Y and Al modified silicide coatings on an Nb–Ti–Si based ultrahigh temperature alloy prepared by pack cementation process

Y and Al modified silicide coatings were prepared on an Nb–Ti–Si based ultrahigh temperature alloy by co-depositing Si, Al and Y at 1150 °C for up to 10 h, respectively. The deposition of Al and Si occurred in a sequential manner during the pack cementation process. At the initial stage, the element deposited was primarily Al with very little Si and an Al₃(Nb,X) (X represents Ti, Cr and Hf elements) layer formed preferentially. After a short period of holding time, Si started depositing and Si–Al co-deposition took place. However, this Si–Al co-deposition period was not long. When the holding time was longer than 1 h at 1150 °C, Si deposition dominated the coating growth process. The coating growth kinetics at 1150 °C followed a parabolic law. The coating prepared at 1150 °C for 10 h had a multi-layer structure, with a thick (Nb,X)Si₂ outer layer, a thin (Ti,Nb)₅Si₄ middle layer and an Al, Cr-rich inner layer. The coating could protect the Nb–Ti–Si based alloy from oxidation at 1250 °C in air for at least 100 h. The excellent oxidation resistance of the coating was attributed to the formation of a dense scale mainly consisted of TiO₂, SiO₂ and Al₂O₃.     

Formation and oxidation resistance of Hf and Al modified silicide coating on Nb–Si based alloy

Codeposition of Si, Al and Hf were prepared by pack cementation at 1300 °C for 10 h. The results show that the coating is composed of a thick (Nb, X)Si₂ outer layer, a (Ti, Nb)₅Si₄ middle layer and a thin discontinuous (Cr, Al)₂(Nb, Ti) inner layer. The mass gain of the coating is only 4.12 mg/cm² after isothermal oxidation at 1250 °C for 100 h. Some “oxide pegs” form at the interface of the oxide scale and coating. The coating exhibits good cyclic oxidation resistance due to the improved adhesion between the oxide scale and coating.     

Strength and fracture of two hot-rolled Ti–Al–Si–Nb dual phase alloys based on Ti3Al

Ti–Al–Si–Nb dual phase alloys are mainly composed of α₂-Ti₃Al matrix and Ti₅Si₃ silicide phases. In this paper, two alloys (402 and 405) whose Si contents are 2 and 5 at% respectively were arc melted and hot-rolled into sheets with different amounts of deformation. The silicide phase (Ti,Nb)₅(Si,Al)₃ was broken up into small pieces and redistributed in the α₂ matrix during the hot-rolling. Improved strength and ductility of the two alloys were observed after hot-rolling, which can be attributed to both the finely distributed reinforcement silicide phase and refinement of the matrix grain size. The mechanical properties of the two alloys are dependent on their volume fractions of the silicide phase: the strength of alloy 405 is higher than that of alloy 402, while alloy 402 is more ductile than alloy 405. The brittle–ductile transition temperature of the two dual phase alloys is between 600 and 800°C. The surface slip on the dual phase alloys was also observed. Obvious separation between the (Ti,Nb)₅(Si,Al)₃ particles and the α₂ matrix is found on the fracture surfaces obtained at high temperature, showing dimple-like morphology.     

A comparative study on corrosion of Mg–Al–Si alloys

Corrosion behavior of various Mg–Al–Si alloys (AS11, AS21, AS41, AS61 and AS91 series), cast under the same cooling conditions and controlled alloying composition, was investigated systematically. Optical microscopy and scanning electron microscopy were used for microstructural examinations. The corrosion behavior was evaluated by immersion tests and potentiodynamic polarization measurements in 3.5% NaCl solution. The results from both immersion tests and the potentiodynamic polarization measurements showed that marginal improvement in corrosion resistance was observed with 2.0% Al (mass fraction) containing alloy (AS21) whereas Al addition above 2.0% (AS41, AS61 and AS91) deteriorated the corrosion resistance which was attributed to β phase, acting as cathode, and the interruption of continuity of the oxide film on the surface of the alloys owing to coarsened β and Mg₂Si phases.     

