Synthesis and characterization of MoSi2-Mo5Si3 nanocomposite by mechanical alloying and heat treatment

The nanocomposite of MoSi₂-Mo₅Si₃ powder was synthesized by mechanical alloying from Mo and Si powder mixture at room temperature. The phase evaluation of powder after various milling durations and heat treatments were assessed via X-ray diffraction (XRD) and a differential thermal analysis (DTA). Morphology and microstructure of powder particles were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results revealed that nanocomposite of MoSi₂-Mo₅Si₃ powder was synthesized by combustion reaction of Mo and Si powder using ball milling. In the early stages of ball milling β-MoSi₂ was produced. However with continued milling for 48 h α-MoSi₂ and Mo₅Si₃ phases were formed. DTA results of 24 h and 48 h as milled mechanical alloyed specimens showed a well-defined peak at 852 °C and 920 °C relating to the formation of α-MoSi₂. The activation energy for 24 h and 48 h milled specimens were –128.6 KJ/mol and –121.4 KJ/mol respectively. Annealing the milled specimens at 1000 °C for 2 h revealed the phase transformation of β-MoSi₂ to α-MoSi₂ and the formation of Mo₅Si₃. The crystallite size of α-MoSi₂ and Mo₅Si₃ were about 9 nm and 12 nm after 48 h mechanical alloying. These values increased slightly to 18 nm and 14 nm after annealing at 1000 °C.     

Thermal expansion of the V5Si3 and T2 phases of the V–Si–B system investigated by high-temperature X-ray diffraction

The thermal expansion anisotropy of the V₅Si₃ and T₂-phase of the V–Si–B system were determined by high-temperature X-ray diffraction from 298 to 1273 K. Alloys with nominal compositions V_62.5Si_37.5 (V₅Si₃ phase) and V₆₃Si₁₂B₂₅ (T₂-phase) were prepared from high-purity materials through arc-melting followed by heat-treatment at 1873 K by 24 h, under argon atmosphere. The V₅Si₃ phase exhibits thermal expansion anisotropy equals to 1.3, with thermal expansion coefficients along the a and c-axis equal to 9.3 × 10^?6 K^?1 and 11.7 × 10^?6 K^?1, respectively. Similarly, the thermal expansion anisotropy value of the T₂-phase is 0.9 with thermal expansion coefficients equal to 8.8 × 10^?6 K^?1 and 8.3 × 10^?6 K^?1, along the a and c-axis respectively. Compared to other isostructural silicides of the 5:3 type and the Ti₅Si₃ phase, the V₅Si₃ phase presents lower thermal expansion anisotropy. The T₂-phase present in the V–Si–B system exhibits low thermal expansion anisotropy, as the T₂-phase of the Mo–Si–B, Nb–Si–B and W–Si–B systems.     

Phase equilibria of the Ni–Si–Zn system at 600 °C

The phase equilibria at 600 °C of the Ni–Si–Zn system were investigated by using 19 alloys and four reaction diffusion couples. The alloys were prepared by melting the pure elements in the alumina crucibles capsulated in evacuated quartz tubes. The samples were examined by means of optical microscopy, X-ray diffraction, scanning electron microscopy with energy dispersive X-ray spectroscopy, and electron probe microanalysis. The presented isothermal section at 600 °C is characterized by the existence of five ternary phases, which are labeled as T₁, T₂, τ₃, τ₄ and β′, respectively. Three previously reported ternary compounds, viz. Ni₃Si_0.33Zn_0.67 (T₂), Ni₂SiZn₃ (τ₃), and Ni₃Si₂Zn (τ₄), were confirmed to exist at 600 °C. β′-NiZn was assumed to be the Si-stabilized β-NiZn phase in the Ni–Zn binary system. T₁ is a newly found ternary compound. It should be noted that the phase relationships at 600 °C are significantly different from the previously reported ones at 800 and 297 °C. γ₁-NiZn₃, the existence of which is still controversial in the literature, was observed at 600 °C. The solubilities of Zn in Ni₃₁Si₁₂, Ni₃Si₂ and NiSi, and those of Si in γ₁-NiZn₃ and γ-Ni₂Zn₁₁ are negligible. The solubilities of Zn in α-NiSi₂ and δ-Ni₂Si were determined to be 8.5 and above 3.1 at.% Zn, respectively. α-NiSi₂ and δ-Ni₂Si extend into the ternary system at constant Ni content with Zn substituting for Si.     

