Reactive sintering of TiAl–Ti5Si3 in situ composites

TiAl with between 0 and 20 vol%Ti₅Si₃ was produced by reactive sintering (700 °C for 15 min in vacuum) of cold pressed compacts of elemental Ti, Al and Si powder. The results show that adding Si to Ti and Al reduces the swelling associated with reactive sintering of TiAl, as composites containing more than 5 vol%Ti₅Si₃ densified during reactive sintering. However, composites containing more than 10 vol%Ti₅Si₃ did not retain their shape and the TiAl+20 vol%Ti₅Si₃ composite completely melted during the sintering process. A thermodynamic analysis indicated that the simultaneous formation of TiAl and Ti₅Si₃ increases the adiabatic flame temperature during the reaction between the powders. In fact, the analysis predicted that the maximum temperature of the reaction associated with the formation TiAl+20 vol%Ti₅Si₃ should exceed the melting point of TiAl, and this was observed experimentally. Differential thermal analysis (DTA) revealed that an Al–Si eutectic reaction occurred in mixtures of Ti, Al and Si powders prior to the formation of the TiAl and Ti₅Si₃ phases. There was no such pre-reaction formation of a eutectic liquid in Ti and Al powder mixtures. The formation of the pre-reaction liquid and the increase in adiabatic flame temperature resulted in the melting that occurred and the enhanced densification (minimization of swelling) during reactive sintering of the in situ composites.     

Formation and characterization of mechanically alloyed Ti–Cu–Ni–Sn bulk metallic glass composites

In the present study, amorphous Ti₅₀Cu₂₈Ni₁₅Sn₇ and its composite powders reinforced with 4, 8, and 12 vol.% of W additions were prepared by mechanical alloying. After 5 h of milling, amorphous powders with homogeneously dispersed W nanoparticles were synthesized. The as-milled Ti₅₀Cu₂₈Ni₁₅Sn₇ and composite powders were then consolidated by vacuum hot pressing into disc compacts with a diameter and thickness of 4 and 10 mm, respectively.The structure of the as-milled powders and consolidated compacts was characterized by X-ray diffraction (XRD), scanning electron microscopy, and transmission electron microscopy. While the thermal stability was examined by differential scanning calorimeter (DSC). In addition, the mechanical property of the consolidated BMGs was evaluated by Vickers microhardness tests. The experimental results showed that W nanoparticles ranged from 20 to 200 nm were embedded within the amorphous matrix. The presence of W nanoparticles did not dramatically change the glass formation ability of amorphous Ti₅₀Cu₂₈Ni₁₅Sn₇ powders. While the thermal stability of amorphous powders differed from those of its composites. A significant hardness increase with W additions was noticed for consolidated composite compacts.     

Formation of TiAl–Ti2AlC in situ composites by combustion synthesis

Preparation of TiAl–Ti₂AlC in situ composites with a broad range of composition was conducted by self-propagating high-temperature synthesis (SHS) with compressed samples from the mixture of elemental powders. When compared with SHS formation of monolithic TiAl, the addition of carbon particles to the Ti–Al powder mixture enhances the sustainability of the reaction. It was found that no prior heating was required for the samples prepared to produce the composites containing more than 20 mol% Ti₂AlC, in contrast to the need of preheating at 200 °C for single-phase TiAl formation. This is attributed to the fact that formation of Ti₂AlC is more exothermic than that of TiAl. As a result, the combustion temperature and combustion wave velocity increase with the content of Ti₂AlC formed in the TiAl–Ti₂AlC composite, and approach the values associated with formation of single-phase Ti₂AlC when considerable amounts of Ti₂AlC are yielded. The XRD analysis of the end products confirms formation of TiAl–Ti₂AlC in situ composites. Moreover, simultaneous formation of Ti₂AlC promotes the phase evolution of the aluminide compounds. That is, the secondary aluminide phase, Ti₃Al, was no longer detected in the TiAl–matrix composites containing Ti₂AlC of 30 mol% or above.     

Preparation of Ti+Ti6Si2B powders by high-energy ball milling and subsequent heat treatment

The present work reports on the preparation of two-phase Ti_SS+Ti₆Si₂B alloys by high-energy milling and subsequent heat treatment. The milled and heat-treated products were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and microanalysis via WDS. Results indicated the dissolution of silicon and boron atoms into the Ti lattice to form supersaturated solid solutions during the ball milling of Ti–10Si–5B and Ti–20Si–10B powders. TiB₂ precipitates were formed during ball milling, and the metastable structures were decomposed due to the released heat from its exothermic formation. After heat treatment at 1100 °C for 4 h, the equilibrium microstructures of the Ti–10Si–5B and Ti–20Si–10B alloys indicated the majority presence of the Ti and Ti₆Si₂B phases. TiB precipitates were found in Ti–10Si–5B and Ti–20Si–10B powders after heat treatment at 1200 °C for 16 h, indicating that the composition was moved from two-phase Ti+Ti₆Si₂B region to the three-phase Ti+Ti₆Si₂B+TiB field.     

