Abnormal growth of TiB<Subscript>2</Subscript> crystals during sintering of TiB<Subscript>2</Subscript>-Ni<Subscript>3</Subscript>(Al,Ti) cemented borides

TiB₂-Ni₃(Al,Ti) cermets present both normal and abnormal growth of faceted titanium diboride (TiB₂) grains during liquid-phase sintering. Abnormal grain growth (AGG) is preferentially found at high sintering temperatures in specimens processed from powder mixtures with a wide particle size distribution. The WC additions to the initial powder mixtures have proved efficient in reducing the number and size of these large TiB₂ grains. However, the sinterability of these materials is dramatically reduced, which suggests that TiB₂ AGG control is obtained by decreasing TiB₂ dissolution kinetics in the liquid phase. On the other hand, an alternative method based on intensive powder milling not only reduces TiB₂ AGG but also the porosity levels obtained by previous powder processing routes. TiB₂ cermets produced by aggressive milling present a higher amount of alumina particles in the matrix after sintering, which, in addition, appear more homogeneously dispersed in the microstructure. The distortion produced by these particles on the facets of TiB₂ growing grains suggests a possible dragging effect responsible for the AGG reduction found in these cermets. Moreover, aggressive milling removes large TiB₂ particles from the powder mixtures, which could act as seeds for TiB₂ uncontrolled growth. TiB₂-Ni₃(Al,Ti) cermets obtained by intensive milling combine hardness over 20 GPa with K _IC of about 10 MPa √m, data clearly out of the range covered so far by other TiB₂-based materials.     

Hipping after sintering of titanium diboride cermets

Abstract

A new fabrication route, an alternative to glass encapsulated hipping (GEHIP), has been developed to produce dense TiB₂ cermets. Key points of this technique, based on hipping after vacuum sintering (VS + HIP), are the use of Ni₃ (Al,Ti) as binder phase and the selection of the proper amount of additions. The main advantage of VS + HIP with respect to GEHIP is the simplification of the sintering procedure which avoids the glass encapsulation step that makes it more adaptable for industrial use. Successful application of VS + HIP requires a minimum binder content about 10 vol.-% below which a significant hardness reduction is observed owing to the presence of residual porosity as compared with GEHIP. The materials produced by this technique combine low density and high stiffness with high hardness and toughness values, thus giving a set of properties especially attractive for applications where inertial loads are responsible for failure.     

Effects of TiB2/TiCx ratios on compression properties and abrasive wear resistance of in situ 50 vol.-% (TiB2–TiCx)/Al–Cu composites

Effects of TiB₂/TiC_x ratios on compression properties and abrasive wear resistance of 50?vol.-% (TiB₂–TiC_x)/Al–Cu composites fabricated via the combustion synthesis (CS) method assisted with hot press from the Al–Ti–B₄C system were studied. With decrease in the TiB₂/TiC_x ratio from 3:1 to 1:3, the shape of TiB₂ changed from platelike to clubbed while the size of TiC_x particles increased, which directly led to different mechanical properties. The 50?vol.-% (TiB₂–TiC_x)/Al–Cu composite with the TiB₂/TiC_x ratio of 2:1 exhibited good compression strength without sacrificing the fracture strain, while the composite possessed the highest abrasive wear resistance when the TiB₂/TiC_x ratio was 3:1. The appropriate TiB₂/TiC_x ratio was recommended to be 2:1 in this research.     

Synthesis and characterisation of Ti6Al4V/xTa alloy processed by solid state sintering

ABSTRACT

This work investigates the effect of Ta particle addition into a Ti6Al4V alloy processed by solid state sintering. The volume fraction of Ta ranged between 0 and 30?vol.-%. The sintering kinetics of powder mixes are evaluated by dilatometry. Sintered materials are characterised by SEM and XRD, and their mechanical properties are obtained from microhardness and compression tests. Sintering behaviour and final microstructure are affected by Ta particles, which slow down the densification, lower the temperature of α-to-β phase transition and stabilise the β phase. Mechanical properties, as microhardness, Young’s modulus and yield stress, depend on the microstructure reached after sintering and on the residual porosity. An equation expressing the Young’s modulus of Ti6Al4V/xTa alloy as function of x and porosity is proposed and validated. The materials with at least 20?vol.-% of Ta exhibited a high strength to modulus ratio, which is suitable for orthopaedic implants.     

Microstructural characterisation and mechanical properties of spark plasma-sintered TiB2-reinforced titanium matrix composite

Titanium and titanium matrix composites, reinforced with TiB₂ particles, have been synthesised by the spark plasma sintering method at 1050°C under 50?MPa pressure, using mixtures of 2.4?wt.-% TiB₂ and 97.6?wt.-% Ti powders. The changes in microstructural features and mechanical properties were investigated. XRD results and SEM observations confirm the formation of TiB whiskers as a result of the reaction between Ti and TiB₂. However, some unreacted TiB₂ particles have remained in the composite owing to the incomplete chemical reaction between matrix and additives. The measured mechanical properties demonstrate that the increase in hardness and tensile strength with TiB₂ addition is mainly attributed to the generation of TiB whiskers, increase in relative density and decrease in grain size, while the reduction in bending strength is possibly due to the plastic restraint imposed on the matrix by the TiB whiskers and unreacted TiB₂ particles.     

