering power on the physical properties of dc magnetron sputtered copper oxide thin films

Cuprous oxide films were deposited on glass substrates using dc magnetron sputtering technique by sputtering of pure copper target in a mixture of argon and oxygen atmosphere under various sputtering powers in the range 0.38–1.50 W cm⁻². The influence of sputtering power on the structural, electrical and optical properties was systematically studied. The films were polycrystalline in nature with cubic structure. The films formed at sputtering powers ≤0.76 W cm⁻² exhibited mixed phase of Cu₂O and CuO while those formed at 1.08 W cm⁻² were single phase Cu₂O. The single-phase Cu₂O films formed at a sputtering power of 1.08 W cm⁻² showed electrical resistivity of 46 Ω cm, Hall mobility of 5.7 cm² V⁻¹ s⁻¹ and optical band gap of 2.04 eV.     

Fabrication of fiber-reinforced functionally graded materials by a centrifugal in situ method from Al–Cu–Fe ternary alloy

Ternary Al–13.8at%Cu–1.6at%Fe alloy was prepared from Al–Cu and Al–Fe alloys at 1000 °C. The ternary Al–Cu–Fe alloy was centrifugally cast to fabricate a new type of functionally graded material (FGM) by a centrifugal in situ method. The structure is expected to differ from that of binary alloys. It was found that the fabricated FGM rings consist of four different phases, namely, Al, Al₂Cu, Al₇Cu₂Fe(ω) and Al₁₃Fe₄ phases. The shape of ω phase was fiber (needle) judging from the observation by a scanning acoustic microscope (SAM). The position dependence of the microstructure was examined on the fabricated FGM rings, and the volume fraction of ω phase was found to increase toward the outer region of the ring. Moreover, orientation and aspect ratio of the ω phase varied in the rings in a gradually graded manner. Therefore, the present study explores a method to produce fiber-dispersed FGMs by applying a centrifugal in situ method to ternary alloys.     

Adsorption of p-chlorophenol from aqueous solutions on bentonite and perlite

The adsorption of p-chlorophenol (p-CP) from aqueous solutions on bentonite and perlite was studied. These materials are available in large quantities in Bulgaria. Model solutions of various concentrations (1–50 mg dm⁻³) were shaken with certain amounts of adsorbent to determine the adsorption capacity of p-CP on bentonite and perlite as well. The influence of several individual variables (initial adsorbate concentration, adsorbent mass) on the rate of uptake of the studied compound on the adsorbent was determined by carrying out experiments at different contact times using the batch adsorber vessel designed according to the standard tank configuration. Rapid adsorption was observed 20–30 min after the beginning for every experiment. After that, the concentration of p-CP in the liquid phase remained constant. The adsorption equilibrium of p-CP on bentonite and perlite was described by the Langmuir and the Freundlich models. A higher adsorption capacity was observed for bentonite (10.63 mg g⁻¹) compared to that for perlite (5.84 mg g⁻¹).     

Sigma-phase formation in Nitronic 50 and Nitronic 50W stainless steels

The formation of the sigma-phase has been studied in two heats of a nitrogen-strengthened austenitic stainless steel, Nitronic 50 and Nitronic 50W, a weld filler wire composition. It was found that the presence of cold-work greatly enhanced the rate of sigma precipitation in the temperature range of 600–1000° C. The nucleation and growth of the sigma-phase was accompanied by recrystallization of the deformed austenite. The final austenite grain size was generally very small (0.5–4m), since grain growth was inhibited by the intergranular sigma precipitates. Precipitation of M₂₃C₆ carbides was suppressed by the rapid formation of the sigma-phase. In samples solution annealed prior to ageing, the kinetics of sigma precipitation were much slower, and for a heat containing 0.04wt% carbon (Nitronic 50W), the sigma precipitation was retarded by the formation of intergranular M₂₃C₆. Grain boundaries in solution annealed and aged samples of both alloys also contained Z-phase (NbCrN) precipitates, which formed in the range of 700–1000° C. Retained delta-ferrite in the Nitronic 50W heat transformed directly to sigma-phase during ageing of the cold-worked specimens, but under-went more complex reactions in the solution annealed plus aged samples. Precipitation sequences at the ferrite-austenite interfaces in these latter specimens varied with ageing temperature. Sigma-phase chemistries in all samples were determined in the analytical electron microscope. In the solution-annealed and aged specimens, the sigma-phase chemistries were generally not of equilibrium values, and a wide variation in sigma composition was seen in each sample. In the cold-worked and aged samples, equilibrium sigma chemistries were attained. On comparing data from different samples, compositional changes in the sigma phase were observed, with molybdenum increasing and chromium decreasing as ageing temperatures increased.