Structure and physicomechanical properties of eutectic Ti-Si-X alloys

We study the structure and physicomechanical properties of various eutectic alloys of Ti-Si-Zr, Ti-Si-B, and Ti-Si-Ga systems. It is shown that Ti-Si-Zr alloys with elevated concentrations of Zr reveal, due to the presence of (Ti, Zr)₂ Si dispersed silicides with sizes of about several hundred nanometers, improved mechanical properties as compared with the properties of alloys based on Ti₅Si₃ silicides. The cast eutectic alloy of the Ti-Si-Zr-Sn system with a plasticity of ∼ 1.7% is obtained for the first time. The formation of superfine eutectics based on the Ti₆Si₂B ternary compound in alloys of the Ti-Si-B system enables one to obtain titanium composites with improved refractory properties and elevated moduli of elasticity (of about 150 GPa or, after additional alloying, as high as 165 GPa). This can be promising for the development of new refractory titanium composite materials with elevated stiffness. The analysis of the combined effect of gallium and silicon in Ti-Si-Ga alloys reveals the possibility of getting titanium materials with high heat resistance, i.e., materials based on the (α-Ti(α₂-Ti₃Ga) + Ti₅ (Si, Ga)₃ binary eutectics. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 3, pp. 35–42, May–June, 2008.     

Influence of N and Fe on α-Ti precipitation in the in situ TiC–titanium alloy composites

High strength with high ductility can be achieved in the titanium alloys by using metal precipitated ceramic particle as reinforcement. In this work, α + β or β-Ti alloy composites were prepared with α-Ti precipitated TiC particles. A series of Ti–Fe–C–N alloys were prepared and a constitutional diagram was constructed as a function of N and Fe contents. Two criteria were identified for the formation of α-Ti precipitation. One is the existence of Ti₂C phase and the other is the presence of α-Ti phase in the matrix. The mechanism of α-Ti formation from the Ti₂C phase is discussed.     

Effect of silicon content on the microstructure and properties of Fe–Cr–C hardfacing alloys

In this study, the surface of St52 steel was alloyed with preplaced powders 55Fe39Cr6C, 49Fe39Cr6C6Si, and 45Fe39Cr6C10Si using a tungsten-inert gas as the heat source. Following surface alloying, conventional characterization techniques, such as optical microscopy, scanning electron microscopy, and X-ray diffraction were employed to study the microstructure of the alloyed surface. Microhardness measurements were performed across the alloyed zone. Room-temperature dry sliding wear tests were used to compare the coatings in terms of their tribological behavior. It was found that the as-deposited coatings contained higher volume fractions of carbides (Cr₇C₃). The presence of 6%Si in the preplaced powders caused an increase in microhardness and wear resistance.     

Microstructural study and wear behavior of ductile iron surface alloyed by Inconel 617

In this research, microstructure and wear behavior of Ni-based alloy is discussed in detail. Using tungsten inert gas welding process, coating of nearly 1–2 mm thickness was deposited on ductile iron. Optical and scanning electron microscopy, as well as X-ray diffraction analysis and electron probe microanalysis were used to characterize the microstructure of the surface alloyed layer. Micro-hardness and wear resistance of the alloyed layer was also studied. Results showed that the microstructure of the alloyed layer consisted of M₂₃C₆ carbides embedded in Ni-rich solid solution dendrites. The partial melted zone (PMZ) had eutectic ledeburit plus martensite microstructure, while the heat affected zone (HAZ) had only a martensite structure. It was also noticed that hardness and wear resistance of the alloyed layer was considerably higher than that of the substrate. Improvement of wear resistance is attributed to the solution strengthening effect of alloying elements and also the presence of hard carbides such as M₂₃C₆. Based on worn surface analysis, the dominant wear mechanisms of alloyed layer were found to be oxidation and delamination.     

