Microstructure and shear strength of ZrB2SiC/Ti6Al4V joint by TiCuZrNi with Cu foam

In this paper, brazing behaviors between ZrB₂SiC and Ti6Al4V by Cu foam interlayer were studied. The microstructure, formation mechanism, mechanical property and fracture surface of the joints were systematically studied. The results showed that the phases in the joints were α+β-Ti, TiCu, Ti₂Cu, Cu(s, s), TiC, TiB₂ and Ti₃SiC₂. An optimum shear strength reached up to 435??MPa?at a brazing temperature of 910?°C and holding time of 20?min. Such a shear strength was 90?MPa higher than the one without the Cu foam. The obtained high shear strength of joint was discussed from microstructure and residual stress. With the increase of brazing time, Cu(s,s) gradually disappeared and the content of Ti₂Cu intermetallic compound increased, which was harmful for the joint. Furthermore, the residual stress of joint with Cu foam was calculated to be 324?MPa, lower than the one without Cu foam interlayer.     

Oxidation Behavior of ZrB2–SiC Composites at Low Pressures

ZrB₂‐60 mol%SiC composite with a eutectic microstructure was oxidized at 1573 to 1873 K with reduced total pressures (P_tot) and low oxygen partial pressures (). The mass change was continuously measured by a thermobalance, and then fit with a multiple paralinear model. Oxidation scale of SiO₂/ZrO₂+SiO₂/ZrO₂/ZrB₂ was formed at  > 0.13 kPa, whereas only porous ZrO₂ remained at  < 0.13 kPa, P_tot < 1.33 kPa and higher than 1773 K. With increasing , the parabolic oxidation constant decreased, whereas the linear oxidation constant increased.     

SiC Depletion in ZrB2–30 vol% SiC at Ultrahigh Temperatures

The formation of a porous SiC‐depleted region in ZrB₂–SiC due to active oxidation at ultrahigh temperatures was characterized. The presence/absence of SiC depletion was determined at a series of temperatures (1300°C–1800°C) and times (5 min–100 h). At T < 1627°C, SiC depletion was not observed. Instead, the formation of a ZrO₂ + C/borosilicate oxidation product layer sequence was observed above the ZrB₂–SiC base material. At T ≥ 1627°C, SiC was depleted in the ZrB₂ matrix below the ZrO₂ and borosilicate oxidation products. The SiC depletion was attributed to active oxidation of SiC to form SiO(g). The transition between C formation in ZrO₂ (T < 1627°C) and SiC depletion in ZrB₂ (T ≥ 1627°C) is attributed to variation in the temperature dependence of thermodynamically favored product assemblage influenced by the local microstructural phase distribution. The growth kinetics of the SiC depletion region is consistent with a gas‐phase diffusion‐controlled process.     

Improvement of SiC–Si/MAS Interface in the Joints via In Situ Synthesizing SiC Nanowires

Improving the SiC–Si/MAS interface is essential for enhancing the SiC–Si/MAS joint strength. In this work, well‐dispersed SiC nanowires (NWs) were in situ synthesized at the SiC–Si/MAS interface in the joints as the anchors, which made the fracture feature of the joints change from SiC–Si/MAS interface fracture to MAS fracture, resulting in an obvious improvement of the shear strength. The results showed that the average shear strength of the joints was increased by 49% by in situ synthesizing SiC NWs, which was primarily attributed to the excellent interface anchoring effects of SiC NWs involving the interfacial bonding anchoring between SiC NWs and SiC–Si layer and the mechanical interlocking anchoring between SiC NWs with the junctions and the surrounding grains. This study may provide new insight into the anchoring mechanisms and suggest a new design methodology for nanowire joints, leading to improved joint strength.     

Microstructure and Mechanical Properties of (TiB2 + SiC) Reinforced Ti3SiC2 Composites Synthesized by In Situ Hot Pressing

Fully dense (TiB₂ + SiC) reinforced Ti₃SiC₂ composites with 15 vol% TiB₂ and 0–15 vol% SiC were designed and synthesized by in situ reaction hot pressing. The increase in SiC content promoted densification and significantly inhibited the growth of Ti₃SiC₂ grains. The in situ incorporated TiB₂ and SiC reinforcements showed columnar and equiaxed grains, respectively, providing a strengthening–toughening effect by the synergistic action of particulate reinforcement, grain's pulling out, “self‐reinforcement,” crack deflection, and grain refining. A maximum bending strength of 881 MPa and a fracture toughness of 9.24 MPam^1/2 were obtained at 10 vol% SiC. The Vickers hardness of the composites increased monotonously from 9.6 to 12.5 GPa.     