A study of the microstructures and oxidation of Nb–Si–Cr–Al–Mo in situ composites alloyed with Ti,Hf and Sn

The effects of alloying on the microstructures, solidification path, phase stability and oxidation kinetics of Nb_ss/Nb₅Si₃ base in situ composites of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system have been investigated in this study. All the studied alloys are classified as hyper-eutectic Nb silicide base in situ composites and have lower densities compared to nickel-based superalloys. The Nb₃Si silicide formed in the Hf-free alloys and transformed to Nb_ss and αNb₅Si₃ during heat treatment at 1500 °C. This transformation was enhanced by the addition of Ti. The Nb_ss and Nb₅Si₃ were the equilibrium phases in the microstructures of the Hf-free alloys. In the presence of Ti, the βNb₅Si₃ only partially transformed to αNb₅Si₃, suggesting that Ti stabilises the βNb₅Si₃ to lower temperatures (at least to 1300 °C). Furthermore, alloying with Hf stabilised the hexagonal γNb₅Si₃ (Mn₅Si₃-type) silicide in the Hf-containing alloys. The addition of Sn promoted the formation of the Si-rich C14 Laves phase and stabilised it at 1300 °C. This is attributed to the Sn addition decreasing the solubility of Cr in the Nb_ss of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system whilst increasing the Si solubility. The Si solubility in the C14 Laves phase was in the range ∼6.6 to 10.5 at%. The lattice parameter of the Nb_ss in each alloy increased after heat treatment signifying the redistribution of solutes between the Nb_ss and the intermetallic phases. The oxidation resistance of the alloys at 800 °C and 1200 °C increased significantly by alloying with Ti and Sn. Pest oxidation behaviour was exhibited by the Nb–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–2Mo (heat treated) and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf (heat treated) alloys at 800 °C. Pesting was eliminated in the alloy Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf–5Sn at 800 °C, indicating that the addition of Sn plays an important role in controlling the pest oxidation behaviour at intermediate temperatures. The oxidation behaviour of all the alloys at 800 °C and 1200 °C was controlled by the oxidation of the Nb_ss and was sensitive to the area fraction of Nb_ss in the alloy.     

Microstructural evolution and mechanical properties of as-cast and directionally-solidified Nb-15Si-22Ti-2Al-2Hf-2V-(2, 14) Cr alloys at room and high temperatures

Arc-melting (AC) and directional solidification (DS) techniques were used to prepare Nb-15Si-22Ti-2Al-2Hf-2V-(2, 14) Cr alloys (hereafter referred as to 2Cr and 14Cr alloys, respectively), and the microstructural evolution and mechanical properties, including Vickers hardness, room temperature fracture toughness and high temperature strength, of the two AC and DS alloys were compared. The results showed that with heat-treatment at 1350 °C for 50 h, the AC-2Cr alloy composed of Nb solid solution (Nb_SS) and α-Nb₅Si₃ silicide, while Laves C15-Cr₂Nb phase arose in the 14Cr alloy. With two-phase Nb_SS/α-Nb₅Si₃ microstructure, the AC-2Cr alloy showed excellent room-temperature fracture toughness (K_Q: 14.2 MPa m^1/2) and 0.2% yield strength at 1250 °C (σ_0.2: 315 MPa) and 1350 °C (σ_0.2: 294 MPa), better than the AC-14Cr alloy with tri-phase Nb_SS/α-Nb₅Si₃/C15-Cr₂Nb microstructure (K_Q: 9.4 MPa m^1/2, σ_0.2: 189 MPa at 1250 °C and 87 MPa at 1350 °C). The DS technique was found not to change the phase constituent of each alloy, but it made the microstructure slightly orient to the growth direction, resulting in a significant improvement in room-temperature fracture toughness (by ∼43%) and high-temperature yield strength σ_0.2 (by ∼55%), as compared with the AC samples.     

