Differential thermal analysis (DTA) on the reaction mechanism in Fe–Ti–B4C system

Differential thermal analysis (DTA) was undertaken to determine the reaction mechanism in the Fe–Ti–B₄C system under argon. When the mixtures were heated to about 786 °C, Fe₂B and C appeared as a result of Fe reacting with B₄C. As the temperature continued to increase, FeTi formed by an interdiffusion between Fe and Ti. When the mixtures were heated to 1089 °C, FeTi reacted with Ti, leading to the formation of a Fe–Ti melt, into which the displaced C and B from B₄C dissolve, forming a Fe–Ti–C–B melt. Finally, when the concentration of C and B attained a certain value, Ti reacted with C and B, yielding TiC and TiB₂ in the melt, and simultaneously considerable heat released.     

Self-propagating high-temperature synthesis of TiCxNy–TiB2 ceramics from a Ti–B4C–BN system

An experimental study on direct formation of TiC_xN_y–TiB₂ ceramics by self-propagating high-temperature synthesis (SHS) was conducted using a Ti–B₄C–BN system. The effects of the C/(C + N) ratio on the combustion behavior and reaction products were investigated. Experimental characterizations of quenched samples show that the combustion reaction started with the formation of highly substoichiometric TiN_y and TiB due to the solid-state reaction between Ti and BN; and then the TiN_y precursor and TiB dissolved back into the titanium melt, forming the Ti–B–N liquid, which in turn transformed to the Ti–B–N–C liquid due to the dissolution of the carbon atoms diffused away from B₄C. Finally, the TiC_xN_y and TiB₂ particles are formed from the melt through the dissolution-precipitation mechanism.     

Al–Ti–B grain refiners via powder metallurgy processing of Al/K2TiF6/KBF4 powder blends

The response to thermal exposure of ball-milled Al/K₂TiF₆/KBF₄ powder blends was investigated to explore the potential of PM processing for the manufacture of Al–Ti–B alloys. K₂TiF₆ starts to be reduced by aluminium as early as 220 °C when ball-milled Al/K₂TiF₆/KBF₄ powder blends are heated. The reaction of KBF₄ with aluminium follows soon after. The Ti and B thus produced are both solutionized in aluminium before precipitating out as Al₃Ti and TiB₂. All these reactions take place below the melting point of aluminium. The ball-milled Al/K₂TiF₆/KBF₄ powder blends heat treated at approximately 525 °C can be compacted to produce Al–Ti–B pellets with in situ formed Al₃Ti and TiB₂ particles. These pellets are shown to be adequate grain refiners for aluminium alloys.     

Wear-resistant ceramic and metal–ceramic ultrafine composites fabricated from combustion synthesised metastable powders

Dense Ti–Al₂O₃–TiC cermet and TiC–TiB₂ ceramic composites have been fabricated by high-pressure high-temperature (HPHT) sintering starting from metastable nanostructured powders obtained by means of a technique based on the self-propagating high-temperature synthesis (SHS) process. The microstructural observations showed that an ultrafine microstructure was retained in the sintered composites thanks to the limited grain growth allowed by the short sintering duration of the HPHT method. The sintered TiC–TiB₂ and Ti–Al₂O₃–TiC fine-grained bulk composites exhibited high values of hardness and Young modulus. The tribological characterization confirmed the good properties of both the materials in terms of wear-resistance and makes them very promising candidates for demanding applications. The influence of the ultrafine grain size on physical and tribological properties of the densified materials is discussed.     

A novel Al–Ti–B alloy for grain refining Al–Si foundry alloys

Al–Ti–B refiners with excess-Ti (Ti:B > 2.2) perform adequately for wrought aluminium alloys but they are not as efficient in the case of foundry alloys. Silicon, which is abundant in the latter, forms silicides with Ti and severely impairs the potency of TiB₂ and Al₃Ti particles. Hence, Al–Ti–B alloys with excess-B (Ti:B < 2.2) and binary Al–B alloys are favored to grain refine hypoeutectic Al–Si alloys. These grain refiners rely on the insoluble (Al,Ti)B₂ or AlB₂ particles for grain refinement, and thus do not enjoy the growth restriction provided by solute Ti. It would be very attractive to produce excess-B Al–Ti–B alloys which additionally contain Al₃Ti particles to maximize their grain refining efficiency for aluminium foundry alloys. A powder metallurgy process was employed to produce an experimental Al–3Ti–3B grain refiner which contains both the insoluble AlB₂ and the soluble Al₃Ti particles. Inoculation of a hypoeutectic Al–Si foundry alloy with this grain refiner has produced a fine equiaxed grain structure across the entire section of the test sample which was more or less retained for holding times up to 15 min.     

