Oxidation behavior of MoSi2 and Mo(Si,Al)2 coated Mo–0.5Ti–0.1Zr–0.02C alloy

MoSi₂ and Mo(Si, Al)₂ coatings were prepared on Mo–0.5Ti–0.1Zr–0.02C alloy using pack cementation process. Oxidation studies revealed that Mo(Si, Al)₂ coating had a much superior oxidation resistance in the temperature range from 400 to 900 °C, where pest disintegration of MoSi₂ occurs due to internal oxidation. The growth kinetics of Al₂O₃ layer formed on Mo(Si, Al)₂ coating was much slower than that of SiO₂ layer formed on MoSi₂ coatings during oxidation.     

Thermal and electrical properties of TiB2–MoSi2

The present communication reports the effect of MoSi₂ addition on high temperature thermal conductivity and room temperature (RT) electrical properties of TiB₂. The thermal diffusivity and the thermal conductivity of the hot pressed TiB₂–MoSi₂ samples were measured over a range from room temperature to 1000 °C using the laser-flash technique, while electrical resistivity was measured at RT using a four linear probe method. The reciprocal of thermal diffusivity of TiB₂ samples exhibit linear dependence on temperature and the measured thermal conductivity of TiB₂-2.5% MoSi₂ composites correlate well with the theoretical predictions from Hashin’s model and Hasselman and Johnson’s model. A common observation is that the thermal conductivity of all the samples slightly increases with temperature (up to 200 °C) and then decreases with further increasing temperature. It is interesting to note that both the thermal conductivity and electrical conductivity of TiB₂ samples enhanced with the addition of 2.5 wt.% MoSi₂ sinter additive. Among all the samples, TiB₂-2.5 wt.% MoSi₂ ceramics measured with high thermal conductivity (77 W/mK) and low electrical resistivity (12 μΩ-cm) at room temperature. Such an improvement in properties can be attributed to its high density and low volume fraction of porosity. On the other hand, both the thermal and electrical properties of TiB₂ were adversely affected with further increasing the amount of MoSi₂ (10 wt.%).     

Effect of WC content on the microstructure and mechanical properties of Ti(C0.5N0.5)–WC–Mo–Ni cermets

Ti(C_0.5N_0.5)–xWC–Mo–Ni system cermets were studied to investigate the effect of WC content on the microstructure and mechanical properties. The results showed that the lattice constant of Ti(C,N) hard phase in the Ti(C_0.5N_0.5)–WC–Mo–Ni system increases with the increase of WC content, but it is smaller than that in Ti(C_0.7N_0.3)–WC–Ni system. The microstructures of the cermets exhibit three kinds of grains: one has black core-grey rim structure, the second has white core-grey rim structure and the third has black core-white rim structure. The thickness of inner rim increases with the increasing of WC content. Additionally, the particle size of black core decreases with the increase of WC content in the range of 0–10 wt.%, and then increases slightly with further increase of WC. The TRS increases with the increase of WC content, but hardness decreases with the increase of WC content.     

Hot deformation behavior of Al–9.0Mg–0.5Mn–0.1Ti alloy based on processing maps

Hot deformation behavior of extrusion preform of the spray-formed Al–9.0Mg–0.5Mn–0.1Ti alloy was studied using hot compression tests over deformation temperature range of 300–450 °C and strain rate range of 0.01–10 s^?1. On the basis of experiments and dynamic material model, 2D processing maps and 3D power dissipation maps were developed for identification of exact instability regions and optimization of hot processing parameters. The experimental results indicated that the efficiency factor of energy dissipate (η) lowered to the minimum value when the deformation conditions located at the strain of 0.4, temperature of 300 °C and strain rate of 1 s^?1. The softening mechanism was dynamic recovery, the grain shape was mainly flat, and the portion of high angle grain boundary (>15°) was 34%. While increasing the deformation temperature to 400 °C and decreasing the strain rate to 0.1 s^?1, a maximum value of η was obtained. It can be found that the main softening mechanism was dynamic recrystallization, the structures were completely recrystallized, and the portion of high angle grain boundary accounted for 86.5%. According to 2D processing maps and 3D power dissipation maps, the optimum processing conditions for the extrusion preform of the spray-formed Al–9.0Mg–0.5Mn–0.1Ti alloy were in the deformation temperature range of 340–450 °C and the strain rate range of 0.01–0.1 s^?1 with the power dissipation efficiency range of 38%–43%.     