Optimization of the Ti3SiC2 MAX phase synthesis

The synthesis of Ti₃SiC₂ MAX phase by self-propagating high-temperature synthesis (SHS) and pressureless argon shielding synthesis has been investigated following different pathways pertaining to the reactant systems Ti/Si/C, Ti/SiC/C and Ti/TiC/Si. Silicon in excess ranging from 10 to 50 mol% was employed to obtain powders mainly constituted by Ti₃SiC₂.Optimizing the excess of silicon and the pressing technique, the resultant powders with Ti₃SiC₂ content near to 100% were obtained. Result was consequent to the use of pressureless argon shielding synthesis obtained with 30 mol% of silicon excess in the examined different systems. The Ti₃SiC₂ was also obtained by SHS, but with lower proportion (88% and 86% from 3Ti + 1.2SiC + 0.8C and 3Ti + 1.3Si + 2C respectively). These results driving from XRD patterns were confirmed by FESEM observations and the EDAX analyses.     

Microstructure and some properties of TiAl-Ti2AlC composites produced by reactive processing

The TiAl–Ti₂AlC composites with and without impurities, Ni, Cl and P, were prepared by combustion reaction from the elemental powders and cast after arc melting. The resulting composites had about 18 vol% Ti₂AlC in the lamellar matrix consisting of γ-TiAl and Ti₃Al (α₂). In the homogenized specimens, the α₂ phase decomposed to γ-TiAl and Ti₂AlC. The composite material had a high strength both at ambient and elevated (1173 K) temperatures; about 800 and 400 MPa, respectively, with an ambient temperature ductility of 0.7% at bending test. The fracture toughness test also proved that the homogenized composite has higher toughness than the as cast one. The toughness value reached to 17.8 MPa m^1/2. The zigzag cracks propagated in the homogenized composite and the reinforcement Ti₂AlC particles and the finely precipitated Ti₂AlC particles were main obstacles to the crack propagation. The composite with impurities showed a marginal improvement in the oxidation resistance over the composites without impurities.     

Creep behaviour of a cast TiAl-based alloy for industrial applications

The creep behaviour of a cast TiAl-based alloy with nominal chemical composition Ti–46Al–2W–0.5Si (at.%) was investigated. Constant load tensile creep tests were performed in the temperature range 973–1073 K and at applied stresses ranging from 200 to 390 MPa. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent n is determined to be 7.3 and true activation energy for creep Q is calculated to be 405 kJ/mol. The initial microstructure of the alloy is unstable during creep exposure. The transformation of the α₂(Ti₃Al)-phase to the γ(TiAl)-phase, needle-like B2 particles and fine Ti₅Si₃ precipitates and particle coarsening are observed. Ordinary dislocations in the γ-matrix dominate the deformation microstructures at creep strains lower than 1.5%. The dislocations are elongated in the screw orientation and form local cusps, which are frequently associated with the jogs on the screw segments of dislocations. Fine B2 and Ti₅Si₃ precipitates act as effective obstacles to dislocation motion. The kinetics of the creep deformation within the studied temperature range and applied stresses is proposed to be controlled by non-conservative motion of dislocations.     

Microstructural stability of a cast Ti–45.2Al–2W–0.6Si–0.7B alloy at temperatures 973–1073 K

Microstructural stability of a cast intermetallic Ti–45.2Al–2W–0.6Si–0.7B (at.%) alloy after ageing in the temperature range from 973 to 1073 K up to 10,000 h was studied. The microstructure of the alloy consists of lamellar and TiAl-rich regions. Lamellar regions consist of fine lamellae of γ(TiAl) and α₂(Ti₃Al) phases, fine B2 and Ti₅Si₃ precipitates on the lamellar interfaces. Fine α₂ lamellae are partially decomposed before ageing. Small volume fraction of coarse B2 particles and few ribbon-like borides were found within lamellar regions. TiAl-rich regions contain coarse α₂ lamellae, rod-like B2 particles, small volume fraction of coarse Ti₅Si₃ precipitates and ribbon-like borides. The as-received microstructure of the alloy is unstable during long-term ageing at temperatures ranging from 973 to 1073 K. The α₂ phase in the lamellar and TiAl-rich regions transforms to the γ phase and fine needle-like B2 precipitates. The microstructural instabilities lead to softening of the alloy. The microhardness decrease is measured to be faster in the lamellar regions compared to that in the TiAl-rich regions.     