Reactive Hot‐Pressed Hybrid Ceramic Composites Comprising SiC(SCS‐6)/Ti Composite and ZrB2–ZrC Ceramic

In this study, hybrid composites comprising SiC(SCS‐6)/Ti and ZrB₂–ZrC ceramics were prepared by sandwiching Ti/SiC(SCS‐6)/Ti sheets and Zr + B₄C powder layers, followed by reactive hot pressing at 1300°C. The microstructure of the obtained hybrid composites was characterized by field‐emission scanning electron microscopy, transmission electron microscopy, and energy‐dispersive X‐ray spectroscopy. The results show that after reactive hot pressing, a highly dense matrix was achieved in the hybrid composites. A Ti‐rich zone was observed only in the hybrid composite prepared using a 10‐μm‐thick Ti foil. Interface reaction occurred during sintering and interface reaction layers were formed between the fibers and the matrix, and the phases were identified. In addition, the mechanical behavior of the hybrid composites was evaluated using by testing under four‐point bend testing. The results indicate that the hybrid composites exhibited greater flexural strengths and noncatastrophic fracture behavior. The flexural strength ranged from 440 to 620 MPa, depending on the thickness of the Ti foils and the fiber volume amount.     

Ti6Al4V／TiC＋TiB双相增强复合材料的力学性能研究

在地壳中的金属元素中,钛元素含量仅次于镁（Mg）、铁（Fe）、铝（Al）,排在第四位。钛合金材料以其轻质、比强度高、中低温性能好、耐腐蚀等优良的性能而成为倍受人们关注的材料,现已被广泛应用于航空、航天领域和其他民用领域,如汽车、体育器材等。Ti6Al4V以其优异的机械性能成为众多钛合金牌号中应用最广泛的一种钛合金材料,而基于TiAl4V的复合材料也成为倍受人们关注的焦点。     

Effect of Si3N4 Addition on Compressive Creep Behavior of Hot‐Pressed ZrB2–SiC Composites

Compressive creep studies have been carried out on hot‐pressed ZrB₂–SiC (ZS) and ZrB₂–SiC–Si₃N₄ (ZSS) composites in air under stress and temperature ranges of 93–140 MPa and 1300°C–1425°C, respectively for time durations of ≈20–40 h. The results of these studies have shown the creep resistance of ZS composite to be greater than that of ZSS. As the temperature is increased from 1300°C to 1425°C, the stress exponent of ZS decreases from 1.7 to 1.1, whereas that of ZSS drops from 1.6 to 0.6. The activation energies for these composites have been found as ≈95 ± 32 kJ/mol at temperatures ≤1350°C, and as ≈470 ± 20 kJ/mol in the range of 1350°C–1425°C. Studies of the postcreep microstructures using scanning and transmission electron microscopy have shown the presence of glassy film with cracks at both ZrB₂ grain boundaries and ZrB₂–SiC interfaces. These results along with calculated values of activation volumes suggest grain‐boundary sliding as the major damage mechanism, which is controlled by O^2? diffusion through SiO₂ at ≤1350°C, and by viscoplastic flow of the glassy interfacial film at temperatures ≥1350°C. Studies by transmission electron microscopy have shown formation of crystalline precipitates of Si₂N₂O near ZrB₂–SiC interfaces in ZSS tested at ≥1400°C, which along with stress exponent values <1 suggests that grain‐boundary sliding involving solution‐precipitation‐type mechanism is operative at these temperatures.     