High-temperature oxidation behavior of multi-phase Mo-containing γ-TiAl-based alloys

Intermetallic titanium aluminides are potential materials for a number of high-temperature components used in aero and automotive engines. In particular, alloys solidifying via the β-phase are of great interest because they possess a significant volume fraction of the disordered body-centered cubic β_o-phase at elevated temperatures ensuring good processing characteristics during hot-working. Nevertheless, the practical use of such alloys at a temperature as high as 800 °C requires improvement of their oxidation resistance. Various attempts have been made including alloying with additional elements such as Nb, Cr, Mo etc. or applying the so-called fluorine effect. However alloying could not provide a sufficient oxidation resistance above 850 °C whereas the fluorine effect protects the base material against environmental degradation up to over 1000 °C. This paper aims to investigate the influence of the phase composition on the oxide scale morphology without and with fluorine effect. The results refer to the oxidation behavior of three β-solidifying γ-TiAl-based alloys in the cast and hot-isostatically pressed condition at 800 °C in air. The behavior of the TNM alloy (Ti–43.5Al–4Nb–1Mo–0.1B, in at.%) was compared with that of two Nb-free TiAl alloys which contain different amounts of Mo (3 and 7 at.%, respectively) and hence a different microstructure (α₂/β_o/γ vs. β_o/γ). During testing in dry synthetic air at 800 °C a mixed oxide scale develops on all three alloys. This behavior was changed via the fluorine effect, as demonstrated for previously investigated TiAl alloys with an Al-content higher than 40 at.% based on α₂/γ and α₂/β_o/γ phases. The oxidation resistance of the fluorine treated samples was significantly improved compared to the untreated samples. The reason for this is the change in the oxidation mechanism triggered by the small additions of fluorine in the subsurface zone of the investigated alloys. The results of isothermal oxidation tests at 800 °C in air are presented and discussed in view of chemical composition and microstructure, along with the impact of the phase composition on the efficiency of the fluorine effect. From a microstructural perspective the fluorine effect leads to the formation of an even thinner oxide scale on the β-phase compared to the γ-phase.     

Preparation of Si3N4-TaC and Si3N4-ZrC composite ceramics and their mechanical properties

Si₃N₄-TaC and Si₃N₄-ZrC composite ceramics with sintering additives were consolidated in the sintering temperature range of 1500–1600 °C using a resistance-heated hot-pressing technique. The addition of 20–40 mol% carbide improved the sinterability of the ceramics. The ceramics were densely sintered under 0–40 mol% TaC or ZrC at 1500 °C, 0–80 mol% TaC at 1600 °C, and 0–60 mol% ZrC at 1600 °C. In ceramics sintered at 1500 °C, the proportion of α-Si₃N₄ was larger than that of β-SiAlON; α-Si₃N₄ transformed mostly to β-SiAlON at 1600 °C. Carbide addition was effective in inhibiting α-Si₃N₄-to-β-SiAlON phase transformation. Young's modulus for the dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics increased with the carbide amount, and the hardness of dense Si₃N₄-ZrC and Si₃N₄-TaC ceramics increased from 14 GPa to 17 GPa with increasing α-Si₃N₄ content. Dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics, with larger quantities of α-Si₃N₄ sintered at 1500 °C, exhibited high hardness; the fracture toughness of these ceramics decreased with increasing α-Si₃N₄ proportion. Both the hardness and fracture toughness of the dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics were strongly related to the proportion of α-Si₃N₄ in the sintered body.     