Microstructure and mechanical behavior of NiAl-based alloy prepared by powder metallurgical route

Rapidly solidified NiAl–28Cr–6Mo–B–Dy prealloyed powder doped with Nb powder was consolidated by hot pressing under 1250 °C for 30 min at 30 MPa. The consolidated material exhibited a different microstructure from the original powder, i.e. the NiAl and Cr(Mo) plates in the eutectic cell tend to break down into short platelets or even particles during hot pressing process. The mechanical behaviors at room temperature and at high temperature of consolidated sample from powder alloy were evaluated by three-point bending technique, tensile test and compressive test. The results showed that the hot pressing alloy possessed a reasonable combination of room temperature ductility and toughness, and elevated temperature strength.     

Oxidation behavior of sulfidation processed TiAl–2 at.%X (X=Si,Mn, Ni,Ge, Y,Zr, La,and Ta) alloys at 1173 K in air

The oxidation behavior of sulfidation processed TiAl–2 at.%X (X=Si, Mn, Ni, Ge, Y, Zr, La, and Ta) alloys was investigated at 1173 K in air for up to 630 ks under a heat-cycle condition between 1173 K and room temperature. During the sulfidation processing the TiAl–2 at.%Ta alloy formed Ta-aluminides on the TiAl₃ layer, while the alloys containing Mn, Ni, Y, and Zr formed a TiAl₃ (TiAl₂ included) layer including a small amount of the third element, like the TiAl binary alloy. The cross-sectional microstructure of the TiAl–2 at.%Ta alloy shows the sequence: oxide scale/TiAlTa/TiAl₂/alloy substrate; and the cross sections of the alloys containing Mn, Ni, Y, and Zr are: oxide scale/Ti₃Al/alloy substrate. The TiAl–2 at.%Ta alloy showed some scale exfoliation at the initial stage of oxidation, but very little exfoliation after long oxidation times, whereas alloys containing other third elements such as Si and Ge showed little exfoliation at the first several cycles and then tended to exfoliate significantly, resulting in very rapid oxidation. The TiAlTa/TiAl₂ layers formed by the reaction between the Ta-aluminide and TiAl₃ improve the oxidation properties of the TiAl–2 at.%Ta alloy.     

Synthesis of high purity titanium silicon carbide from elemental powders using arc melting method

The objective of this study is to investigate the formation of Ti₃SiC₂ from Ti/Si/C powders using the arc melting method. The results show that the sample sintered at 80 s produced a near single-phase of Ti₃SiC₂ (99.2 wt.%) with a relative density of 88.9%. These results were confirmed by phase determination using XRD analysis and were supported with micrographs from FESEM/EDX analyses. The relative density and porosity of all samples were dependent on the formation of macropores in bulk samples and micropores in TiC_x grains. The proposed reaction mechanisms for the synthesis of Ti₃SiC₂ by arc melting is that Ti₃SiC₂ might be formed from TiC_x + Si, Ti₅Si₃C_x + C, and Ti₅Si₃C_x + TiC_x at early arcing time (≤ 10 s), while TiC_x + TiSi₂ take place at 15 s to 80 s. After 80 s, decomposition of Ti₃SiC₂ into TiC_x, TiSi₂ and C was observed.     

Effect of silicidation pretreatment with gaseous SiO on sinterability of TiC powders

Silicidation pretreatment with gaseous SiO at 1350 °C for 30 min is employed for chemically modifying commercially available TiC powder. Phase composition and microstructural features of the pretreated powder are discussed. Densification behavior of the pretreated TiC powder during hot pressing is studied in comparison with that of non-pretreated one. Significantly improved densification behavior and sinterability of TiC powder after silicidation pretreatment are explained by the effect of Ti₃SiC₂ acting as a solid lubricant. Nearly fully dense TiC-based ceramics having flexural strength of 370 MPa, fracture toughness of 5.6 MPa m^½, and microhardness of 24 GPa is obtained by hot pressing under conditions as mild as 1600 °C and 20 MPa.     