Microstructure and wear properties of silicide based coatings over Mo–30W alloy

The paper deals with the microstructure and wear properties of the Mo-30W alloy, coated with silicide based oxidation resistant coatings using pack cementation technique. The microstructure of the coating revealed that the coating was free of cracks or pores, adherent to the substrate and comprised of either single layer or double layer depending the coating temperature. Scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) was used to determine diffusion profiles for Mo, W and Si. Reciprocating sliding wear and friction experiments were performed on the uncoated and coated alloy. Double layer coating showed an improved friction coefficient as compared to base alloy as well as a single layer coated alloy. The wear tests also showed a marked improvement of wear resistance of coated alloy as compared to uncoated alloy. The coating was found to be wear resistant at 7 N.     

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.     

Microstructure and mechanical properties of Al–Si alloy modified with Al–3P

Al–Si alloy was modified with Al–3P master alloy at 740 °C. The effects of Si content (7, 8, 9, 10 and 11 wt.%) and adding amount of alterant Al–3P (0, 0.1, 0.3, 0.6, 1.0 and 1.5 wt.%) on microstructures and tensile properties of the alloy were investigated with optical microscope (OP), Image Pro Plus 6.0, scanning electron microscope (SEM) and universal testing machine. When the content of Al–3P is 0.6 wt.%, the area fraction of primary α(Al) in the Al–Si alloy increases more compared to the unmodified alloy with an increase in Si content, which could be explained by the movement of non-equilibrium eutectic point. When the Si content is constant (Al–10Si), with the increase of Al–3P content, the increased rate in area fraction of primary α(Al) phase in the Al–10Si alloy increases first and then decreases. And when 0.6 wt.% Al–3P is added, the increase in area fraction of primary α(Al) phase is the largest. Compared to the unmodified Al–10Si alloy, the tensile strength and elongation of Al–10Si alloy increase by 2.3% and 47.0%, respectively, after being modified with 0.6 wt.% Al–3P alloy. The fracture mode of the modified Al–10Si alloy is ductile fracture.     

Microstructural evolution of Ti–47Al–2Cr–2Nb–0.8B alloy prepared by semi-solid process

The refinement of large boride ribbons in the as-cast TiAl alloy is the essential issue for aerospace industry application,which is difficult to avoid by classical casting techniques.The present paper seeks to explore the possibility of the semi-solid process in manufacturing Ti-47Al-2Cr-2Nb-0.8 B(at%) alloy.An important result is that,except forming a nondendritic globular structure,the semi-solid process also plays a crucial role in refining large borides for the TiAl alloys with boron.In the ...     

Process optimization of Ti–Nb alloy coatings on a Ti–6Al–4V plate using a fiber laser and blended elemental powders

A fiber laser was used to modify the surface composition of a Ti–6Al–4V plate through deposition of the blown powder mixture of Ti–45 wt.%Nb. Scanning electron microscopy and energy dispersive spectroscopy (EDS) were employed to examine the clad sections microstructure and chemical composition. The optimized set of laser processing parameters, including the laser power of 1100 W, the laser scan speed of 350 mm/min (or ∼5.83 mm/s), the laser spot diameter of 2 mm and the powder feed rate of 0.1 g/s was found with the identification of combined parameters, the laser specific energy, the powder density and the newly defined laser supplied energy (i.e. representing the amount of energy given to the unit mass of the blown powder). It is shown that, with these parameters, continuous beads can be formed with pore-free sections and a homogeneous composition corresponding to that of β (Ti, Nb) solid solution phase. Furthermore, Al and V elements are thoroughly replaced with a more biocompatible element, Nb, in the second layer of a Ti–Nb cladding build-up on the surface of the Ti–6Al–4V plate (i.e. after ∼1 mm in clad thickness from the clad/substrate interface).     