High temperature oxidation of Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C alloy

A newly developed Ti–46Al–6Nb-0.5W-0.5Cr-0.3Si-0.1C alloy was oxidized isothermally and cyclically in air, and its high-temperature oxidation behavior was investigated. When the alloy was isothermally oxidized at 700 °C for 2000 h, the weight gain was only 0.15 mg/cm². The parabolic rate constant, k_p (mg²/cm⁴·h), measured from isothermal oxidation tests was 0.002 at 900 °C and 0.009 at 1000 °C. Such excellent isothermal oxidation resistance resulted from the formation of the dense, continuous Al₂O₃ layer between the outer TiO₂ layer and the inner (TiO₂-rich, Al₂O₃-deficient) layer. The alloy also displayed good cyclic oxidation resistance at 900 °C. Some noticeable scale spallation began to occur after 68 h at 1000 °C during the cyclic oxidation test.     

Influence of Al on the microstructure and mechanical properties of Cr–Zr–(Al–)N coatings with low and high Zr content

Low Zr (S1) and high Zr (S2) quaternary Cr–Zr–(Al–)N coatings with increasing Al content were deposited by d.c. reactive magnetron sputtering. The structure, fracture cross-section morphology and mechanical properties of the coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), nanoindentation, scratch testing and Vickers micro-indentation testing. All the coatings present an fcc NaCl-type B1 structure; in the low Zr content coatings, the diffraction peaks shift towards higher angles as the Al content increases. The grain size is approximately constant in a range from 6 to 8 nm, except for high Zr content films where a significant decrease in crystalline order is observed (grain size ~ 2.5 nm). In both series, the microstructure changed from equiaxed to columnar with increasing Al content. The highest hardness and strongest adhesion values were achieved in coatings with lower Zr and Al content. Conversely, the coatings with high Zr and the highest Al content exhibited an abrupt decrease in hardness, adhesion strength and toughness.     

Formation and growth mechanism of a manganese phosphate coating on an as-cast Zr–Al binary alloy

The preparation of a manganese phosphate coating on an as-cast Zr–Al alloy is described. The alloy consisted of an α phase (α-Zr) and a β phase (Zr₂Al), where α-Zr is the matrix and Zr₂Al spreads along the grain boundaries discontinuously. The coating was divided by cracks to exhibit a network structure. The manganese phosphate coating showed a combination of crystalline and amorphous structure. The film consists of small particles densely packed. It is uniform and compact. The formation of the phosphate coating on substrates treated for short periods was investigated. Mn₅(PO₄)(OH)₂·H₂O and MnZr(PO₄)₂·4H₂O phases were found in the manganese phosphate coatings. The grain particles are preferentially deposited on the β phase rather than on the α phase.     

High-Temperature Corrosion of Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C Alloy in N2/0.1%H2S Gas

Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C alloy was corroded at 800–1100 °C for 200 h in N₂/0.1%H₂S gas to characterize its corrosion behavior in an aggressive H₂S-containing environment. The alloy displayed superior corrosion resistance because Ti and Al preferentially reacted with impurity oxygen in the gas to form TiO₂ and Al₂O₃. It corroded primarily by outward diffusion of Ti, Al, W, and Cr in addition to inward transport of sulfur, nitrogen, and oxygen. Scales were adherent and consisted of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner (TiO₂, Al₂O₃)-mixed layer. TiN and Ti₂AlN formed at the scale/matrix interface where sulfur, Nb, W, and Cr segregated.