Laser surface modification of carburized and borocarburized 15CrNi6 steel

The paper presents the results of laser heat treatment (LHT) of carburized and borocarburized 15CrNi6 low-carbon steel. Laser tracks were arranged by CO₂ laser beam as multiple tracks formed in the shape of a helical line. The microstructure and properties of these diffusion layers were compared with those obtained after through-hardening. The microstructure after carburizing and LHT consists of adjacent characteristic zones: re-melted zone (coarse-grained martensite), carburized layer with heat affected zone (fine acicular martensite), carburized layer without heat treatment and the substrate (ferrite and pearlite). The highest measured microhardness (about 820 HV) was observed in re-melted and heat affected zones. The increase of distance from the surface was accompanied by a gradual decrease of microhardness up to 400 HV beneath the HAZ and up to 250 HV in the core of steel. The carburized layer after LHT exhibited a higher resistance to frictional wear compared to a carburized layer after through-hardening. The microstructure after borocarburizing and LHT consists of the following characteristic zones: iron borides of laser-modified morphology (FeB and Fe₂B), carburized layer with heat affected zone (martensite and alloyed cementite), carburized layer without heat treatment and the substrate (ferrite and pearlite). The highest microhardness was obtained in the iron boride zone. The microhardness of FeB boride extended up to 2200 HV and for the Fe₂B boride up to about 1300–1600 HV. With increased distance from the surface, the microhardness gradually decreases to 800 HV in HAZ, 400–450 HV in the carburized layer without heat treatment and to 250 HV in low-carbon substrate. The iron borides after LHT assume a globular shape, which leads to a lower texture and porosity of the borided layers. The increased resistance to friction wear of the borocarburized layers is certified in comparison with the borided layer after conventional heat treatment (through-hardening).     

Effects of Si addition on mechanical properties and superelasticity of Ti–7.5Nb–4Mo–2Sn shape memory alloy

Effects of 0–2.1 at.% Si additions on microstructure and mechanical properties of a Ni-free biomedical superelastic β-Ti alloy, Ti–7.5 at.%Nb–4 at.%Mo–2 at.% Sn (Ti–7.5Nb–4Mo–2Sn), were investigated. The alloys after annealing at 973 K mainly contain β and α″. As the concentration of Si is higher than 1 at.%, Ti₅Si₃ particles can be found in the alloys, and the number density of the particles increases with the increasing of silicon’s concentration. The addition of Si promotes the strength of the Ti–7.5Nb–4Mo–2Sn due to the Si solid solution strengthening effect and fine Ti₅Si₃ precipitates. However, as the Si concentration reaches 2.1%, the alloy exhibits a brittle fracture. The 0.5–1.6 at.% Si additions improve the superelasticity of the Ti–7.5Nb–4Mo–2Sn alloy by increasing the critical stress for inducing martensite (σ_SIM).     

Improving the wear resistance of AA 6061 by laser surface alloying with NiTi

To improve the wear resistance of a popular aluminum alloy AA 6061, a 1.5 mm thick hard surface layer consisting of Ni–Al and Ti–Al intermetallic compounds was synthesized on the alloy by laser surface alloying technique. NiTi powder was preplaced on the aluminum alloy substrate and irradiated with a high-power CW Nd:YAG laser in an argon atmosphere. With optimized processing parameters, a modified surface layer free of cracks and pores was formed by reaction synthesis of Al with Ni and Ti. X-ray diffractometry (XRD) confirmed the main phases in the layer to be TiAl₃ and Ni₃Al. The surface hardness increased from below 100 HV for untreated AA 6061 to more than 350 HV for the laser-treated sample. Accompanying the increase in hardness, the wear resistance of the modified layer reached about 5.5 times that of the substrate.     

Microstructure characteristics of products in Ti–Si system via combustion synthesis reaction

The products of combustion synthesis reaction in Ti–Si system with molar ratios of Ti:Si = 3:1, 5:3, 5:4, 1:1, and 1:2 were investigated. The phase composition of products and degree of completion in the reaction considerably depend on the initial stoichiometric ratios of reactants. During the SHS reaction of Ti–Si system, the degree of completion follows the order of 5Ti + 3Si > Ti + Si > Ti + 2Si > 5Ti + 4Si > 3Ti + Si. Besides, the micro-structural morphology of Ti–Si compounds, i.e., TiSi₂, TiSi, Ti₅Si₄, and Ti₅Si₃ were also characterized in this study.     