Phase Evolution and Microstructural Studies in CaZrTi2O7–Nd2Ti2O7 System

A series of compositions with general stoichiometry Ca_1?xZr_1?xNd_2xTi₂O₇ has been prepared by high‐temperature solid‐state reaction of component oxides and characterized by powder X‐ray diffraction and electron probe for microanalyses (EPMA). The phase fields in CaZrTi₂O₇–Nd₂Ti₂O₇ system and distribution of ions in different phases have been determined. Four different phase fields, namely monoclinic zirconolite, cubic perovskite, cubic pyrochlore, and monoclinic Nd₂Ti₂O₇ structure types are observed in this system. The 4M‐polytype of zirconolite structure is stabilized by substitution of Nd³⁺ ion. The addition of Nd³⁺ ions form a cubic perovskite structure‐type phase and thus observed in all the compositions with 0.05 ≤ x ≤ 0.80. Cubic pyrochlore structure‐type phase is observed as a coexisting phase in the nominal composition with 0.20 ≤ x ≤ 0.90. Only a subtle amounts of Ca²⁺ and Zr⁴⁺ are incorporated into the perovskite‐type Nd₂Ti₂O₇ structure. EPMA analyses on different coexisting phases revealed that the pyrochlore and perovskite phases have Nd³⁺‐rich compositions.     

Brazing of SiO2-BN modified with in situ synthesized CNTs to Ti6Al4V alloy by TiZrNiCu brazing alloy

Brazing SiO₂-BN ceramic to Ti6Al4V is often associated with the problem of brittle continuous phases and high residual stress at the reaction layer of SiO₂-BN, inducing low joining strength. To overcome these problems, SiO₂-BN ceramic modified with in situ synthesized carbon nanotubes (CNTs) were joined to Ti6Al4V by TiZrNiCu alloy. Results show that CNTs can improve the wettability of TiZrNiCu on SiO₂-BN rapidly, and break the continuous reaction layer of SiO₂-BN into fine sizes. Residual stress can also be reduced by low CTE of CNTs and fine size phases among reaction layer. The shear strength of SiO₂-BN/Ti6Al4V joints with CNTs modified is 35.3?MPa at 970?°C for 10?min, which is 3 times higher than that of the joints without CNTs modified.     

Potential dependent selective dissolution of Ti–6Al–4V and laser treated Ti–6Al–4V in acid/chloride media

The corrosion properties of Ti–6Al–4V and laser surface melted (LSM) Ti–6Al–4V samples were investigated in 0.05 M H₂SO₄/0.05 M NaCl solution. Laser surface treatment was found to increase the corrosion potential and decrease the corrosion rates of the alloy. The current–potential profile of the LSM was found to be generally noisy below 0.5 V, indicating an unstable surface, which undergoes continuous dissolution and repassivation. However, above 0.5 V the LSM specimen exhibited higher corrosion current compared to the untreated alloy. Inductively coupled plasma (ICP) analysis of metals in solution was carried out after controlled potential electrolysis. Generally, the aluminium percentage was found to be the highest in solution compared to titanium and vanadium. The aluminium percentage in solution reached 94% compared to titanium and vanadium upon polarization in the passive region at 1.01 V. SEM showed that some local and shallow pitting to occur in both the untreated and LSM alloy. EDS results showed that aluminium composition of the electrolysed alloy surface is lower than the original material composition, and decreased from 6% in the original alloy to 0.18% after two hours of electrolysis of the LSM specimen.     

TEM Study of the High‐Temperature Oxidation Behavior of Hot‐Pressed ZrB2–SiC Composites

The oxidation behaviors of ZrB₂‐ 30 vol% SiC composites were investigated at 1500°C in air and under reducing conditions with oxygen partial pressures of 10⁴ and 10 ^{? 8} Pa, respectively. The oxidation of ZrB₂ and SiC were analyzed using transmission electron microscopy (TEM). Due to kinetic difference of oxidation behavior, the three layers (surface silica‐rich layer, oxide layer, and unreacted layer) were observed over a wide area of specimen in air, while the two layers (oxide layer, and unreacted layer) were observed over a narrow area in specimen under reducing condition. In oxide layer, the ZrB₂ was oxidized to ZrO₂ accompanied by division into small grains and the shape was also changed from faceted to round. This layer also consisted of amorphous SiO₂ with residual SiC and found dispersed in TEM. Based on TEM analysis of ZrB₂ – SiC composites tested under air and low oxygen partial pressure, the ZrB₂ begins to oxidize preferentially and the SiC remained without any changes at the interface between oxidized layer and unreacted layer.     