The effect of Cr and Zr on the structure and phase composition of TNM gamma titanium aluminide alloy

Quantitative analysis of the zirconium and chromium cooperative effect on the structure and phase composition, after solidification and heat treatment, of the TNM type alloy, was performed using both experimental techniques and thermodynamic calculations. It is shown that the joint alloying by chromium and zirconium up to 1 at.% does not change the sequence of phase transformations during solidification and subsequent cooling of the TNM alloy. Rather, it significantly decreases the α₂-phase amount and increases β-phase amount. Increasing the amount of β-phase is associated with a high beta-stabilizing effect of chromium, whose partition coefficients k(β/γ) and k(β/α) are significantly higher than 1 (ranging from 1.67 to 3.8). Zirconium is distributed quite uniformly with partition coefficients k(β/γ) and k(β/α) close to 1. HIP treatment of the alloy with chromium and zirconium up to 1 at.% at 170 MPa and 1250 °C leads to a significant reduction of the α(γ/α₂)-phase fraction, and increases the amount of γ-globular in comparison to vacuum annealing at 1250 °C. The higher content of the softest γ-globular decrease the hardness from 365 HV to 333 HV, increases the total elongation of the alloy from 0.5% to 0.8% at room temperature and substantially decreases the ultimate tensile strength from 750 MPa to 715 MPa.     

Study of the role of Ta and Cr additions in the microstructure of Nb–Ti–Si–Al in situ composites

Niobium silicide-based in situ composites are Nb-base alloys with high Si content that have the potential for higher temperature capability than the Ni-base superalloys. Microstructure-property studies of these alloys have been the subject of many research programmes, where the differentiation between the αNb₅Si₃ and βNb₅Si₃ is usually not clear, even though it is essential to understanding the solidification of the alloys and the stability of their microstructures at high temperatures. In this work, the effects of Cr (5 or 8 at.%) and Ta (6 at.%) in the microstructures of as-cast and heat-treated Nb–24Ti–18Si–5Al in situ composites have been studied. The main phases observed in the as-cast and heat-treated (100 h at 1400 or 1500 °C) alloys were the niobium solid solution, (Nb,Ti)_ss, the niobium 5–3 silicides, αNb₅Si₃ and βNb₅Si₃, and a Cr-rich C14 silicide Laves phase. During solidification, Al additions promoted the formation of βNb₅Si₃, while the Cr additions caused the appearance of the C14 silicide Laves phase that was probably formed congruently from the remaining liquid. During heat treatment, the βNb₅Si₃ phase transformed to αNb₅Si₃ according to the reaction βNb₅Si₃→αNb₅Si₃+(Nb,Ti)_ss. The Cr addition lowered the melting temperature of the alloys as liquation was observed after 100 h at 1500 °C in the two Cr-rich alloys. Ta and Cr retard the βNb₅Si₃→αNb₅Si₃+(Nb,Ti)_ss transformation. Solid state diffusion was sluggish in the presence of Ta, but the Ta addition did not destabilize the three-phase equilibrium among (Nb,Ti)_ss, αNb₅Si₃ and the C14 silicide Laves phase, in the Nb–24Ti–18Si–6Ta–8Cr–4Al alloy.     

Preparation of bulk β-FeSi2 by synthetical control of melt cyclical superheating,solidification and cooling process

A new method of melt cyclical superheating combined with the control of solidification and cooling process as well as appropriate heat treatment was proposed to prepare bulk β-FeSi₂. FeSi₂ precursor of φ18 × 17 mm in size was obtained under the conditions of melt superheating temperature 1550 °C, superheating time 10 min, recycling times 3, solidification rate 30 °C/s, cooling rate 12 °C/min from solidification temperature to 700 °C and cooling naturally from 700 °C to room temperature. The precursor had homogeneous and complete α + ε eutectic structure, with the rod-like ε phase of 1–2 μm in diameter. After the precursor with complete α + ε eutectic structure were annealed at 900 °C for 150 h, both the α and ε phases totally disappeared and were transformed into β-FeSi₂ except few residual Si-rich phase.     

