CeO2 induced dispersive distribution of TiB2 particles in in situ TiB2/Al composite

Abstract

The roles of CeO₂ additive during preparation of in situ TiB₂/Al composite, alleviating particle settlement in composite melt and significantly improving particle dispersion in final microstructure, are studied in this paper. It is evidenced that the CeO₂ additive reacts with Al melts to release Ce solute into the melts, and the released surface active Ce is absorbed in the Al/TiB₂ interfaces without any other reaction products. First principles calculations show that the interfacial energy of Al/TiB₂ interfaces is reduced owing to the presence of Ce in Al/TiB₂ interfacial area. Therefore, the wettability of molten Al on TiB₂ surface is increased and the dispersion of TiB₂ particles in Al matrix is eventually improved.     

Explosive shock-compression processing of titanium aluminide/titanium diboride composites

Explosive shock-compression processing is used to fabricate Ti₃Al and TiAl composites reinforced with TiB₂. The reinforcement ceramic phase is either added as TiB₂ particulates or as an elemental mixture of Ti + B or both TiB₂ + Ti + B. In the case of fine TiB₂ particulates added to TiAl and Ti₃Al powders, the shock energy is localized at the fine particles, which undergo extensive plastic deformation thereby assisting in bonding the coarse aluminide powders. With the addition of elemental titanium and boron powder mixtures, the passage of the shock wave triggers an exothermic combustion reaction between titanium and boron. The resulting ceramic-based reaction product provides a chemically compatible binder phase, and the heat generated assists in the consolidation process. In these composites the reinforcement phase has a microhardness value significantly greater than that of the intermetallic matrix. Furthermore, no obvious interface reaction is observed between the intermetallic matrix and the ceramic reinforcement.     

Design aspects of processing of aluminium 6061 based metal matrix composites via powder metallurgy

Abstract

Acceptance of metal matrix composites for industrial applications depends upon improving properties using an economic production route, which includes the processing design. Two powder metallurgical routes have been used in the manufacture of Al 6061 metal matrix composites. The first involves blending, vacuum canning, and hot pressing from prealloyed powders and the second involves blending of elemental powders, liquid phase sintering, and subsequent hot rolling. These composites comprise 7·5 or 15 vol.-% of 7, 23, or 45 μm SiC particles. In this paper, the composite microstructure at each stage of the different processing routes has been examined and the aging behaviour investigated. Effects on the tensile properties of fabrication techniques, SiC particle size, and volume fraction are presented and discussed.

MST/3020     

Effect of re-melting on particle distribution and interface formation in SiC reinforced 2124Al matrix composite

The interface between metal matrix and ceramic reinforcement particles plays an important role in improving properties of the metal matrix composites. Hence, it is important to find out the interface structure of composite after re-melting. In the present investigation, the 2124Al matrix with 10 wt.% SiC particle reinforced composite was re-melted at 800 °C and 900 °C for 10 min followed by pouring into a permanent mould. The microstructures reveal that the SiC particles are distributed throughout the Al-matrix. The volume fraction of SiC particles varies from top to bottom of the composite plate and the difference increases with the decrease of re-melting temperature. The interfacial structure of re-melted 2124Al–10 wt.%SiC composite was investigated using scanning electron microscopy, an electron probe micro-analyzer, a scanning transmission electron detector fitted with scanning electron microscopy and an X-ray energy dispersive spectrometer. It is found that a thick layer of reaction product is formed at the interface of composite after re-melting. The experimental results show that the reaction products at the interface are associated with high concentration of Cu, Mg, Si and C. At re-melting temperature, liquid Al reacts with SiC to form Al₄C₃ and Al–Si eutectic phase or elemental Si at the interface. High concentration of Si at the interface indicates that SiC is dissociated during re-melting. The X-ray energy dispersive spectrometer analyses confirm that Mg- and Cu-enrich phases are formed at the interface region. The Mg is segregated at the interface region and formed MgAl₂O₄ in the presence of oxygen. The several elements identified at the interface region indicate that different types of interfaces are formed in between Al matrix and SiC particles. The Al–Si eutectic phase is formed around SiC particles during re-melting which restricts the SiC dissolution.     