Establishing multilayered coating on Mo–12Si–8.5 B alloy by Si pack cementation and its oxidation behavior at 900°C–1300°C

A multilayered oxidation protection coating consisting of MoSi₂ outer layer, Mo₅Si₃ internal layer, and Mo₅SiB₂/MoB inner layer was developed on the surface of Mo–12Si–8.5B 1.0 wt% ZrB₂ alloy via Si pack cementation. The multilayered coating significantly enhanced the oxidation resistance of the alloy at 900°C, 1100°C, and 1300°C in the air by exhibiting negligible oxidation recession. MoSi₂ outer layer provided admirable oxidation protection for the alloy at high temperatures by forming a thin and protective SiO₂-rich glass scale on its surface. This was supplemented by the Mo₅Si₃ internal layer and Mo₅SiB₂/MoB inner layer that reduced the thermal expansion mismatch between the MoSi₂ outer layer and substrate, and therefore no obvious cracks were found in the MoSi₂ outer layer. More importantly, the Mo₅SiB₂/MoB layer as an in situ barriers of Si interdiffusion ensured the stable existence of MoSi₂ and Mo₅Si₃ layers without obvious thickness change during oxidation at 900°C and 1100°C. Mechanical property test indicated that the formation of the coating layers could not affect the fracture toughness of the alloy.     

Effect of Nb addition on microstructure evolution and nanomechanical properties of a glass-forming Ti–Zr–Si alloy

The glass-forming Ti₇₅Zr₁₀Si₁₅ alloy is regarded as a potential material for implant applications due to its composition of non-toxic, biocompatible elements and some interesting mechanical properties. The effects of partial substitution of 15 at.% Ti by Nb on the microstructure and the mechanical behaviour have been investigated by X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray analysis, transmission electron microscopy and nanoindentation techniques. Copper mold casting and melt-spinning methods have been applied to study the influence of the cooling rate on the properties of both alloys, Ti₇₅Zr₁₀Si₁₅ and Ti₆₀Zr₁₀Nb₁₅Si₁₅. As a result of different cooling rates, significant microstructural variations from multiphase crystalline states in cast rods to nanocomposite structures in ribbons were observed. The limited glass-forming ability (GFA) of the Ti₇₅Zr₁₀Si₁₅ alloy results for melt-spun ribbons mainly in nanocomposite structures with β-type nanocrystals being embedded in a glassy matrix. Addition of Nb increases the glass-forming ability. Raising the overheating temperature of the melt prior to melt-spinning from 1923 K to 2053 K yields for both alloys a higher amorphous phase fraction. The mechanical properties were investigated using compression tests (bulk samples) and the nano-indentation technique. A decrease of hardness (H), ultimate stress and reduced Young's modulus (E_r) is observed for Ti₆₀Zr₁₀Nb₁₅Si₁₅ rods as compared to Ti₇₅Zr₁₀Si₁₅ ones. This is attributed to an increase of the fraction of the β-type phase. The melt-spun ribbons show an interesting combination of very high hardness values (H) and moderate reduced elastic modulus values (E_r). This results in comparatively very high H/E_r ratios of >0.075 which suggests these new materials for applications demanding high wear resistance.     

Microstructure and tensile properties of magnesium alloy modified by Si/Ca based refiner

Microstructure and mechanical properties of pure magnesium and AZ31 alloy with Ca/Si based refiner addition were investigated. The results indicate that addition of Ca/Si based refiners to pure magnesium and AZ31 alloy results in remarkable microstructure refinement. With proper amount of refiner addition, the grain size in as cast ingots can be one order of magnitude lower than that without refiner addition. Small amount of refiner addition to AZ31 alloy increases both ultimate strength and yield strength significantly,while the ductility of the alloy with refiner addition is similar to that without refiner addition. Addition of refiner improves the deformability of AZ31 alloy and extruded or hot rolled specimens （rods or sheets） with refiner addition exhibit higher surface quality and mechanical properties than those without refiner addition.     

Effect of Y2O3 crucible on contamination of directionally solidified intermetallic Ti–46Al–8Nb alloy

