Microstructure and oxidation behaviors of near-α Ti-6.5Al-4Sn-4Zr-0.5Mo-based alloys with Ir addition

The microstructure and oxidation behaviors of near α-Ti-based alloys with small amount of iridium (Ir) additions were investigated. The microstructure of both Ir-free and Ir-containing alloys was observed to consist of α + β Widmanstätten colonies. The β lamellae gradually became continuous with increasing Ir additions since Ir acted as a β-stabilizer in the alloys. Isothermal oxidation test indicated that Ir addition reduced the oxidation resistance at 650 °C; while at 750 °C, the adherence of thermally grown oxides was enhanced, and a thin Al₂O₃-enriched layer on the oxide scale was promoted in the Ir-containing alloy, which suggests that Ir addition was effective in improving oxidation resistance of near-α-based alloys at 750 °C.     

Effect of microstructure on the fatigue crack growth behaviour of a near-α Ti alloy

Fatigue crack growth behaviour of Ti–6Al–2Zr–1.5Mo–1.5V (VT-20 a near-α Ti alloy) was studied in lamellar, bimodal and acicular microstructural conditions. Fatigue crack growth tests at both increasing and decreasing stress intensity factor range values were performed at ambient temperature and a loading ratio of 0.3 using compact tension samples. Lamellar and acicular microstructures showed lower fatigue crack growth rates as compared to the bimodal microstructure due to the tortuous nature of cracks in the former and the cleavage of primary α in the latter. The threshold stress intensity factor range was highest for acicular microstructure.     

Rolling texture and its effect on tensile property of a near-α titanium alloy Ti60 plate

Rolling texture and its effect on tensile properties of Ti60 alloy plates were investigated in the present study. The plates wereβ-rolled at 1070℃ and(α+β)-rolled at 980℃, using uni-directionally rolling(UDR) and cross-directionally rolling(CDR) processes, respectively.β-rolled plates exhibited weak textures, which were attributed to the dispersive orientations of secondary α during the β→α phase transformation. Strong deformation textures formed in(α+β)-rolled plates as a result of slipping mechanisms: the strong T-type texture in UDR plate was related to {10 1 0}[11 2 0] slipping, while the B-type texture in CDR plate was relevant with {0001}[11 2 0] slip. Strong T-type textures led to anisotropic tensile properties. B-type textures would decrease such an anisotropy. The(α+β)-CDR process was found to be a candidate process for reducing anisotropy of Ti60 alloy plates.     

High temperature strength,fracture toughness and oxidation resistance of Nb–Si–Al–Ti multiphase alloys

Nb–Si–Al–Ti quaternary phase diagram around three-phase region, which consists of niobium solid solution (Nb_ss), Nb₃Al and Nb₅Si₃,is constructed in this study. The three-phase region exists up to titanium content of about 20 mol%. Based on the quaternary phase diagram, three-phase alloys containing Nb_ss from about 50 to 75% in volume are prepared to improve high temperature strength, room temperature fracture toughness and oxidation resistance simultaneously.

When microstructure and composition are optimized (Nb_ss + Nb₃Al + Nb₅Si₃) three-phase alloy with the addition of titanium exhibits higher compressive strength than nickel-based superalloys at room temperature to 1573 K. Fracture toughness at room temperature of (Nb_ss + Nb₃Al + Nb₅Si₃) three-phase alloys is increased to over 12 MPa m^1/2 by the addition of titanium without sacrificing high temperature strength. Oxidation resistance of (Nb_ss + Nb₃Al + Nb₅Si₃) three-phase alloys is improved by the addition of titanium.     

High temperature strength,fracture toughness and oxidation resistance of Nb–Si–Al–Ti multiphase alloys

Nb–Si–Al–Ti quaternary phase diagram around three-phase region, which consists of niobium solid solution (Nb_ss), Nb₃Al and Nb₅Si₃, is constructed in this study. The three-phase region exists up to titanium content of about 20 mol%. Based on the quaternary phase diagram, three-phase alloys containing Nb_ss from about 50 to 75% in volume are prepared to improve high temperature strength, room temperature fracture toughness and oxidation resistance simultaneously.When microstructure and composition are optimized (Nb_ss+Nb₃Al+Nb₅Si₃) three-phase alloy with the addition of titanium exhibits higher compressive strength than nickel-based superalloys at room temperature to 1573 K. Fracture toughness at room temperature of (Nb_ss+Nb₃Al+Nb₅Si₃) three-phase alloys is increased to over 12 MPa m^1/2 by the addition of titanium without sacrificing high temperature strength. Oxidation resistance of (Nb_ss+Nb₃Al+Nb₅Si₃) three-phase alloys is improved by the addition of titanium.     