Effect of nickel addition on the exothermic reaction of the Ti–C–BN system

The effect of Ni addition on the exothermic reaction in the Ti–C–BN system was investigated using differential thermal analysis (DTA). Based on DTA and X-ray diffraction (XRD) analyses, a reaction path for the formation of TiC_xN_y and TiB₂ in the Ni–Ti–C–BN system was proposed in which the reaction initiated with the solid-state diffusion between Ti and BN. The resultant TiN_x then reacted with Ni to form Ni–Ti compounds (e.g., Ti₂Ni). With increasing temperature, a Ni–Ti–N eutectic liquid phase formed between the Ni–Ti compounds and TiN_x at about 1110 °C, enabling the dissolution of C and B in the melt to form Ni–Ti–N–C–B liquid. Finally, TiC_xN_y and TiB₂ formed and precipitated out of the liquid.     

Microstructure and grain refining performance of a new Al–Ti–C–B master alloy

A kind of Al–Ti–C–B master alloy with a uniform microstructure is prepared using a melt reaction method. It is found that the average grain size of α-Al can be reduced from 3500 to 170 μm by the addition of 0.2 wt.% of the prepared Al–5Ti–0.3C–0.2B and the refining efficiency does not fade obviously within 60 min. It is considered that the TiC_xB_y and TiB_2−mC_n particles found at the grain center are the effective and stable nucleating substrates for α-Al during solidification, which accounts for the good grain refining performance.     

Wear-resistant Ti–B–N nanocomposite coatings synthesized by reactive cathodic arc evaporation

Wear-resistant Ti–B–N coatings have been synthesized by reactive arc evaporation of Ti–TiB₂ compound cathodes in a commercial Oerlikon Balzers Rapid Coating System. Owing to the strong non-equilibrium conditions of the deposition method, a TiN–TiB_x phase mixture is observed at low N₂ partial pressures, as determined by elastic recoil detection analysis, X-ray diffraction, X-ray spectroscopy, transmission electron microscopy and selected area electron diffraction. The indicated formation of a metastable solid solution of B in face-centered cubic TiN gives rise to a maximum in hardness (>40 GPa) and wear resistance on the expense of increased compressive stresses. A further saturation of the nitrogen content results in the formation of a TiN–BN nanocomposite, where the BN phase fraction was tailored by the target composition (Ti/B ratio of 5/3 and 5/1). However, the amorphous nature of the BN phase does not support self-lubricious properties, showing friction coefficients of 0.7 ± 0.1 against alumina. The effect of an increased bias voltage on structure and morphology was investigated from −20 to −140 V and the thermal stability assessed in Ar and air by simultaneous thermal analysis up to 1400 °C.     

Microstructure and mechanical properties of B4C-TiB2 composite ceramic fabricated by reactive spark plasma sintering

B₄C-TiB₂ composite ceramic was prepared by reactive spark plasma sintering, using amorphous B, Ti, and graphite as the raw materials. The reaction process and the phase composition in the process of sintering were studied. The effects of the ratio of raw materials and sintering process on the microstructure and mechanical properties of B₄C-TiB₂ composite ceramic were investigated. The composition of the sintered sample was B₄C, TiB₂, and bits of residual unreacted graphite. B and Ti preferentially reacted to form TiB₂ at 800 °C, and then B and graphite reacted to form B₄C at 1250 °C. The 75 vol% B₄C-25 vol% TiB₂ composite ceramic synthesized with 60.6 wt% B, 25.8 wt% Ti, and 13.6 wt% graphite and sintered at 1900 °C for 15 min resulted in nearly full densification and optimal mechanical properties. The relative density, Vickers hardness, fracture toughness, and flexural strength were 98.6 ± 0.01%, 26.6 ± 0.01 GPa, 5.9 ± 0.13 MPa·m^1/2, and 605 MPa, respectively.     