Effect of substitution of Si by Al on microstructure and synthesis behavior of Ti5Si3 based alloys fabricated by mechanically activated self-propagating high-temperature synthesis

The effect of substitution of Si by Al and mechanical activation on microstructure, phase composition, ignition and combustion temperature of Ti₅Si₃ based alloys and composites that were prepared by mechanically activated self-propagating high-temperature synthesis (MASHS) method was investigated. For this purpose elemental powders of titanium, silicon and aluminum were mixed according to the 5Ti + 3(1 − X)Si + 3XAl formula, where X = 0, 0.2, 0.4, 0.6. The samples were characterized by X-ray diffraction (XRD) analytical technique and scanning electron microscope (SEM) equipped with an energy-dispersive spectrum (EDS) analyzer. The results have shown that formation of Ti₅Si₃ during milling stage is postponed by adding Al into the system. Presence of Al in the Ti–Si system have a significant effect on the phase composition of the final products. Substitutional solid solution of Ti₅(Si, Al)₃ and Ti₅Si₃–Ti₃Al composite are formed by increasing Al amount in the system. Furthermore combustion temperature and crystallites size of Ti₅Si₃ is reduced with addition of Al into the Ti–Si system. Moreover, solubility of Al in Ti₅Si₃ is increased with enhancing the X up to 0.4, after that, the solubility of Al in Ti₅Si₃ is ceased, due to achieving the solubility limit of Al in the Ti₅Si₃. The average crystallites size of Ti₅Si₃ are decreased with increasing milling time prior to the reaction.     

Microstructural studies and wear assessments of Ti/TiC surface composite coatings on commercial pure Ti produced by titanium cored wires and TIG process

Tungsten Inert Gas (TIG) process and titanium cored wires filled with micro size TiC particles were employed to produce surface composite coatings on commercial pure Ti substrate for wear resistance improvement. Wire drawing process was utilized to produce several cored wires from titanium strips and titanium carbide powders. Subsequently, these cored wires were melted and coated on commercial pure Ti using TIG process. This procedure was repeated at different current intensities and welding travel speeds. Composite coating tracks were found to be affected by TIG heat input. The microstructural studies using optical and scanning electron microscopy supported by X-ray diffraction showed that the surface composite coatings consisted of α′-Ti, spherical and dendritic TiC particles. Also, greater volume fractions of TiC particles in the coatings were found at lower heat input. A maximum microhardness value of about 1100 HV was measured which is more than 7 times higher than the substrate material. Pin-on-disk wear tests exhibited a better performance of the surface composite coatings than the untreated material which was attributed to the presence of TiC particles in the microstructure.     

Laser-induced self-propagating reaction synthesis of Ti–Fe alloys

Ti_70.5Fe_29.5 alloy is synthesized using laser-induced self-propagating reaction synthesis (LSRS). The product mainly consists of β-Ti + TiFe eutectic. However, a given amount of oxygen-stabilized Ti₂Fe phase is also found in the product due to high cooling rate and oxygen existence. The formation of the fine eutectic structure makes the alloy exhibit high hardness (9.34 GPa), high compressive strength (2609 MPa), and good relative compressibility (8.5%). The phase formation during LSRS can be divided into four stages: melting of Fe particle periphery, formation of a liquid-state Ti–Fe diffusion layer, eutectic reaction, and formation of oxygen-stabilized Ti₂Fe phase.     

Microstructure and Properties of Laser Surface Alloying 304 Stainless Steel with Si

Silicon was added to the surface of 304 stain-less steel by laser melting the sprayed preplaced Sipowders.The optical microscopy,X-ray diffractionand EPMA were employed to investigate themicrostructure and chemical composition of thelaser surface alloyed layer.The hardness of the al-loyed layer was measured by microhardness testand wear resistance was evaluated by scratch test.Corrosion resistance of laser alloyed sample wasstudied in 1N H_2SO_4+0.1N NaCl and0.5N HCl+0.5N NaCl aqueous solutions.Amicrocrystalline intermetallic compound coatingwith smooth surface and good chemicalhomogeneity without porosity and crack is ob-tained.The hardness is about 720 HV.The wear re-sistance is 2 times better than that of the substrate.The corrosion resistance of the laser alloyed sampleis much better than that of 304 stainless steelsample.     

Improving the wear resistance of AZ91D magnesium alloys by laser cladding with Al-Si powders

To improve the wear resistance of AZ91D magnesium alloy, laser surface cladding with Al and Si powders was investigated using a Nd:YAG pulsed laser. With appropriate processing parameters and the suitable weight ratio of Al to Si in powders, a modified surface layer free of cracks and pores was formed by reaction synthesis of Mg with Al and Si. X-ray diffractometry (XRD) confirmed the main phases in the layer to be Mg₂Si and Mg₁₇Al₁₂. The surface hardness increased from 35 HV for as-received magnesium alloy to more than 170 HV for laser treated sample. Accompanying the increase in hardness, the wear resistance of the clad layer increased more than 4 times that of the substrate.     