Effects of heat treatments on microstructure and tensile properties of as-extruded TiBw/near-α Ti composites

The effects of different heat treatments on the microstructure and tensile properties of the as-extruded TiBw/Ti60 composites have been investigated. Without solution treatment, the α₂ phase precipitated from α phase and mounts of elliptical S₂ silicide are formed at the interface of α/β phases after aging at 700 °C for 5 h, which results in clear enhancement of strength at 700–800 °C. After the solution and aging treatment, more α₂ phase precipitates from α_p phase and less spherical S₂ silicide precipitated from the body of both α_p and transformed β phase. Moreover, some small silicide precipitates are formed from α matrix near the TiBw due to the stacking faults and the interface effect. The TiBw are stable without any interfacial reaction during the heat treatments. The results show that the composites performed by solution and aging treatment exhibit high strength (1377 MPa) and low elongation (2%). The annealing treatment can soften the composites and increase the elongation to 7%.     

A comparative evaluation of friction and electron beam welds of near-α titanium alloy

Electron beam and friction welding studies have been carried out on a TIMET 834 a near-α titanium alloy. The study was also focused on the post-weld heat treatment aspects and their influence on mechanical properties. The influence of parent metal microstructure on the toughness has been evaluated to explain the trends observed on the toughness of welds. Electron beam welds contained micro-porosity while friction welds were free from such solidification related defects. Friction welds exhibited very fine equiaxed prior β grains as compared to electron beam welds. Electron beam welds exhibited better creep and stress rupture properties and marginally higher strength. The impact toughness of both the welds was comparable. Post-weld ageing of welds resulted in reduction in impact toughness due to precipitation of silicides along the martensite lath boundaries.     

The effect of Ni additions on the oxidation behaviour of Co–Re–Cr high-temperature alloys

Abstract

The present study focused on the influence of Ni on the microstructure and oxidation behaviour of Co–Re–Cr-based alloys. Alloys with three different Ni contents were tested in laboratory air at 800–1100 °C. A refinement and a reduction of the σ phase volume fraction as well as a change in the matrix microstructure were observed. Thermogravimetric measurements showed that the alloys with higher Ni contents possess a better oxidation resistance when exposed to higher temperatures. All alloys suffered from continuous mass loss during oxidation at 800 °C due to the formation of porous oxides scales, consisting of Co₃O₄, Co(Ni)O and Ni-doped CoCr₂O₄, which allow the evaporation of Re-oxides. At 900–1100 °C, only the alloy with 25 at. % Ni showed parabolic oxidation kinetics after a short period of transient oxidation. This is a result of the fast formation of a protective Cr₂O₃ layer. It was also found that exposure to air at 1000 °C leads to a phase transformation of the bulk material; an oxidation-induced formation of fine hexagonal close-packed (hcp) grains was observed near the oxide scales. It is supposed that the improved oxidation resistance of Ni-containing Co–Re–Cr alloys is a result of enhanced Cr diffusion caused by the Ni addition. The extensive formation of the fcc phase in the alloy matrix had a detrimental effect on the oxidation behaviour of the Ni-containing Co–Re–Cr-based alloys.     

Microstructure and grindability of as-cast Ti–Sn alloys

In this study, the structure, microhardness, and grindability of a series of binary Ti–Sn alloys with tin contents ranging from 1 to 30 wt% were investigated. Commercially pure titanium (c.p. Ti) was used as a control. The experimental results indicated that all the Ti–Sn alloys showed hcp α structure, and the hardness values of the Ti–Sn alloys increased with greater Sn contents, ranging from 246 HV (Ti–1Sn) to 357 HV (Ti–30Sn). Among these Ti–Sn alloys, the alloy with 30 wt% Sn content showed the highest hardness value. The grindability of each metal was found to be largely dependent on the grinding conditions. The addition of Sn to c.p. Ti did contribute to improving the grindability of c.p. Ti. The Ti–Sn alloys with a higher Sn concentration could be ground more readily. The grinding rate of the Ti–20Sn alloy at 1200 m/min was about 2.8 times higher than that of c.p. Ti. Additionally, the grinding ratios of the Ti–10Sn, Ti–20Sn, and Ti–30Sn alloys at 1200 m/min were about 2.8, 2.7, and 3.4 times that of c.p. Ti, respectively. Our research suggests that the Ti–Sn alloys with Sn contents of 10 wt% and greater developed here are good candidates for machining by the CAD/CAM method.     

Synthesis of (Ti1−xWx)3SiC2 MAX phase solid solution and its high-temperature oxidation performance

Journal of Materials Science: Materials in Electronics - In this study, (Ti1?xWx)3SiC2 (x?=?0–0.3) MAX phase ceramics were prepared via a solid-phase reaction synthesis...     