TiB2 and TiC stainless steel matrix composites

Stainless steel matrix composites reinforced with TiB₂ or TiC particulates have been in situ produced through the reactive sintering of Ti, C and FeB. X-ray diffraction analysis confirmed the completion of reaction. The TiB₂, TiC and steel were detected by X-ray diffraction analysis. No other reaction product or boride was found, indicating the stability of TiB₂ and TiC in steel matrix. The SEM micrographs revealed the morphology and distribution of in situ synthesized TiB₂ and TiC reinforcements in steel matrix. During sintering the reinforcements TiB₂ and TiC grew in different shapes. TiB₂ grew in hexagonal prismatic and rectangular shape and TiC in spherical shape.     

Reaction synthesis of TiC–TiB<Subscript>2</Subscript>/Al composites from an Al–Ti–B<Subscript>4</Subscript>C system

The TiC–TiB₂/Al composites were fabricated by self-propagating high-temperature synthesis (SHS) from Al–Ti–B₄C compacts. The addition of Al to the Ti–B₄C reactants facilitates the ignition occurrence, lowers the reaction exothermicity, and modifies the resultant microstructure. The maximum combustion temperature and combustion wave velocity decrease with the increase in the Al amount. The B₄C particle size exerts a significant effect on the combustion wave velocity and the extent of the reaction, while that of Ti has only a limited influence. The reaction products are primarily dependent on the B₄C particle size and the Al content in the reactants. Desired products consisting of only the TiC, TiB₂, and Al phases could be obtained by a cooperative control of the B₄C particle size and the Al content.     

Synthesis of titanium diboride TiB2 and Ti-Al-B metal matrix composites

Titanium diboride TiB₂ and TiAl aluminide composites reinforced with in situ borites have been synthesized from the elemental powders of Ti and B, and Ti, Al and B respectively using mechanical alloying technique. No progressive diffusion between Ti and B was observed. The formation of TiB₂ was found to be governed by strong and fast exothermic heat release. This indicates that the formation of TiB₂ compound in local area of mechanically alloyed powder generated high energy which in turn ignited and promoted the formation of new compound in the rest of the area. Because of the presence of Al in Ti-Al-B system, the concentration of Ti or B was diluted. The exothermic reaction between Ti and B was consequently delayed. However, grain refinement of Ti and Al in this system down to nanometer scale is faster than that in Ti-Al system due to the contribution of B. Using X-ray analysis, strong but broad TiAl, and weak TiB and TiB₂ peaks had been detected at 50 h of mechanical alloying indicating the formation of nano TiAl composite reinforced by TiB and TiB₂. However, TiB was, however, not a stable phase; it later was transformed into equilibrium phase of TiB₂ after annealing at 800 °C.     

Dry sliding wear of TiB2 particle reinforced aluminium alloy composites

Abstract

Al–4 wt-%Cu alloy and composites reinforced with 10 and 20 vol.-% of TiB₂ particles were prepared by powder metallurgy followed by hot isostatic pressing. The dry sliding wear behaviour of specimens of these materials was investigated. Pin-on-disc measurements showed that the wear resistance of Al–4Cu alloy can be improved dramatically by the addition of 20 vol.-%TiB₂ particles. This was due to the high hardness of the TiB₂ particles, and to strong particle–matrix bonding. The wear data were found to correlate with SEM observations.     