Room-temperature sliding wear properties of laser melt deposited Cr13Ni5Si2/γ alloy

A wear-resistant alloy consisting of Cr₁₃Ni₅Si₂ ternary silicide dendrites and the interdendritic nickel-base solid solution (γ) was designed and fabricated by the laser melting/continuous deposition (LMCD) process. The wear resistance of Cr₁₃Ni₅Si₂/γ alloy was evaluated on an MM-200 block-on-wheel dry sliding wear tester at room temperature. Results indicate that the Cr₁₃Ni₅Si₂/γ alloy has excellent wear resistance and extremely low load-sensitivity of wear under dry sliding wear test conditions due to the high toughness and the high strength, as well as the transferred cover-layer on the worn surface of the alloy. Translated from Acta Metallurgica Sinica, 2006, 42(2): 181–185 [译自: 金属学报]     

Development and characterization of laser clad high temperature self-lubricating wear resistant composite coatings on Ti–6Al–4V alloy

To enhance the wear resistance and friction-reducing capability of titanium alloy, a process of laser cladding γ-NiCrAlTi/TiC + TiWC₂/CrS + Ti₂CS coatings on Ti–6Al–4V alloy substrate with preplaced NiCr/Cr₃C₂–WS₂ mixed powders was studied. A novel coating without cracks and few pores was obtained in a proper laser processing. The composition and microstructure of the fabricated coating were examined by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) techniques, and tribological properties were evaluated using a ball-on-disc tribometer under dry sliding wear test conditions at 20 °C (room-temperature), 300 °C, 600 °C, respectively. The results show that the coating has unique microstructure consisting of α-Ti, TiC, TiWC₂, γ-NiCrAlTi, Ti₂CS and CrS phases. Average microhardness of the composite coating is 1005 HV_0.2, which is about 3-factor higher than that of Ti–6Al–4V substrate (360 HV_0.2). The friction coefficient and wear rate of the coating are greatly decreased due to the combined effects of the dominating anti-wear capabilities of reinforced TiC and TiWC₂ carbides and the CrS and Ti₂CS sulfides which have excellent self-lubricating property.     

Monotonic and cyclic deformation behavior of the Al–Si–Cu–Mg cast alloy with micro-additions of Ti,V and Zr

Monotonic and cyclic tests were used to assess the influence of micro-additions of Ti, V and Zr on the deformation behavior of the Al–7Si–1Cu–0.5Mg (wt.%) alloy in as-cast and T6 heat treated conditions and to compare the results with alloys of similar chemistry described in the literature. The microstructure of the as-cast alloy consisted of α-Al, eutectic Si, and Cu, Mg and Fe based phases Al_2.1Cu, Al_8.5Si_2.4Cu, Al_7.2Si_8.3Cu₂Mg_6.9 and Al₁₄Si_7.1FeMg_3.3. In addition, the micro-size Zr–Ti–V-rich phases Al_21.4Si_4.1Ti_3.5VZr_3.9, Al_6.7Si_1.2TiZr_1.8, Al_2.8Si_3.8V_1.6Zr and Al_5.1Si_35.4Ti_1.6Zr_5.7Fe were present in the as-cast state. During solution treatment, Cu based phases were completely dissolved, while the eutectic silicon, Fe- and Zr–Ti–V-rich intermetallics experienced only partial dissolution. The monotonic test results showed that the T6 heat treated alloy achieved a tensile strength of 343 MPa and a compressive strength of 418 MPa. Also, the cyclic yield stress of the studied alloy in the T6 temper condition was higher than the monotonic value and reached 335 MPa. The fatigue life of the studied alloy was substantially longer than that of the reference alloy with the same base but lower additions of V, Zr and Ti, reported in the literature. The fractography revealed the tensile crack propagation through the eutectic Si and primary phases, exhibiting intergranular fracture along with some cleavage-like features of the plate-shape Zr–Ti–V-rich intermetallics with a presence of fatigue striations on the latter, indicating their ductile nature. It is believed that the intermetallic precipitates containing Zr, Ti and V improve the fatigue life of the studied alloy in the T6 condition.     