Phase transformations in a welded near-α titanium alloy as a function of weld cooling rate and post-weld heat treatment conditions

In - titanium alloys, the high-temperature phase can decompose in several ways, depending on alloy composition and cooling rate. In the case of welded joints, cooling rates can vary widely as a function of heat input. In the current work, a dilute - Ti-Al–Mn alloy was welded over a range of heat inputs using electron beam and gas tungsten-arc welding processes. A major part of the rapidly cooled electron beam weld could be identified as lath-type or massive martensite. In the slower cooled gas tungsten-arc welds, the transformation product was a mixture of lamellar and phases formed entirely by diffusion. Post-weld heat treatment resulted, in all cases, in an - structure that coarsened with annealing temperature and time. Tensile elongation in the as-welded condition was poor on account of a large prior- grain size and an acicular microstructure. The ductility improved as the structure coarsened on heat treatment. Tensile fracture was always microscopically ductile, but the presence of a grain boundary layer tended to induce intergranular rupture, especially when a hard, intragranular matrix confined slip to occur in the grain boundary regions.     

Design and development of powder processed Fe–P based alloys

The present investigation deals with designing Fe, Fe–P binary and Fe–P–Si ternary alloys produced by an in-house developed powder metallurgical technique based on ‘Hot Powder Preform Forging’. Proper soaking of preforms at high temperature (1050 °C) eliminates iron-phosphide eutectic and brings entire phosphorus into solution in iron. Attempting hot forging thereafter completely eliminates hot as well as cold shortness and thereby helps to form these preforms (alloys) into very thin sheets of 0.5 mm. The use of costly hydrogen atmosphere during sintering has been eliminated by the addition of carbon as a reducing agent to form CO gas within the compact by reacting with oxygen of iron powder particles. The glassy ceramic coating applied over the compact serves as a protective coating to avoid atmospheric oxygen attack over the compact held at high temperature. These alloys so formed were subjected to density examination at various stages. Microstructural study has been carried out to estimate the grain size, volume percentage of porosity in the alloys, and uniform distribution of phosphorus and silicon in an iron matrix. X-ray diffraction studies of these alloys revealed the presence of only ferrite as product phase. Addition of alloying elements such as P and Si has improved the resistivity and magnetic properties of iron. Fe–0.07C–0.2O–0.3P–0.5Si alloy showed a resistivity as high as 31.7 μΩ cm. Coercivity values of the alloys ranged from 0.51 to 1.98 Oe. The total magnetic loss of Fe–0.07C–0.2O–0.3P–0.5Si alloy was the lowest (2.03 W/kg) amongst the alloys developed owing to its high resistivity combined with its low coercivity. These alloys which are drawn to thin sheets could find their possible application in the manufacturing of transformer cores.     

Formation mechanism of lamellae during β→α transformation in polycrystalline dual-phase Ti alloys

Phase field simulations incorporating contributions from chemical free energy and anisotropic interfacial energy are presented for the β→α transformation in Ti-6Al-4 V alloy to investigate the growth mechanism ofα lamellae of various morphologies from undercooled β matrix.The α colony close to realistic microstructure was generated by coupling the Thermo-Calc thermodynamic parameters of α and β phases with the phase field governing equations.The simulations show that α lamellar side branches with feathery morphology can form under a certain combination of interfacial energy anisotropy and temperature.α lamellae tend to grow slowly at high heat treatment temperature and become wider and thicker as temperature increase from 800 to 900 ℃ provided that the interfacial energy anisotropy ratio kx: ky was set as 0.1: 0.6.Besides,higher interfacial energy anisotropy can accelerate the formation of α lamellae,and the equilibrium shape of α lamellae changes from rod to plate as the interface energy anisotropy ratio kx∶ ky vary from 0.1∶ 0A to 0.1∶ 0.8 under 820 ℃.Experiments were conducted to study the α lamellar side branches in Ti-6Al-4 V (Ti-6.01Al-3.98 V,wt.％) and Ti-4211 (Ti-4.02Al-2.52V-1.54Mo-1.03Fe,wt.％) alloys with lamellar microstructure.Electron backscatter diffraction (EBSD) results show that α lamellar side branches and their related lamellae share the same orientation.The predicted temperature range for α lamellar side branches formation under various interfacial energy anisotropy is consistent with experimental results.     

Remarkable superelasticity of sintered Ti–Nb alloys by Ms adjustment via oxygen regulation

Nickel-free Ti–Nb alloys were fabricated by conventional powder metallurgy sintering method. The oxygen content of sintered Ti–Nb alloys were successfully decreased to around 0.8 wt.% by putting TiH₂ powders aside the samples during the sintering process. For the first time, the phase transformation behaviors of these sintered Ti–Nb alloys were observed by differential scanning calorimetry (DSC) measurement. The relationship between martensitic transformation start temperature (M_s) and mechanical properties, including elastic modulus and recoverable strain, was established by compression tests carried out at room temperature. While M_s was close to test temperature, an obvious drop of elastic modulus was observed, which was related to the “β-α” transformation. In the meantime, a remarkable recoverable strain as high as 5% was obtained at room temperature, which is the highest value reported in sintered Ti–Nb alloys until now. The results of this study reveal that the recoverable strain can be further improved by adjusting M_s close to the service temperature, which was neglected in the reported studies before and will provide some guidance for the future design and fabrication of porous Ni-free Ti-based shape memory alloys for biomedical application.     