Chemical stability of titanium diboride reinforcement in nickel aluminide matrices

Chemical stability of titanium diboride (TiB₂) reinforcement in NiAl (45 at% Al) and Ni₃Al (24 at% Al) matrices has been theoretically and experimentally investigated. Calculations were made using thermodynamic properties of the systems to predict behaviour at temperatures between 1173 and 1573 K. Experimental investigation of hot-press consolidated TiB₂ particulate/prealloyed matrix powder blends were conducted using energy dispersive X-ray analysis, X-ray diffraction analysis, Auger electron spectroscopy and transmission electron microscopy. The theoretical and experimental analyses suggest that TiB₂ is chemically stable in both matrices up to 1573 K, however, TiB₂ was found to be less active in NiAl than in Ni₃Al due to lower nickel activity in NiAl.     

The Cu matrix cermets remarkably strengthened by TiB2 “in situ” synthesized via self-propagating high temperature synthesis

The TiB₂–Cu cermets with predominant concentration of superhard TiB₂ (from 45 to 90 vol.%) were fabricated using elemental powders by means of SHS (self-propagating high-temperature synthesis) process and simultaneously densified by p-HIP (pseudo-isostatic pressing technique). The heat released during highly exothermic SHS reaction was “in situ” utilized for sintering. The combustion occurred even for 50 vol.% Cu dilution. According to XRD metallic copper binder was formed in those cermets in whole range of investigated compositions. The TiB₂ volume fraction significantly influenced the properties of fabricated materials, especially grain size and hardness. Both the average grain size and hardness significantly increased with TiB₂ content, so the maximum value of 18 GPa was measured for TiB₂–5 vol.%Cu composite. Coarse grains of 6.4 μm in size were observed for this composite while TiB₂-based submicro-composites were formed for 40–50% of Cu where the average grain size did not exceed 0.6 μm. The Vickers hardness of 16–18 GPa obtained for cermets containing from 85 to 90 vol.% of TiB₂ and no radial cracks in Vickers hardness test proved that in term of hardness and fracture toughness the composites might be competitive to WC–Co cermets.     

Production of titanium particulate metal matrix composite by mechanical milling

Abstract

Mechanical milling is an established production method for aluminium particulate metal matrix composites (MMCs). There are examples of its use for high performance automotive applications and within the aerospace industry. The production of a titanium particulate MMC is still in the developmental stage. However, compared to conventional titanium alloys such materials offer improvements in stiffness, strength, fatigue and creep properties, high temperature capability, and wear resistance. This paper describes the use of mechanical milling for the production of titanium particulate MMCs with the addition of 10 vol.-%TiB. Gas atomised titanium powders with additions of either boron or TiB₂ were milled in a high purity argon atmosphere to avoid contamination of the powders by oxygen or nitrogen. The distribution of the boron or TiB₂ with increasing milling time is discussed along with the effect of the alloy composition. Gas atomised, hydride dehydride, and sponge fine powder blends are also compared. The powders were subsequently hot isostatically pressed at 500°C for 2 h at 150 MPa followed by 900°C for 2 h at 150 MPa. During this consolidation process TiB was formed by an in situ reaction between either the TiB₂ or boron and the titanium matrix.     

Microstructure and polarisation behaviour of TiN–TiB2 matrix coating prepared by reactive plasma spraying

Abstract

Two TiN–TiB₂ matrix coatings were prepared by reactive plasma spraying with two spraying powders (Ti+B₄C+Cr, Ti+B₄C). Their microstructure, phases, microhardness and polarisation behaviours in 3·5 wt-% NaCl solution have been investigated by SEM, XRD, hardness tester and electrochemical analyser. The thermodynamics analysis for reactive plasma spraying was discussed and the effect of addition of Cr on adiabatic temperature of TiB₂ forming reaction was also analysed. The thermodynamics analysis shows that the reaction temperature during reactive spraying process is >2030 K. To satisfy the criterion of self-propagating high temperature TiB₂ forming reaction, the addition of Cr should be >192·6 wt-% of the total weight of Ti and B₄C. The experimental results show that the addition of Cr decreases the stress in the coating so as to the cracks of coating at the expense of microhardness, yet the microhardness of both two coatings is a high value. The corrosion resistance of TiN–TiB₂ matrix coating was greatly improved. The anodic polarisation curve of TiN–TiB₂ coating includes a narrow activation zone and a broad passivation zone, which indicates the stability of coating is very excellent. The addition of Cr increases the corrosion potential of coating, yet an overpassivation zone appears, which is due to the breakage of passivation film.     