Mechanochemical synthesis of TiN/TiB2/Ti-silicide nanocomposite powders and their thermal stability

Extremely fine and homogeneous TiN/TiB₂/Ti-silicide composite powders have been synthesized from mixtures of Ti, BN and Si₃N₄ powders by high-energy ball milling through a mechanically activated self-sustaining reaction. They have a microstructure consisting of TiN and TiB₂ crystallites of less than 15 nm embedded in amorphous TiSi₂ or Ti₅Si₃ matrix. When these nanocomposite powders were annealed at high temperatures, the microstructure did not change significantly and TiN and TiB₂ mutually suppressed the grain growth of both phases effectively.     

Manufacturing of Al–Mg–Si alloy foam using calcium carbonate as foaming agent

Aluminium foams can be manufactured by two main methods: casting and powder metallurgy. When the latter route is used, a foaming agent (usually TiH₂) is mixed with the aluminium or aluminium alloy powders, followed by powder mixture consolidation (usually hot extrusion) into a precursor and finally its foaming treatment. In this research, two calcium carbonate powders were used as foaming agents on an Al–Mg–Si (AA6061) alloy. Their different characteristics (particle size and chemical composition) modified the manufacturing process to achieve the final foam. AA6061 powders were then mixed with 10% calcium carbonate and, after cold isostatic pressing into green cylinders, hot extruded at different temperatures (475–545 °C). The foaming treatment was carried out in a furnace preheated to 750 °C using several heating times. The density changed from 2.03 to 2.10 g/cm³ after cold isostatic pressing to 2.64–2.69 g/cm³ in precursor materials obtained by hot extrusion. Foaming behaviour depends on the carbonate powder as well as the extrusion temperature. Thus, natural carbonate powder (white marble) produces a foam density close to 0.65 g/cm³ after a shorter time than when chemical carbonate is used. The foam structure showed a low degree of aluminium draining, no wall cell cracks and a good fine cell size distribution. Compressive strength of 6.11 MPa and 1.8 kJ/m³ of energy absorption were obtained on AA6061 foams with a density between 0.53 and 0.56 g/cm³.     

Role of nitrogen on the oxidative stability of Ti5Si3 based alloys at elevated temperature

Ti₅Si₃ has been extensively studied as a candidate material for high temperature application due to its high melting point (2130 °C), low density (∼4.3 g/cm³) and excellent oxidation resistance in oxygen above 1000 °C. However, stoichiometric Ti₅Si₃ alloy experiences accelerated oxidation during exposure in air above 1000 °C. It was proposed that nitrogen was responsible for the increased oxidation in air. In the present study, the isothermal reaction kinetics of Ti₅Si₃ in nitrogen at 1000 °C was investigated. Compared to a slow parabolic oxidation rate in oxygen, a faster linear reaction rate was observed when Ti₅Si₃ is exposed to nitrogen. Further studies on the oxidation behavior for changing nitrogen/oxygen atmospheres showed that Ti₅Si₃ is stable for exposure up to 400 h at 1000 °C when the gas contained 50% N₂. Breakaway oxidation occurs after short exposures when the gas contained at least 75% N₂, and the reaction rate increased as the concentration of N₂ increased. Furthermore, time to breakaway oxidation decreases with the increasing nitrogen partial pressure. Extensive analysis of the oxidation products with SEM and XRD revealed that the formation and fast growth of a nitride-containing subscale interferes with the establishment of the continuous protective silica scale and contributes to the breakaway oxidation.     

Ordering process of Al5Ti3, h-Al2Ti and r-Al2Ti with f.c.c.-based long-period superstructures in rapidly solidified Al-rich TiAl alloys

Change in microstructure and stability of superstructural phases in Al-rich TiAl alloys containing 58.0–62.5 at.% Al were investigated using melt-spun ribbons. Ordering processes of long-period ordered phases such as Al₅Ti₃, h-Al₂Ti and r-Al₂Ti in the L1₀ matrix during annealing were examined. The presence of Al₅Ti₃ and h-Al₂Ti phases in the L1₀ matrix was confirmed in melt-spun Ti–60.0 at.% Al and Ti–62.5 at.% Al ribbons by electron diffraction patterns, while diffuse scattering corresponding to the Al₅Ti₃ superstructure appeared in Ti–58.0 at.% Al ribbon. In Ti–58.0 at.% Al ribbon, the Al₅Ti₃ phase developed as an island in the L1₀ matrix having an obscure coherent boundary at and below 800°C, while it dissolved during annealing above 800°C. Although the r-Al₂Ti phase was finally formed as an equilibrium phase, the ordering of Al₅Ti₃ and metastable h-Al₂Ti phases in Ti–60.0 at.% Al and Ti–62.5 at.% Al ribbons occurred prior to the precipitation of the r-Al₂Ti during annealing below 800°C. The priority for the ordering process is discussed on the basis of crystal symmetry and periodicity of Al layers parallel to the (002) plane. The anti-phase boundaries (APBs) based on the Al₅Ti₃-type ordering were observed along {110) planes in Ti–62.5 at.% Al ribbon annealed at 700°C and their energies were calculated using the interaction energy between neighbouring atoms.     