     

Linear friction welding of Ti–5Al–2Sn–2Zr–4Mo–4Cr alloy with dissimilar microstructure

Dissimilar linear friction welding of Ti–5Al–2Sn–2Zr–4Mo–4Cr with bimodal and lamellar microstructures was produced. The microstructure evolution of the joint was investigated via OM, SEM, XRD, TEM and microhardness analysis. The temperature field of joint was calculated by a numerical model. The typical microstructures of weld center were recrystallized β grains with some acicular α′′ martensites. In the case of thermo-mechanically affected zone, some partial re-crystallization grains formed in severely deformed microstructures, where a mass of dislocations were observed. However, dislocations were rarely found in the recrystallized β grains of weld center, the temperature field of weld joints calculated was consistent with the microstructural evolution.     

Effect of predeformation on globularization of Ti–5Al–2Sn–2Zr–4Mo–4Cr during annealing

The microstructure evolution during annealing of Ti–5Al–2Sn–2Zr–4Mo–4Cr alloy was investigated. The results show that for the alloy compressed at 810 °C and 1.0 s^?1 deformation amount (height reduction) 20% and 50% and annealed at 810 °C, thermal grooving by penetration of β phase is sufficient during the first 20 min annealing, resulting in a sharp increase in globularization fraction. The globularization fraction continuously increases with the increase of annealing time, and a height reduction of 50% leads to a near globular microstructure after annealing for 4 h. For the alloy with deformation amount of 50% by compressing at 810 °C, 0.01 s^?1 and then annealed at 810 °C, thermal grooving is limited during the first 20 min of annealing and large quantities of high-angle grain boundaries (HABs) remain. With long time annealing, the chain-like α grains are developed due to the HABs, termination migration and Ostwald ripening. The present results suggest that a higher strain rate and a larger height reduction are necessary before annealing to achieve a globular microstructure of Ti–5Al–2Sn–2Zr–4Mo–4Cr.     

Refining performance of Al–3Ti–0.2C–5Sr on A356 alloy and electron microanalysis of ternary Al–Ti–Sr phases

The effect of Al-3Ti-0.2C-5Sr(wt%) grain refiner on the refining performance and modification of A356 alloy was investigated using optical microscope(OM).The morphology and crystal structure of ternary Al-Ti-Sr phases in Al-3Ti-0.2C-5Sr refiner were analyzed by scanning electron microscopy(SEM) and transmission electron microscopy(TEM).The results show that the ternary Al-TiSr phases in Al-3Ti-0.2C-5Sr refiner can promote the grain refining efficiency of A356 alloy.The ternary Al-Ti-Sr phases co-exist in two morphologies,i.e.,blocky-like phase and surround-like phase,besides,which both have the same chemical composition of Al_(34)Ti_3Sr.The crystal structure of Al_(34)Ti_3Sr is face-centered cubic,and the lattice parameter is determined to be about 1.52 nm.     

Grain and dendrite refinement of A356 alloy with Al–Ti–C–RE master alloy

Commercial A356 alloy was refined with a homemade Al-5Ti-0.25C-2RE master alloy, and the microstructure and macrostructure of the refined alloy were investigated. The results show that the grain refining effect of A356 is poor by the addition level of 0.5 wt% master alloy, but when the level reaches 3.0 wt% the grain can get a satisfactory refining effect. Dendrite of A356 can be effectively refined by addition of 0.5 wt% master alloy; however, the refining effect is not significantly improved by further increasing the addition of master alloy. Grain and dendrite refining effects are compared in this article, and the results show that the grain and dendrite exhibit different refining effects with the same addition level of master alloy. Dendrite is easier to reach the optimal refining effect than grain.     