Microstructures of in situ Al/TiB2 MMCs prepared by a casting route

In situ Al/TiB₂ metal matrix composites (MMCs) have been successfully produced by Salt-Metal reactions. This is a novel low-cost reactive approach, which involves adding Ti and B bearing salts to molten Al. The reactions between the salts lead to the formation of the reinforcing TiB₂ particles in the Al matrix. The in situ formed TiB₂ particles are very fine (below 1 m in size). Strings and clusters of particle agglomerates are distinct microstructural features of all the composites with pure Al as the matrix. The effects of processing parameters on the kinetics of TiB₂ formation and on the final microstructures are studied in detail. Besides, efforts are made to improve the distribution of TiB₂ particles in the Al matrix by means of chemical additions; it is found that a homogeneous distribution is obtained by using a eutectic Al-Si alloy as the matrix material.     

The mechanism of thermal explosion (TE) synthesis of TiC-TiB2 particulate locally reinforced steel matrix composites from an Al-Ti-B4C system via a TE-casting route

TiC-TiB₂ particulate locally reinforced steel matrix composites were fabricated by a novel TE-casting route from an Al-Ti-B₄C system with various B₄C particle sizes. The formation mechanism of TiC and TiB₂ in the locally reinforced regions was investigated. The results showed that TiC and TiB₂ are formed and precipitated from Al-Ti-B-C melt resulting from the dissociation of B₄C into Al-Ti melt when the concentrations of B and C atoms in the Al-Ti-B-C melt become saturated. However, in the case of coarse B₄C powders (≥40 μm) used, the primary reaction in the Al-Ti-B-C melt is quite limited due to the poor dissociation of B₄C. The poured steel melt infiltrates into the primary reaction product and thus leads to the formation of Al-Fe-Ti-B-C melt, thanks to the favorable reaction of molten Fe with remnant B₄C, and then TiC and TiB₂ are further formed and precipitated from the saturated Al-Fe-Ti-B-C melt. The relationship between the mechanisms of thermal explosion (TE) synthesis of TiC and TiB₂ in the electric resistance furnace and during casting was proposed.     

Microstructural Analysis of the Reinforced Al‐Cu5mgti/Tib2 5 wt % Alloy for Investment Casting Applications

The paper describes the influence of 5 wt % titanium diboride (TiB₂) particles on the microstructure of an Al‐Cu alloy produced by plaster casting process. The elaboration route leads to a composite material with 1% of in situ TiB₂ particles and 4% ex situ of TiB₂ particles. The comparison of the reinforced alloy with the corresponding non‐reinforced counterpart makes clear that the presence of TiB₂ particles has a large influence in the observed microstructure. The presence of TiB₂ particles decreases the grain sizes and the porosity level. It is also found that TiB₂ particles play an important role in the precipitation events of Al₂Cu precipitates that are formed during solidification at the TiB₂/aluminum matrix interfaces.     

Nano-sized aluminum oxide reinforced commercial casting A356 alloy matrix: Evaluation of hardness,wear resistance and compressive strength focusing on particle distribution in aluminum matrix

Achieving a uniform distribution of reinforcement within the matrix is a challenge which impacts directly on the properties and quality of the composite material. In the present study a fabrication and evaluation approach was used focusing on particle distribution in metal matrix. Al and Cu powders were separately milled with nano-Al₂O₃ particles and incorporated into A356 alloy via vortex method to produce cylindrical A356/nano-Al₂O₃ composites. The stirring was carried out in various durations. The variations of density, hardness, compressive strength, and wear resistance were measured throughout the cylindrical samples. The evaluation of mechanical properties and microstructural studies showed that an increase in stirring time led to a more uniform dispersion of particles in the matrix and also led to a decrease in mechanical properties due to an increase in porosity content of the composites compared with those of the samples stirred for shorter durations. Moreover, milling process affected particle distribution. Nanoparticles more uniformly dispersed in the Al₂O₃–Cu reinforced samples compared with that of the samples reinforced with Al₂O₃–Al or pure alumina powders.     