Synthesis and characterization of the Mg-based amorphous/nano ZrO2 composite alloy

Mg-based composites are fabricated through mechanical alloying (MA) in the planetary mill, using amorphous Mg₆₅Cu_25−xY₁₀Ag_x (x=0, 5, 10) matrix alloy prepared by melt spun and 1–5 vol.% spherical nano-sized ZrO₂ particles. The melt spun amorphous matrix ribbons are ground into powders and mixed with the ZrO₂ nano particles in the planetary mill, after then formed by hot pressing in Ar atmosphere under different pressures at the temperature 5 K above the glass transition temperature (T_g). The microstructure characterizations of the resulting specimens are conducted by means of XRD, FEG-SEM, and TEM techniques. It is found that the nano-sized ZrO₂ dispersed Mg-based composite alloy powders can reach to a homogeneous size distribution (about 80 nm) after 50 h mechanical alloying. After hot pressing of these composite alloy powders under the pressure of 1100 MPa at 409 K, a 96% dense bulk specimen can be formed. Throughout the MA and hot pressing, the amorphous nature of the Mg₆₅Cu_25−xY₁₀Ag_x matrix is maintained. The hardness of the formed bulk Mg-based composites (with 5 vol% nano-sized ZrO₂ particles) can reach to 360 in H_v scale. In addition, the microstructure near the interface between the matrix and nano particles presents a well-bonded condition.     

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 site occupancies of alloying elements in TiAl and Ti3Al alloys

The site occupancies of V, Cr, Mn, Fe, Ni, Zr, Nb, Mo, Ta, Ga and Sn (1–5 at.%) in TiAl alloys with different compositions, and in Ti₃Al with the compositions of Ti–26 at.%Al–(1–2 at.%)X, were measured by the atom location channelling enhanced microanalysis (ALCHEMI) method. For TiAl alloys, the results show that Zr, Nb and Ta atoms invariably occupy Ti sites, while Fe, Ni, Ga and Sn atoms occupy Al sites, the alloy composition having no significant influence on their site preference. By contrast, the site preference of V, Cr, and Mn changes considerably with alloy composition (the Ti/Al ratio in particular), the probability of these elements substituting for Ti decreasing in the above order. For quaternary Ti–Al–V–Cr alloys, the site occupancies of V and Cr do not show much mutual influence. In general, with increasing atomic number, elements in the same period show increasing tendency to substitute for Al, as is the tendency to substitute for Ti for elements in the same group of the periodic table. For Ti₃Al alloys, Ga and Sn atoms occupy Al sites, while V, Cr, Mn, Zr, Nb, Mo and Ta atoms occupy Ti sites, the site preference of V, Cr, Mn and Mo in TiAl alloys being different from that in Ti₃Al. The experimental results are interpreted in terms of a Bragg–Williams-type model and bond-order data obtained from electronic structure calculation. Qualitative agreement between the model and measurements is reached.     

The effect of zirconium addition on microstructure and properties of ball milled and hot compacted powder of Al–12 wt% Zn–3 wt% Mg–1.5 wt% Cu alloy

Elemental powders of the composition Al–12 wt% Zn–3 wt% Mg–1.5 wt% Cu with addition of 1 and 2 wt% Zr were ball milled in a planetary high-energy ball mill and then hot pressed in vacuum under 600 MPa pressure at 380 °C. The effect of ball milling and hot pressing on the microstructure was investigated by means of X-ray diffraction measurements (XRD), light microscopy, analytical and scanning transmission electron microscopy (TEM). Ball milling for 80 h leads to homogenous, highly deformed microstructure of aluminium solid solution with grain size below 100 nm. In the powder with zirconium addition, some part of the Zr atoms diffused in aluminium up to 0.3 wt% Zr. The remaining was found to form Zr-rich particles identified as face centered cubic (fcc) phase. Good quality samples without pores and cracks obtained by hot pressing composed of grains and subgrains of size below 200 nm. The particles of MgZn₂ phase were identified which were located mainly between compacted particles of milled powder. Hot pressed powder showed Vickers microhardness of about 195 HV (0.2 N) and ultimate compression strength in the range 611–658 MPa in the compression test. Addition of zirconium had no influence on the strength of the compacted powders.