Isothermal Section of the Nb-Si-B System at 1700 °C in the Nb-NbSi<Subscript>2</Subscript>-NbB<Subscript>2</Subscript> Region

Me-Si-B (Me—metal) alloy systems have been evaluated to aid the development of ultra high temperature structural materials. In the present work, the phase relations at 1700 °C in the Nb-NbSi₂-NbB₂ region of the Nb-Si-B system have been experimentally determined. The alloys were prepared via arc melting and powder metallurgy routes. All samples were characterized via scanning electron microscopy (SEM) and x-ray diffraction (XRD) and selected samples were also scanned via wave-length dispersive spectroscopy (WDS). The high B solubility in the αNb₅Si₃ phase has been confirmed with solubility range larger than that proposed in the 1600 °C isothermal section of Nowotny et al. (Aufbau und Zunderverhalten von Niob-Bor-Silicium-Legierungen, Mon. Chem., 1960, 91, p 975-990). In contrast to Nowotny’s isothermal section, it has been observed that the D8₈-phase should be nearly stoichiometric with composition close to 57Nb35Si8B (at.%). It was also found that the D8₈-phase does not equilibrate with NbB₂-phase at 1700 °C. A negligible Si solubility in the boride phases as well as that of B in NbSi₂ has been noticed.     

Microstructures and mechanical properties of conventionally solidified Al63Cu25Fe12 alloy

In this study, the microstructures and mechanical properties of conventionally solidified Al₆₃Cu₂₅Fe₁₂ alloy after different heat-treatments were investigated. The microstructures of the as-cast and subsequently heat-treated samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential thermal analysis (DTA). The XRD results showed the presence of quasicrystalline icosahedral phase (i-phase) together with crystalline phases corresponding to β-AlFe(Cu) solid solution phase (β-phase) and τ-AlCu(Fe) solid solution phase (τ-phase). The SEM investigations clearly showed the formation of i-phase with pentagonal dodecahedra structure. However, the i-phase together with β-phase was also observed in the heat-treated samples and the peak intensity of the β-phase decreased with increasing heat-treatment temperature. From the DTA curves, the melting point of i-phase was determined as 890 °C for this alloy composition. Mechanical properties of the as-cast and subsequently heat-treated samples were measured by a Vickers indenter. Results showed that the microhardness (HV) and the elastic modulus (E) of the as-cast sample were around 598 kg fmm^?2 (5.86 GPa) and 104 GPa, respectively. In addition, the characteristic of material plasticity (δ_H) value was calculated to be 0.54.     

700 °C Isothermal Section of the Al-Ti-Si Ternary Phase Diagram

The 700 °C isothermal section of the Al-Ti-Si ternary phase diagram has been determined experimentally by means of scanning electron microscopy coupled with energy dispersive x-ray spectroscopy and x-ray powder diffraction. Fourteen three-phase regions have been determined experimentally in the isothermal section at 700 °C. The ternary phases τ₁ (I4₁/amd, Zr₃Al₄Si₅-type) and τ₂ (Cmcm, ZrSi₂-type) are confirmed in the system at 700 °C. The compositions of τ₁ and τ₂ are found as Al_6.2-9.3Ti_32.4-34.0Si_57.5-60.9 and Al_10.0-11.6Ti_34.2-34.5Si_53.9-55.6, respectively. The τ₃ and Ti₃Al₅ phases are not found in the section. The Ti-rich corner at 700 °C shows the presence of three three-phase equilibriums, i.e., (TiAl + Ti₃Al + Ti₅Si₃), (α-Ti + Ti₃Si + Ti₅Si₃) and (α-Ti + Ti₃Al + Ti₅Si₃). The maximum solubility of Al in Ti₅Si₃, Ti₃Si and α-Ti is 6.0, 1.5 and 13.9 at.% at 700 °C, respectively. The maximum solubility of Si in L-Al, TiAl₃, TiAl₂, TiAl, Ti₃Al and α-Ti is 24.1, 13.6, 1.5, 0.8, 2.3 and 2.3 at.%, respectively.     