Densification and compressive strength ofin-situ processed Ti/TiB composites by powder metallurgy

In the present study, the densification of Ti/TiB composites, the growth behavior ofin-situ formed TiB reinforcement, the effects of processing variables — such as reactant powder (TiB₂, B₄C), sintering temperature and time — on the microstructures and the mechanical properties ofin-situ processed Ti/TiB composites have been investigated. Mixtures of TiB₂ or B₄C powder with pure titanium powder were compacted and presintered at 700°C for 1 hr followed by sintering at 900, 1000, 1100, 1200, and 1300°C, respectively, for 3hrs. Some specimens were sintered at 1000°C for various times in order to study the formation behavior of TiB reinforcementin-situ formed within the pure Ti matrix. TiB reinforcements were formed through different mechanisms, such as the formation of fine TiB and the formation of coarse TiB by Ostwald ripening or the coalescence of fine TiB. There was no crystallographic relationship between TiB reinforcement and the matrix. There were voids at the interface between the TiB reinforcement and the Ti matrix due to the preferential growth of coarse TiB without a particular crystallographic relationship with pure Ti matrix and the surface energy between the Ti matrix and TiB reinforcements. Therefore, the densification of Ti/TiB₂ compacts was hindered by the preferential growth of coarse TiB reinforcements. The mechanical properties ofin-situ processed composites were evaluated by measuring the compressive yield strength at ambient and high temperatures. The compressive yield strength of thein situ processed composites was higher than that of the Ti-6A1-4V alloy. It was also found that the compressive yield strength of the composite made from TiB₂ reactant powder was higher than that of the composite made from B₄C at the same volume fraction of reinforcement. A crack path examination suggested that the bonding nature of interface between matrix and reinforcement made from TiB₂ reactant powder was better than that made from B₄C.     

Ti 元素对B4C/Al复合材料力学性能的改善作用研究

B₄C/Al复合材料是目前最理想的中子吸收材料,广泛用于乏燃料储存。本文利用液态搅拌法制备B₄C/Al复合材料,通过添加Ti元素,探讨了界面反应对材料的界面结构和力学性能的影响。研究发现,Ti元素通过参与界面反应,改变了界面结构,在B₄C颗粒表面形成了紧密结合的纳米TiB₂界面层;Ti的添加消除了界面微裂纹、微孔、分离等缺陷。随着界面反应程度的加强,材料强度提高,尤其反应脱落的纳米TiB₂颗粒作为原位第二强化相进一步增强基体。B₄C/Al复合材料断裂过程表现为韧窝延性断裂;TiB₂界面层增强了B₄C颗粒与基体的结合,断裂行为从B₄C-Al界面脱落转变为B₄C颗粒断裂;但过渡的界面反应会形成微韧窝,引起材料延伸率下降。     

Synthesis of ultrafine (Ti, W, Mo, V)(C, N)–Ni composite powders by low-energy milling and subsequent carbothermal reduction–nitridation reaction

Ultrafine (Ti, W, Mo, V)(C, N)–Ni composite powders with globular-like particles of 50–300 nm were synthesized at static nitrogen pressure from oxides by a simple and cost-effective route which combines traditional low-energy milling plus carbothermal reduction–nitridation (CRN) techniques. Reaction path of the (Ti, W, Mo, V)(C, N)–Ni system was discussed by X-ray diffraction (XRD) and thermogravimetry–differential scanning calorimetry (TG–DSC), and microstructure of the milled powders and final products was studied by scanning electron microscopy (SEM) and transmission electron microscope (TEM), respectively. The results show that CRN reaction has been enhanced by nano-TiO₂ and nano-carbon powders. Thus, the preparation of (Ti, 15W, 5Mo, 0.2V)(C, N)–20Ni is at only 1300 °C for 1 h. During synthesizing reaction, Ni solid solution phase forms at about 700 °C and reduction–carbonization of WO₂ and MoO₂ occurs below 900 °C. The reactions of TiO₂ → Ti₃O₅, Ti₃O₅ → Ti(C, O) and Ti(C, O) → Ti(C, N) take place at about 930 °C, 1203 °C and 1244 °C, respectively.     