Effects of Mg content on aging behavior of Al4CuXMg PM alloy

Starting with elemental (pre-mixed) powders for producing shaped powder metallurgy alloys provides some advantages over a pre-alloyed system. The premixed powders are softer than prealloyed powders and therefore by using premixed powders it is possible to have higher compact densities and within a longer die life. In this research work, elemental aluminum powder was mixed with copper and magnesium in various ratios. They were compacted, sintered and heat treated in order to produce light but strong Al-based powder metallurgy alloys. The main focus of this paper is on the effects of micro to macro scale addition of magnesium on the aging response of Al4Cu alloys. Four per cent Cu gives Al powder metallurgy alloy a good control of sintering and a large space for solution treatment. Minor addition of Mg with little amount of Fe, comes from the based Al and Cu powders, enhances the hardness values of Al4Cu powder metallurgy alloys. Highest hardness value was 118 HB obtained from 24 h aged Al4Cu2Mg alloy.     

Preparation and electrical erosion resistance of TiB2/Cu nanocomposites

A procedure is described for producing nanocomposite TiB₂/Cu powders containing up to 57 vol % TiB₂. Using shock compression of composite powders, we have prepared electrode materials offering enhanced electrical erosion resistance at high arc discharge currents. The effect of titanium diboride nanoparticles embedded in the copper matrix on the erosion behavior of the nanocomposites is examined. The nanoparticles are shown to suppress the copper droplet entrainment during the service of the electrode. TiB₂/Cu nanocomposite electrodes containing more than 10 vol % TiB₂ retain their shape and dimensions in the course of electrical erosion tests and offer enhanced service life.     

Solidification and interfacial structure of in situ Al-4.5Cu/TiB2 composite

In situ particle reinforced Al-4.5Cu/TiB₂ composite was fabricated with TiO₂, H₃BO₃, Na₃AlF₆ powders and Al-4.5Cu alloy by reaction in melt. The composite can be directly casted into moulds to make composite parts. TiB₂ particles distribute uniformly in the matrix. The average size of TiB₂ particles is 0.93 m. At the atomic scale, TiB₂ is hexagonal, and exhibits hexagon or quadrilateral shape. The orientation relationships exist in the interfaces between TiB₂ particle and -Al, and between the reinforced small Al₂Cu phase and -Al in the composite. They are . TiB₂ particle is nucleation site for -Al matrix growth in the composite. The interface between TiB₂ particles and the matrix is clean and well bonded. No reaction product has been found through HREM observation. This is beneficial to the strength of the composite. The as-cast Al-4.5Cu/TiB₂ composite exhibits mechanical excellent properties: the tensile strength is 416.7 MPa, the yield strength is 316.9 MPa, and the elongation is 3.3 pct.     

Vacuum diffusion bonding of TiB2 cermet to TiAl based alloys

Abstract

Vacuum diffusion bonding of TiB₂ cermet to TiAl based alloys was carried out at 1123 – 1323 K for 0.6 – 3.6 ks under 80 MPa. The microstructural analyses indicate that a compound Ti(Cu, Al)₂ is formed in the interface of the TiB₂ /TiAl joints, and the width and quantity of the Ti(Cu, Al)₂ compound increase with the increase of the bonding temperature and bonding time. The experimental results show that the shear strength of the diffusion bonded TiB₂ /TiAl joint is 103 MPa, when TiB₂ cermet is bonded to TiAl based alloy at 1223 K for 1.8 ks under 80 MPa.