The oxidation behaviour of uniaxial hot pressed MoSi2 in air from 400 to 1400 °C

MoSi₂ samples were prepared by hot uniaxial pressing from a 2 μm grain-size powder of commercially available MoSi₂. The oxidation behaviour of MoSi₂ was systematically studied from 400 °C to 1400 °C, which includes the pest-oxidation temperature range. It was observed that the rate and mechanism for oxidation of MoSi₂ change significantly with increasing temperature. Five temperature regimes have to be considered regarding both kinetic results and cross-sections: i) 400 < T < 550 °C; ii) 550 ≤ T < 750 °C; iii) 750 ≤ T < 1000 °C; iv) 1000 ≤ T < 1400 °C; v) T ≥ 1400 °C. In the first range, pesting did not occur in samples that were free of cracks and residual stresses and the oxidation kinetics were governed by surface or phase boundary reactions. Above 550 °C, there was a change in the physical properties of the oxidation products due to the evaporation of MoO₃. The formation of Mo₅Si₃ was observed above 800 °C showing that the thermodynamic previsions were satisfied above this temperature. At higher temperatures (>1000 °C), the oxide scale became very protective and transport in the silica scale (amorphous and β cristobalite) governed the oxidation kinetics. The Mo₅Si₃ phase did not appear anymore at 1400 °C, indicating that another oxidation mechanism has to be proposed.     

Effect of wear debris of Al2O3, ZrO2 and WC balls on Y-α/β-SiAlON ceramics consolidated by spark plasma sintering

Three types of Y-α/β-SiAlON powders were prepared by mixing and milling the raw materials for 20 h using Al₂O₃, ZrO₂, and WC balls, thereafter denoted as SNA, SNZ and SNW, respectively. The three types of specimens were sintered using spark plasma sintering (SPS) at 1510 °C for 5 min under 30 MPa in a vacuum. α-phases (SiAlON and Si₃N₄) and β-SiAlON phase were observed in the SNA and SNZ specimens, but only the β-SiAlON phase existed in the SNW specimen. The wear debris of the WC balls affected the grain boundary properties and eventually promoted the full phase transformation of α-Si₃N₄ into α- and β-SiAlON phases. However, SNA and SNZ were partially transformed into α- and β-SiAlON phases. The Vickers hardness of SNA was the highest (18.7 GPa), due to its having the highest content of α-phase, but its fracture toughness was the lowest (4.09 MPam^1/2) due to its having the lowest content of β-SiAlON phase. The wear debris and secondary phases existed at the grain boundary, mostly at the triple junction, and also affected the color. The color of the sintered specimen was quite different depending on the milling media.     

Effect of quenching temperature on structure and properties of titanium alloy: Structure and phase composition

X-ray diffraction analysis and transmission and scanning electron microscopy have been used to study regularities of the formation of structure and phase composition of a VT16 alloy during its quenching. The formation of an athermal ω phase in the VT16 alloy with the initial (α + β) structure during quenching of the alloy from 800°C was found to be possible. Quenching temperatures (T _q) at which various metastable phase compositions, such as the metastable β solid solution, β + α″ + ω, β + α″, and α″ martensite, are formed have been determined to be 750, 800, 750–850, and ≤850°C, respectively. Dependences of variations in the volume fractions of phases were plotted. It has been shown that, at quenching temperatures close to the β-transus, the active growth of β-phase grains takes place at the expense of a decrease in the α-phase volume fraction.     