High temperature oxidation behavior of Ti0.5Al0.5N coating and Ti0.5Al0.4Si0.1N coating

Good oxidation resistance of hard coatings is important for cutting tools. Ti_0.5Al_0.5N coating and Ti_0.5Al_0.4Si_0.1N coating were deposited by cathodic arc evaporation and their oxidation behavior at 850 °C, 900 °C and 1000 °C was compared. The effect of Si addition on the oxidation resistance of Ti_0.5Al_0.4Si_0.1N was investigated. Results show that the oxidation resistance of Ti_0.5Al_0.4Si_0.1N coating at 850-1000 °C is superior to Ti_0.5Al_0.5N coating. The improved oxidation resistance of Ti_0.5Al_0.4Si_0.1N coating can be ascribed to the combined action of Al₂O₃ and SiO₂ barrier layer, the reduction of columnar structure and the refinement of grains. In particular, Si addition increases the diffusion coefficient of Al and promotes the preferential formation of Al₂O₃ barrier layer.     

Enhancement of wear resistance of ductile iron surface alloyed by stellite 6

This paper deals with the improvement of the wear resistance of ductile iron surface alloyed by a hypoeutectic stellite 6 alloy. In this regard, the surface alloyed layer with 3 mm thickness deposited on ductile iron using tungsten inert gas (TIG) surface processing. The microstructure, hardness and wear resistance of surface alloyed layer were investigated using optical microscopy, scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction analysis, Vickers hardness (HV0.3) and pin-on-plate tests. The results showed that the microstructure of the surface alloyed layer consisted of carbides dispersed in a Co-based solid solution matrix with dendritic structure. This microstructure was responsible for the improvement of the hardness and wear resistance of the coating. Further investigations showed that the dominant mechanism of the wear in the coated and uncoated samples was delamination wear.     

Direct bonding of Cu to oxidized silicon nitride by wetting of molten Cu and Cu(O)

When pressureless sintered silicon nitride with the main additives Y₂O₃ and Al₂O₃, having a thermal conductivity K = 20 W/m K, was oxidized at 1240–1360 °C in still air, the resulting surface oxide layer easily bonded to a copper plate in the temperature region between 1065 and 1083 °C, and in the oxygen concentration range of 0.008–0.39 wt%, as shown in a Cu–O phase diagram. The oxide on the silicon nitride was characterized as Y₂O₃·2SiO₂ and mixed silicate glass with additives and impurities that diffused through the grain boundary. The bonding strength of Cu/Si₃N₄ depends on the amount or layer thickness of silicate glass and reaches as high as 100 MPa by shear at room temperature. Detailed analysis of the oxidation layer and the peeled-off surfaces of directly bonded Si₃N₄/Cu reveal that the main mechanism of bonding is wetting to glassy silicate phase by mixtures of molten Cu and α-solid solution Cu(O), which solidify to α + Cu₂O below 1065 °C by a eutectic reaction. The direct reactive wetting of molten Cu, supplied from the grain boundary of a Cu plate, on the glassy phase enables very tight chemical bonding via oxygen atoms.     

Influence of Cu addition on the self-propagating high-temperature synthesis of Ti5Si3 in Cu–Ti–Si system

The self-propagating high-temperature synthesis (SHS) reactions can take place in Cu–Ti–Si systems with Cu additions of 10–50 wt.%, and the products only consist of Ti₅Si₃ and Cu phases, without any transient phase. In Ti–Si system, most of the Ti₅Si₃ grains synthesized exhibit the polygon-shaped coarse appearance with an obviously sintered morphology. When Cu content increases from 10 to 50 wt.%, however, the Ti₅Si₃ exhibits cobblestone-like shape with a relatively smooth surface, and its average size decreases significantly from 15 to 2 μm or less. The formation mechanism of Ti₅Si₃ in Cu–Ti–Si system is characterized by the solution, reaction and precipitation processes. Furthermore, the addition of Cu has a great influence on the volume change between green and reacted preforms. The volume change increases with Cu content increasing from 0 to 20 wt.%, and then decreases with the content further increasing from 20 to 50 wt.%. The addition of Cu to Ti–Si system significantly decreases the onset temperature of the reaction during differential scanning calorimetry process, which is even much lower than the α → β transition temperature of Ti (882 °C), suggesting that the reaction could be greatly facilitated by Cu addition. As a result, the role of Cu serves not only as a diluent but also as a reactant and participates in the self-propagating high-temperature synthesis reaction process.