The effect of Y₂O₃ crucible on contamination of Ti–46Al–8Nb (at.%) alloy directionally solidified (DS) in a Bridgman-type apparatus was studied. Directional solidification experiments were performed in dense Y₂O₃ crucibles using different growth rates, melt temperatures and various reaction time between the melt and the crucible. The main mechanism responsible for the contamination of the DS samples is diffusion controlled dissolution of the Y₂O₃ crucible in the melt which leads to an increase of oxygen and yttrium content in γ(TiAl) + α₂(Ti₃Al) matrix and precipitation of non-metallic particles in interdendritic region. Transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD) showed that these particles are Y₂O₃ phase. The oxygen content and volume fraction of Y₂O₃ particles increase with increasing melt temperature and reaction time. The activation energy for increase of oxygen content is calculated to be Q_O = 412.1 kJ/mol and the kinetics of this process is suggested to be controlled by long-range diffusion with the oxygen content exponent of 3. The activation energy for Y₂O₃ particle formation is calculated to be Q_Y = 421.8 kJ/mol and the time exponent is determined to be m = 0.55. Vickers microhardness measurements in lamellar γ + α₂ matrix without Y₂O₃ particles can be used as an indirect evidence of the level of contamination of DS samples with statistically identical α₂–α₂ interlamellar spacing.     

A comparative study on the electrochemical behaviour of amorphous and nanocrystalline cobalt–tungsten electrodeposited coatings

Amorphous and nanocrystalline cobalt–tungsten coatings were electrodeposited from a citrate-ammonia bath on copper substrates. Both coatings showed a nodular surface morphology, but a microcrack network was observed in the amorphous coating. The cyclic voltammograms of both deposits revealed anodic and cathodic low-current plateaus around the open circuit potential, exhibiting a passive behaviour. Mott–Schottky analysis showed that the passive films exhibit n-type semiconductivity behaviour and that formed on the amorphous coating showed higher crystal defects. Electrochemical impedance spectroscopy revealed that the amorphous coating has higher corrosion resistance than the nanocrystalline one at both open circuit and anodic potentials. This was attributed to the higher pore resistance of passive film formed at the open circuit potential and more chemical stability of the amorphous coating which reduces its dissolution at the anodic potential. The plugging of the microcrack network in the amorphous coating by corrosion products eliminated the negative effect of microcracks.     

Preparation of Al–La master alloy by ultrasonic method and modification on Al alloy

In this study, two kinds of Al–La master alloys were prepared by ultrasonic method and ordinary mixmelting method, and both of the alloys were added into the Al alloy.Then, the microstructure of Al–La master alloy and the modification effect on the Al alloy were investigated using optical microscope(OM), X-ray diffraction(XRD), scanning electron microscope(SEM), and energy disperse spectroscopy(EDS).The results show that because of ultrasonic cavitation and streaming, the grain size of Al–La master alloy prepared by the ultrasonic method is refined, which distributes evenly.And, there is no gas hole,inclusion, or segregation in the Al–La master alloy with aAl, La, and La3Al11.Furthermore, Al–La master alloys show a nice modification for Al alloy, which is much better than that prepared by ordinary mix-melting method.     

Effect of Si content on surface hardening of Al–Si alloy by shot peening treatment

The effect of Si content was investigated for Al–Si alloys (Al-7%, 11%, 18%Si) by shot peening process. The hardness increment by shot peening increased as the Si contents in Al–Si alloys increased. Finer Si particles and more dense distribution of those were observed in Al–18%Si than Al–7%Si. As Si contents of Al–Si alloys increased, grain size at the surface area of Al–Si alloy decreased. Higher hardness of Al–Si alloy with higher Si content could be attributable to more dense and refine Si particles and accelerated grain refinement during severe deformation.     

Effect of neutron irradiation and postradiation annealing on the microstructure and properties of an Al–Mg–Si alloy

The effect of long-term neutron irradiation and postradiation thermal-induced aging on the microstructure and mechanical properties of an aluminum-based reactor Al–Mg–Si alloy grade SAV-1 has been studied. The material under study is the shell of an automatic fine-control rod used to control the reactivity of the core of a VVR-K research reactor. Successive 1-h annealings of specimens of the SAV-1 alloy irradiated to doses of 0.001 and 5 dpa in the temperature range of 100–550°C have been carried out. The evolution of the fine structure of the material and changes in its mechanical characteristics have been studied. The phenomenon of the acceleration of the aging of the SAV-1 alloy under the effect of a high neutron fluence at an irradiation temperature of 80°C has been observed, which involves the formation of numerous lineage (stitch) Guinier–Preston zones in the alloy. It has been shown that the strength characteristics of the SAV-1 alloy depend significantly on the degree of its radiation- and thermal-induced aging.