Electrophoretic deposition of nanocrystalline hydroxyapatite on Ti6Al4V/TiO2 substrate

Hydroxyapatite is a bioactive material that is the main inorganic constituent of human hard tissue (Ca/P ratio of 1.67) whose coatings provide requisite surface bioactivity to the bone implants. In the current work, the characteristics of nanocrystalline hydroxyapatite (HA) coatings, electrophoretically deposited on Ti6Al4V substrate, have been investigated. To enhance the coating’s compatibility, a 0.75 μm thick TiO₂ layer was thermally grown as a diffusion barrier prior to electrophoretic deposition of HA. Subsequently, HA was electrophoretically deposited (EPD) at different deposition voltages (100–250 V) while keeping the deposition time as 10 s. Both anodic oxidation during EPD for 10 s and thermal oxidation during sintering at 1000°C for 2 h resulted in the growth of a TiO₂ layer thickness of more than 25 μm. Enhancement of voltage also has shown significant influence on the mechanism of the evolution of biphasic microstructures, attributed to the simultaneous growth of TiO₂ and HA phases. Optimized distribution of HA and TiO₂ phases was evidenced at 200 V, with explicit HA retention as observed via transmission electron microscopy. An empirical relationship is developed to relate the voltage with the suppression of cracking in the deposited coatings.     

Microstructural Evolution and Grain Growth Kinetics in ZrB2–SiC Composites During Heat Treatment

This work reported the microstructural evolution and grain growth kinetics of ZrB₂–SiC composites containing 10, 20, and 30 vol% SiC during heat treatment at 2000°C. The coarsening of ZrB₂ occurred in the three systems, whereas the obvious coarsening for SiC appeared only in the composite with 30 vol% SiC. The kinetics analysis showed the ZrB₂ grain growth rate in the ZrB₂–30 vol% SiC was 25 times lower than that for ZrB₂–10 vol% SiC during heat treatment. Furthermore, the grain growth controlling mechanisms of ZrB₂ and SiC were discussed. In addition, it was found that the heat treatment had little effect on Vickers hardness and fracture toughness of ZrB₂–SiC.     

Microstructure and mechanical properties of B4C–TiB2–SiC composites toughened by composite structural toughening phases

B₄C–TiB₂–SiC composites toughened by composite structural toughening phases, which are the units of (TiB₂–SiC) composite, were fabricated through reactive hot pressing with B₄C, TiC, and Si as raw materials. The units of (TiB₂–SiC) composite with the size of 10‐20 μm are composed of interlocking TiB₂ and SiC with the size of 1‐5 μm. The addition of TiC and Si can effectively promote the sintering of B₄C ceramics. The relative densities of all the B₄C composites with different contents of TiB₂ and SiC are close to completely dense (98.9%‐99.4%), thereby resulting in superior hardness (33.1‐36.2 GPa). With the increase in the content of TiB₂ and SiC, the already improved fracture toughness of the B₄C composite continuously increases (5.3‐6.5 MPa·m^1/2), but the flexure strength initially increases and then decreases. When cracks cross the units of the (TiB₂–SiC) composite, the cracks deflect along the interior boundary of TiB₂ and SiC inside the units. As the crack growth path is lengthened, the crack propagation direction is changed, thereby consuming more crack extension energy. The cumulative contributions improve the fracture toughness of the B₄C composite. Therefore, the composite structural toughening units of the (TiB₂–SiC) composite play an important role in reinforcing the fracture toughness of the composites.     

Non-isothermal nitridation kinetics of Ti6Al4V in Al2O3-based refractories

To establish a kinetic model of nitridation of Ti6Al4V in Al₂O₃-based refractories, the non-isothermal nitridation of Ti6Al4V–Al₂O₃ composite refractories at various heating rates was investigated using a thermogravimetric (TG) analyzer for large samples. The activation energy (E) and kinetic model (G(α)) for the nitridation of Ti6Al4V were determined using the isoconversional and master plots methods, respectively. The nucleation and growth of nitriding products of the TiN solid solution was the controlling step in the nitridation of Ti6Al4V in Al₂O₃-based refractories. The Avrami-Erofeev kinetic model, depicted by the G(α) = [-ln (1-α)]⁴ equation, is the most rational kinetic model. The values of E and A for the nitridation of Ti6Al4V were calculated to be 214.99 kJ/mol and 1.46 × 10⁷ (S^?1), respectively.     