Effect of composition and milling parameters on the critical ball milling of Ti–Si–B powders

The absence of brittle phases and elevated temperature during ball milling of a powder mixture containing a large amount of ductile component can contribute to reach an excessive agglomeration denoting a critical ball milling (CBM) behavior. This work reports in the effect of composition and milling parameters on the CBM behavior of Ti–Si–B powders. High-purity elemental Ti–Si–B powder mixtures were processed in a planetary ball mill in order to prepare the Ti₆Si₂B compound and two-phase Ti + Ti₆Si₂B alloys. TiH₂ chips instead of titanium powder were used as a starting material. To avoid elevated temperature in the vials during ball milling of Ti–Si–B powders the process was interrupted after each 10 min followed by air-cooling. Following, the milled powders were hot-pressed at 900 °C for 1 h. Samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS). Short milling times followed by air-cooling contributed to obtain a large amount of powders higher than 75% in the vials. Only Ti and TiH₂ peaks were observed in XRD patterns of Ti–Si–B and TiH₂–Si–B, respectively, suggesting that extended solid solutions were achieved. The large amount of Ti₆Si₂B and Ti + Ti₆Si₂B structures were formed during hot pressing from the mechanically alloyed Ti–Si–B and TiH₂–Si–B powders.     

Effects of TiC and Al4C3 addition on combustion synthesis of Ti2AlC

Preparation of the ternary carbide Ti₂AlC was conducted by combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) from the elemental powder compacts of Ti:Al:C = 2:1:1, TiC-containing samples with TiC of 6.67–14.3 mol%, and Al₄C₃-containing samples with Al₄C₃ of 1.96–10 mol%. Effects of TiC and Al₄C₃ addition were studied on combustion characteristics and the degree of phase conversion. Due to the growth of laminated Ti₂AlC grains, the reactant compact was subjected to an axial elongation during the SHS process. Because the addition of TiC and Al₄C₃ led to a decrease in the reaction temperature, the flame-front propagation velocity was correspondingly reduced for the TiC- and Al₄C₃-containing samples when compared with the elemental reactants. Based upon the XRD analysis, formation of Ti₂AlC along with a secondary phase TiC was identified in the synthesized products. The grains of Ti₂AlC are typically plate-like with a size of 10–20 μm and several laminated Ti₂AlC grains form a layered structure. The content of Ti₂AlC yielded from the elemental powder compacts is about 85 wt%. The addition of TiC was found to facilitate the formation mechanism and therefore to enhance the extent of Ti₂AlC conversion approaching 90 wt%. As a result of the reduced exothermicity of the reaction, however, the content of Ti₂AlC decreased slightly in the products synthesized from the Al₄C₃-added samples.     

Strong bonding of infrared brazed α2-Ti3Al and Ti–6Al–4V using Ti–Cu–Ni fillers

Infrared dissimilar brazing of α₂-Ti₃Al and Ti–6Al–4V using Ti–15Cu–25Ni and Ti–15Cu–15Ni filler metals has been performed in this study. The brazed joint consists primarily of Ti-rich and Ti₂Ni phases, and there is no interfacial phase among the braze alloy, α₂-Ti₃Al and Ti–6Al–4V substrates. The existence of the Ti₂Ni intermetallic compound is detrimental to the bonding strength of the joint. The amount of Ti₂Ni decreases with increasing brazing temperature and/or time due to the depletion of Ni content from the braze alloy into the Ti–6Al–4V substrate during brazing. The shear strength of the brazed joint free of the blocky Ti₂Ni phase is comparable with that of the α₂-Ti₃Al substrate, and strong bonding can thus be obtained.     

Influence factors and microstructure evolution during preparation of nanocrystalline (Ti, W, Mo, V)(C, N)–Ni composite powders

Nanocrystalline (Ti, W, Mo, V)(C, N)–Ni composite powders with crystalline size of about 35 nm were synthesized at 1300 °C from oxides by a simple and cost-effective route which combines traditional low-energy milling plus carbothermal reduction–nitridation techniques. Influence of main technological parameters was investigated by X-ray diffraction, and microstructure of the milled powders and reaction products was studied by scanning electron microscopy. The results show that the phase evolution of TiO₂ follows TiO₂ → Ti₃O₅ → Ti(C, N), and (Ti, W, Mo, V)(C, N)–Ni composite powders with higher nitrogen content and smaller crystalline size can be produced by introducing high nitrogen pressure. By contrast with high nitrogen pressure, high synthesizing temperature and long isothermal time can contribute to dissolution of W, Mo and V atoms into Ti(C, N). In addition, synthesizing temperature has a significant effect on the microstructure evolution of (Ti, W, Mo, V)(C, N)–Ni composite powders.     