Effects of ultrasonic treatment on the formation of iron-containing intermetallic compounds in Al-12%Si-2%Fe alloys

In general, the iron impurity is detrimental to the mechanical properties of Al–Si alloys. The α-phase and β-phase are the most important and common iron-containing intermetallic compounds (IMCs) in Al–Si alloys. During conventional casting, the acicular β-phase is stable, and considered to be harmful. In this paper, the Al-12%Si-2%Fe alloy was treated by power ultrasound and solidified under different cooling conditions. The effects of ultrasonic treatment (UST) and cooling rate on morphology and composition of IMCs were investigated. The results showed that UST can change the morphology and composition of iron-containing IMCs and promote the formation of metastable α-phase. When the ultrasound was applied at 720 °C, the amount of starlike α-phase increases and the acicular β-phase decreases with increasing applied time of UST. In addition, the polygonal α-phase is formed and substitutes for the β-phase when quenching after UST for 60 s and 120 s, suggesting that the formation of β-phase can be suppressed under this condition. For the case of UST at 610 °C which the β-phase has been nucleated, the β-phase transforms from an acicular shape to the rod-like morphology, indicating that the cavitation-induced fracture of β-phase.     

Effect of long-term ageing on microstructure stability and lattice parameters of coexisting phases in intermetallic Ti–46Al–8Ta alloy

The lamellar α₂(Ti₃Al) + γ(TiAl) microstructure of intermetallic Ti–46Al–8Ta (at.%) alloy is thermodynamically unstable and transforms to α₂ + γ + τ type during long-term ageing at 750 °C. A new ternary τ-phase with B8₂ type structure (space group P6₃/mmc, Pearson symbol hP6) is identified by XRD analysis and its occurence is included to thermodynamic modelling of Ti–Al–Ta phase diagram by CALPHAD approach. The lattice parameters of the coexisting γ, α₂ and τ phases change during ageing.     

Structure,magnetic and magnetocaloric properties of RE11Ge8In2 (RE = Gd–Tm)

A new tetragonal Gd₁₁Ge₈In₂ phase has been obtained by arc-melting and annealing at 800 °C. The structure has been determined and refined from single crystal X-ray diffraction data in the I4/mmm space group with a = 11.2091(6) and c = 16.3994(9) Å. Phases with the RE₁₁Ge₈In₂ (RE = Gd–Tm) composition were subsequently synthesized and their structures were refined using X-ray powder diffraction methods. Magnetic measurements carried out on RE₁₁Ge₈In₂ (RE = Gd–Tm) indicated a ferromagnetic ordering in all phases. The magnetocaloric effect in terms of the magnetic entropy change, ΔS_mag, was evaluated for the Gd-, Tb- and Tm-containing samples, and the largest |ΔS_mag| value of 10.6 J/kg K was obtained for Tm₁₁Ge₈In₂.     

The effects of Ti and Mo additions on the microstructure of Nb-silicide based in situ composites

The effects of Ti and Mo additions on phase selection, phase transformations and microstructure development in as cast and heat-treated Nb–Si–Cr–Al in situ composites have been investigated. The βNb₅Si₃ was formed in all the as cast alloys, which are classified as hyper-eutectic alloys. After heat treatment at 1500 °C, the βNb₅Si₃ transformed to αNb₅Si₃ completely or partially. The lattice parameter of the bcc Nb solid solution (Nb_ss) decreased with the addition of Ti and with increasing Mo concentration. The C14–Cr₂Nb Laves phase was absent in the alloys with Ti addition at 24at.% in the presence of Mo (≤5 at.%). The eutectoid decomposition of Nb₃Si to Nb_ss and αNb₅Si₃ was very sluggish in the alloy without Ti but it was enhanced with the addition of Ti. The partitioning of Ti between the Nb_ss and (Nb,Ti)₅Si₃ led to the formation of Ti-rich (Nb,Ti)₅Si₃, where the concentration of Ti was about 30.3 at.% in the heat treated microstructure. Nb_ss particles precipitated inside αNb₅Si₃ in the heat treated alloys.