Strength anomaly of the B2 phase in Ti–25Al–25Zr alloy

Mechanical behaviour of the single B2 phase in the alloy Ti–25Al–25Zr has been studied under compression with different strain rates at elevated temperatures. The alloy Ti–25Al–25Zr exhibits yield strength anomaly similar to those of the typical B2 intermetallics such as FeAl. The stress–strain curves of the alloy tested at 400 °C show type C serrations which are considered to be due to dislocation unlocking. Intersecting and straight slip lines are observed in specimens tested at 400 °C and 600 °C, respectively.     

Effects of grain refiner additions (Zr,Ti–B) and of mould variables on hot tearing susceptibility of recently developed Al–2 wt-%Cu alloy

Abstract

The hot tearing susceptibility of the new Al–2 wt-%Cu based alloys prepared using Zr and Ti–B additions was tested using the constrained rod casting mould under different mould variables. The 206 alloy type was used to evaluate the results obtained from this new alloy. It was found that the hot tearing susceptibility of the alloys under investigation decreases proportionally as the mould temperature is increased; thus, the hot tearing susceptibility of the Al–2 wt-%Cu and 206 alloys decreases from 21 for both the alloys to 3 and 9 respectively as the mould temperature is increased from 250 to 450°C. This beneficial effect of elevated mould temperatures may be attributed to a reduction in the contraction strain rate and in the porosity level. Grain refinement additions of Ti–B or Zr–Ti–B enhance the hot tearing resistance of the Al–2 wt-%Cu to a significant level.     

Structural characteristics and high-temperature tribological behaviors of laser cladded NiCoCrAlY–B4C composite coatings on Ti6Al4V alloy

In order to improve the hardness and tribological performance of Ti6Al4V alloy, NiCoCrAlY–B₄C composite coatings with B₄C of 5%, 10% and 15% (mass fraction) were fabricated on its surface by laser cladding (LC). The morphologies, chemical compositions and phases of obtained coatings were analyzed using scanning electronic microscope (SEM), energy dispersive spectrometer (EDS), and X-ray diffraction (XRD), respectively. The effects of B₄C mass fraction on the coefficient of friction (COF) and wear rate of NiCoCrAlY–B₄C coatings were investigated using a ball-on-disc wear tester. The results show that the NiCoCrAlY–B₄C coatings with different B₄C mass fractions are mainly composed of NiTi, NiTi₂, α-Ti, CoO, AlB₂, TiC, TiB and TiB₂ phases. The COFs and wear rates of NiCoCrAlY–B₄C coatings decrease with the increase of B₄C content, which are contributed to the improvement of coating hardness by the B₄C addition. The wear mechanisms of NiCoCrAlY–B₄C coatings are changed from adhesive wear and oxidation wear to fatigue wear with the increase of B₄C content.     

Experiment and modeling of TiB2/TiB boride layer of Ti−6Al−2Zr−1Mo−1V alloy

The influence of the boriding conditions on the boride layers was examined by boriding Ti−6Al−2Zr− 1Mo−1V alloy in the temperature range of 920−1120°C. The experimental results show that the boride layers were composed of a continuous thin outer layer of TiB₂ and a thick inner layer of TiB with whiskers or needle-like morphologies that extended into the substrate. Thick and compact boride layers were obtained when the boriding temperatures were 1000−1080 °C, and the treatment time exceeded 8 h. The boride layer depth increased with the boriding temperature and time, and the growth kinetics of the boride layers was characterized by a parabolic curve. The growth kinetics of the boride layers, including both TiB₂ and TiB layers, were predicted by establishing a diffusion model, which presented satisfactory consistency with the experimental data. As a result, the activation energies of boron in the TiB₂ and TiB layers were estimated to be 223.1 and 246.9 kJ/mol, respectively.     