Interfacial reaction studies of B4C-coated and C-coated SiC fiber reinforced Ti–43Al–9V composites

The interfacial reactions of B₄C-coated and C-coated SiC fiber reinforced Ti–43Al–9V composites were investigated by scanning electron microscope and transmission electron microscope. The detailed microstructures as well as the chemical composition throughout the reaction zone were identified. For SiC_f/B₄C/TiAl composite, the reaction zone from B₄C coating to TiAl matrix is composed of 4 layers, namely, a carbon-rich layer, a mixed layer of TiB₂ + amorphous carbon, a TiC layer and a mixed layer of TiB + Ti₂AlC. For SiC_f/C/TiAl composite, the reaction zone from C coating to TiAl matrix is composed of 3 layers, namely, a fine-grained TiC layer, a coarse-grained TiC layer and a thick Ti₂AlC layer. For both kinds of composites, the reaction mechanisms of the interfacial reactions were analyzed, and the corresponding reaction kinetics were calculated. The activation energies of interfacial reaction in SiC_f/B₄C/TiAl composite and SiC_f/B₄C/TiAl composite are 308.1 kJ/mol and 230.7 kJ/mol, respectively.     

Reaction synthesis of TiB2–TiC composites with enhanced toughness

In situ toughened TiB₂–TiC_x composites were fabricated using reaction synthesis of B₄C and Ti powders at high temperatures. The resulting materials possessed very high relative densities and well developed TiB₂ plate-like grains, leading to a rather high fracture toughness, up to 12.2 MPa⋅m^1/2. The microstructure was examined by means of XRD, SEM, TEM and EDAX. The reaction products mainly consisted of TiB₂ and TiC_x. No other phases, e.g. Ti₃B₄, TiB, Ti₂B₅ and free Ti, were observed regardless of whether the starting composition was Ti:B₄C=3:1 or 4.8:1, and whether the sintering temperature was 1700 or 1800°C. The microstructural morphology is characterised by TiB₂ plate-like grains distributed uniformly in the TiC_x matrix. Some inclusions and defects were found in TiB₂ grains. The very high reaction temperature was believed to be responsible for the formation of plate-like grains, which, in turn, is responsible for the much improved mechanical properties. The main toughening mechanisms were likely to be crack deflection, platelet pull-out and the micro-fracture of TiB₂ grains.     

Powder metallurgical processing of Ni–Ti–Zr alloys undergoing martensitic transformation: part I

Ni–Ti–Zr materials with Zr 12–25 at.% and Ni 42–50 at.% have been produced by powder metallurgy. Suitable temperatures for sintering in Ar-atmosphere Ni–Ti–Zr compacts are within the range 900–1000 °C. Sintering at temperatures above 1000 °C causes melting of the compacts with high Zr content. The presence of ZrC, ZrO₂, Zr₃O, TiO₂ and TiO of different modifications, complex oxides such as Ni₅TiO₇, Ti_0.5Zr_0.5O_0.2 and equilibrium phases after sintering at temperatures above 1000 °C in alloys with low Zr-content was derived from X-ray diffractometry. During sintering at temperatures below 1000 °C the phases belonging to the binary Ti–Ni and Ti–Zr systems were formed. Long-term sintering and slow furnace cooling allowed the precipitation of Ni₄Ti₃ and Ni₂Ti. The process of sintering is controlled by the diffusion of Ni in Ti and Zr particles during the early stages of sintering. Slow diffusion of Zr atoms in Ti₂Ni, Ti–Ni and diffusion of Ti atoms in Zr₂Ni, Ni–Zr controls the later stages of sintering.     

In situ impedance spectroscopy study of the electrochemical corrosion of Ti and Ti–6Al–4V in simulated body fluid at 25 °C and 37 °C

The corrosion resistance of Ti and Ti–6Al–4V was investigated through electrochemical impedance spectroscopy, EIS, potentiodynamic polarisation curves and UV–Vis spectrophotometry. The tests were done in Hank solution at 25 °C and 37 °C. The EIS measurements were done at the open circuit potential at specific immersion times. An increase of the resistance as a function of the immersion time was observed, for Ti (at 25 °C and 37 °C), and for Ti–6Al–4V (at 25 °C), which was interpreted as the formation and growth of a passive film on the metallic surfaces.