Effect of AlN Substitutions on the Oxidation Behavior of ZrB2–SiC Composites at 1600°C

The oxidation behavior of ZrB₂–SiC composites, with varying amounts of AlN substituting for ZrB₂, was studied isothermally under static ambient air at 1600°C for up to 5 h. Small amounts of AlN substitutions (≤10 vol%) were found to result in marginal improvement in the oxidation resistance, whereas larger amounts resulted in a significant deterioration. The size of ZrO₂ clusters formed on the oxidized surface was found to be a function of the AlN content. This effect was more pronounced after longer oxidation times (~1 h) as opposed to shorter durations (~5 min). It was postulated that presence of AlN results in the formation of Al₂O₃ during the oxidation process, subsequently resulting in a lowering of viscosity of the glassy silica scale, which facilitates the coarsening of ZrO₂ clusters. This also increases oxygen permeation through the scale which adversely affects the oxidation resistance of the high AlN content composites.     

ZrB2–SiC Nano‐Powder Mixture Prepared Using ZrSi2 and Modified Spark Plasma Sintering

ZrB₂–SiC nano‐powder mixture was synthesized using ZrSi₂ source material and a modified spark plasma sintering apparatus. The particle size of ZrB₂ and SiC was about 80 and 20 nm, respectively. The molecular‐level homogeneity of Zr/Si source and fast heating/cooling rate by SPS caused the formation of homogeneously intermixed nano‐powders. A strong exothermal reaction occurred at around 860°C, which caused strong agglomeration and growth of the synthesized powder mixture. The rapid reaction could be controlled by adding 20 wt% of NaCl, which acted as an inert filler.     

TiB2涂层刀具制备及干铣削Ti-6Al-4V磨损行为研究?

由于切削过程中产生高温、刀具粘结与氧化严重,钛合金切削尤其是干切削,一直是刀具行业的重大挑战之一,而在刀具表面添加涂层是提高钛合金切削刀具寿命的有效途径。利用脉冲磁控溅射技术制备了TiB2涂层刀具,以相同基体的无涂层刀具为对照,干铣削Ti-6Al-4V钛合金,切削速度从30~100 m/min变化,研究TiB2涂层刀具的切削性能与失效机理。所制备的TiB2涂层具有(100)择优取向的六方晶体结构,组织致密。涂层硬度可高达4000 HV。切削实验发现,在30 m/min的低速时,TiB2涂层刀具的切削寿命超过无涂层刀具57%之多,当切削速度加倍到60 m/min时,刀具寿命未见下降。当切削速度增加到100 m/min时,TiB2涂层刀具与无涂层刀具切削寿命相当。TiB2涂层刀具表面氧化所产生的B2O3液化膜,起自润滑作用,可充分减少钛合金的粘结,降低摩擦力。因此,在TiB2或B2O3消失之前,TiB2涂层刀具均有良好表现。在100 m/min时,切削高温造成B2O3强烈挥发,且TiB2被氧化为多孔疏松的TiO2,刀具寿命急剧下降到无涂层刀具的水平。     

In situ TiC particle reinforced TiCuZrNi brazing alloy for joining C/C composites to Ti6Al4V

Chromium carbide modified C/C and Ti6Al4V were successfully joined using a TiCuZrNi brazing alloy in powder form. The braze/composite interface and the mechanical strength of C/C‐Ti6Al4V joints were evaluated. The apparent shear strength of chromium carbide modified C/C joined to Ti6Al4V, measured by single lap test in compression, was 52 ± 6 MPa, which was highest among that without chromium carbide modification (15 ± 2 MPa) and the intrinsic C/C shear strength. The fractography of joints without chromium carbide modification indicated that failure mainly occurred at the TiC layer formed at the composite/braze interface while the joints with chromium carbide modification failed within the C/C.