Thermal oxidation of CP Ti — An electrochemical and structural characterization

In the present study commercially pure titanium (CP Ti) samples were oxidized thermally at three different temperatures (500, 650 and 800 °C) for 24 h and evaluation of their morphological and structural characteristics, microhardness and corrosion resistance in Ringer's solution was done. The corrosion protective ability of thermally oxidized materials shows a strong dependence on the nature and thickness of the surface oxide layer. Based on the corrosion protective ability, the untreated and thermally oxidized samples can be ranked as follows: CP Ti (800 °C) > CP Ti (650 °C) > CP Ti (500 °C) > untreated CP Ti.     

Transient oxidation of alumina forming Ti–Al–Ag-based alloys and coatings studied by SEM,AFM, XPS and LRS

Abstract

γ-TiAl based intermetallics possess poor oxidation properties at temperatures above approximately 700°C. Previous studies showed that protective alumina scale formation on γ-TiAl can be obtained by small additions (around 2 at.%) of Ag. Recently, this type of materials has therefore been proposed as oxidation resistant coatings for high strength TiAl alloys. In the present study, a number of cast Ti–Al–Ag alloys and magnetron sputtered Ti–Al–Ag coatings were investigated in relation to transient oxide formation in air at 800°C. After various oxidation times the oxide composition, microstructure and morphology were studied by combining a number of analysis techniques, such as SEM, ESCA, AFM and LIOS-RS. The γ-TiAl–Ag alloys and coatings appear to form an α-Al₂O₃ oxide scale from the beginning of the oxidation process, in spite of the relatively low oxidation temperature of 800°C. The formation of metastable alumina oxides seems to be related to the presence of Ag-rich precipitates in the alloy matrix.     

Microstructure and internal friction of Ni–Ti alloys absorbing hydrogen

The microstructure and internal friction of Ni–Ti alloys after hydrogen absorption have been investigated by means of optical microscopy, X-ray diffraction, differential scanning calorimetry and low frequency torsional internal friction apparatus. The results show that, after hydrogen absorption, the grains tend to be elongated, and the nucleation and growth of hydrides are mainly concentrated at the grain boundaries. In addition, a new phase ultimately identified as Ti₂NiH_0·5 phase forms after absorption of hydrogen. The hydrogen induced martensite promotes the emergence of a two-stage transformation. However, the growth of hydrides causes a reduction of the hydrogen induced martensite. The hydrides act as strong pinning points, resulting in a dramatic increase in the internal friction. In addition, the marked change of the internal stress, caused by the microscopic strain and the mismatching of the volumes, also improves the internal friction.     

Enhanced oxidation resistance of Mo-12Si-8.5B alloys with ZrB_2 addition at 1300℃

Mo-12Si-8.5B and Mo-12Si-8. B-1.0 wt%ZrB_2 alloys were fabricated using mechanical alloying, followed by hot-pressing. Both alloys exhibited uniform microstructure, with the Mo_3 Si and Mo_5SiB_2 phases distributing dispersedly in the α-Mo matrix. Mo-12Si-8.5B-1.0 wt%ZrB_2 showed a finer-grained microstructure than Mo-12 Si-8.5 B alloy owing to the addition of ZrB_2. The results of isothermal oxidation tests at 1300℃ in air revealed that Mo-12Si-8.5B and Mo-12Si-8.5B-1.0 wt%ZrB_2 alloys initially suffered a transient stage with high mass loss due to the volatilization of MoO_3, and then achieved a steady stage owing to the formation of a protective borosilicate scale on the alloy surface. Especially, the transient stage of Mo-12Si-8.5B-1.0 wt%ZrB_2 alloy was shortened to be less than 300s, and the mass loss of this stage was reduced by at least 88% compared with that of Mo-12Si-8.5B alloy, indicating a significant improvement in the oxidation resistance. The addition of ZrB_2 not only resulted in a continuous borosilicate scale quickly covering the entire base alloy during the transient stage, but also improved the protectiveness of the borosilicate scale of the steady stage by bringing out a large number of ZrO_2/ZrSiO_4 particles embedded discontinuously in the borosilicate scale, which effectively restricted the inward diffusion of oxygen by acting as diffusion barriers and decreased the thickness of inner oxide layers in particular.