Microstructure Evolution and Phase Transformation of Ti–6.5Al–2Zr–Mo–V Alloy During Thermohydrogen Treatment

Thermohydrogen treatment(THT) is an effective way to refine microstructure and improve the mechanical properties of the titanium alloys.In the current work,as-cast Ti–6.5Al–2Zr–Mo–V alloy was hydrogenated with different hydrogen contents and processed solution aging.Accordingly,the microstructure evolution and phase transformation were analyzed.Results show that during solution aging,eutectoid decomposition occurs and the product is a mixture of coarse primary a,fine eutectoid product and undecomposed bH.The size of primary a is closely dependent on the hydrogen content,and large primary a can be obtained at medium hydrogen content.Further,the influence of hydrogen content on the growth of primary a phase was revealed.The primary a is much fine,and the eutectoid product is relatively homogeneous with 0.984 wt% H.After THT,the ultimate strength is beyond 1,100 MPa that has increased by 23.15% compared with that in as-cast state.     

Structure and properties of micro-arc calcium phosphate coatings on pure titanium and Ti–40Nb alloy

The microstructure, physical and mechanical, and chemical properties of micro-arc calcium phosphate (CaP) coatings deposited under different process voltages in the range of 150–400 V on the commercially pure titanium (Ti) and Ti–40%Nb (Ti–40Nb) (mass fraction) alloy were investigated by the SEM, TEM, XRD and EDX methods. The coating thickness, roughness, and sizes of structural elements were measured and showed similar linear character depending on the process voltage for the coatings on both substrates. SEM results showed the porous morphology with spherical shape structural elements and rough surface relief of the coatings. XRD and TEM studies exhibited the amorphous structure of the CaP coating. With increasing the process voltage to 300–400 V, the crystalline phases, such as CaHPO₄ and β-Ca₂P₂O₇, were formed onto the coatings. The annealing leads to the formation of complex poly-phase structure with crystalline phases: CaTi₄(PO₄)₆, β-Ca₂P₂O₇, TiP₂O₇, TiNb(PO₄)₃, TiO₂, NbO₂, and Nb₂O₅. The applied voltage and process duration in the ranges of 200–250 V and 5–10 min, respectively, revealed the coating formed on Ti and Ti–40Nb with optimal properties: thickness of 40–70 μm, porosity of 20%–25%, roughness (R_a) of 2.5–5.0 μm, adhesion strength of 15–30 MPa, and Ca/P mole ratio of 0.5–0.7.     

Wear and thermal behaviour of CVD α-Al2O3 and MTCVD Ti(C,N) coatings during machining

The wear properties of CVD α-Al₂O₃ layers with , and (0 0 0 1) growth textures were compared with MTCVD Ti(C,N) layers in single point turning of AISI 4140 steel. The experimental coatings were investigated with respect to thermal behaviour, deformation and wear. Substantial texture effects on wear performance of the α-Al₂O₃ layers were observed. A clearly enhanced ability of (0 0 0 1) textured layer to undergo uniform plastic deformation was confirmed. The Ti(C,N) layer exhibited a more uniform plastic deformation than the α-Al₂O₃ layers. The observations were interpreted in the light of thermal, mechanical and frictional conditions occurring at the tool-chip contact interface.     

Formation of adiabatic shear band and deformation mechanisms during warm compression of Ti–6Al–4V alloy

Adiabatic shear band(ASB) was narrow region where softening occurred and concentrated plastic deformation took place. In present study, the effects of height reduction and deformation temperature on ASB were investigated by means of optical microscopy(OM) and scanning electron microscopy(SEM). And the deformation mechanisms within the shear band were discussed thoroughly with the help of transmission electron microscopy(TEM). There is a critical strain for the formation of ASB during warm compression of Ti–6Al–4V alloy. The width of ASB increases with height reduction increasing. Elongated alpha grains within shear band grow up with deformation temperature increasing. Some ultrafine grains that confirm the occurrence of dynamic recrystallization are observed within shear band during warm compression of Ti–6Al–4V alloy.