Grain refinement in aluminum alloyed with titanium and boron

The aluminum corner of the ternary Al-B-Ti diagram was explored. A eutectic: Liq — Al + TiAl₃ + (Al, Ti)B₂ was found at approximately 0.05 wt pct Ti, 0.01 wt pct B; 659.5‡C. TiB₂ and A1B₂ form a continuous series of solid solutions, but no distinct ternary phase was found. The addition of boron to aluminum-titanium alloys expands the field of primary crystallization of TiAl₃ toward lower titanium contents and steepens the liquidus. In equilibrium conditions, pronounced grain refinement is found only in alloys in which TiAl₃ is primary and nucleates the aluminum solid solution before any other impurity can act. The peritectic reaction facilitates this priority but it is not necessary for grain refinement. Because of the low diffusivity of titanium and boron in aluminum, equilibrium is seldom attained and in commercial practice grain refinement by TiAl₃ is found also outside its equilibrium field of primary crystallization.     

Microstructural analysis of multilayered titanium aluminide sheets fabricated by hot rolling and heat treatment

This study is concerned with the microstructural analysis of multilayered or bulk Ti aluminide sheets fabricated by the self-propagating high-temperature synthesis (SHS) reaction using hot rolling and heat treatment. Multilayered Ti/Al sheets were prepared by stacking thin Ti and Al sheets alternately, and a good Ti/Al interfacial bonding was achieved after rolling at 500 °C. When these sheets were held at 1000 °C, spheroidal TiAl₃ phases were formed by the SHS reaction at Ti/Al interfaces and inside Al layers. Microstructural analysis on the hot-rolled, multilayered Ti/TiAl₃ sheets revealed that intermetallic phases such as TiAl₂, TiAl, and Ti₃Al were formed at Ti/TiAl₃ interfaces due to interaction between Ti and TiAl₃ and that pores formed in the TiAl₃ layer were significantly reduced during hot rolling. When multilayered Ti/Ti aluminide sheets were heat treated at 1000 °C, Ti₃Al, TiAl, and TiAl₂ were grown as Ti and TiAl₃ were consumed. As the heat treatment proceeded, TiAl grew further, eventually leading to the fabrication of multilayered sheets composed of Ti₃Al and TiAl. Bulk Ti aluminide sheets, having a lamellar structure of Ti₃Al and TiAl, instead of multilayered sheets, were also fabricated successfully by heat treatment at 1400 °C. This fabrication method of the bulk sheets had several advantages over the method by hot forging or rolling of conventional cast Ti aluminides. From these findings, an idea to fabricate multilayered or bulk Ti aluminide sheets by hot rolling and heat treatment is suggested as an economical and continuous fabrication method, and the formation and growth mechanisms of interfacial phases are elucidated in this study.     

The effect of gravity on the combustion synthesis of metal-ceramic composites

The effects of gravity on the combustion characteristics and microstructure of metal-ceramic composites (HfB₂/Al and Ni₃Ti/TiB₂ systems) were studied under both normal and low gravity conditions. Under normal gravity conditions, pellets were ignited in three orientations relative to the gravity vector. Low gravity combustion synthesis (SHS) was carried out on a DC-9 aircraft at the NASA-Lewis Research Center. It was found that under normal gravity conditions, both the combustion temperature and wave velocity were highest when the pellet was ignited from the bottom orientation; i.e., the wave propagation direction was directly opposed to the gravitational force. The SHS of 70 vol pct Al (in the Al-HfB₂ system) was changed from unstable, slow, and incomplete when ignited from the top to unstable, faster, and complete combustion when ignited from the bottom. The hydrostatic force (height × density × gravity) in the liquid aluminum was thought to be the cause of formation of aluminum nodules at the surface of the pellet. The aluminum nodules that were observed on the surface of the pellet when reacted under normal gravity were totally absent for reactions conducted under low gravity. Buoyancy of the TiB₂ particles and sedimentation of the Ni₃Ti phase were observed for the Ni₃Ti/TiB₂ system. The possibility of liquid convective flow at the combustion front was also discussed. Under low gravity conditions, both the combustion temperature and wave velocity were lower than those under normal gravity. The distribution of the ceramic phase, i.e., TiB₂ or HfB₂, in the intermetallic (Ni₃Ti) or reactive (Al) matrix was more uniform.     

On sintering of Ti–Ni (–TiB2) alloys to near full density

Abstract

Using a combination of mixed elemental powders and TiB₂, a series of Ti–Ni and Ti–Ni–B alloys were optimised for sintering by varying the nickel and boron contents, the particle size of the elemental powders and the compaction pressure. The sintering temperature was maintained at 1200°C to limit the costs of a potential commercial sintering operation. For Ti–Ni alloys, a density of 99% was attained in Ti–7Ni made using fine Ti and Ni powders sintered in the solid state, and from liquid phase sintering of Ti–8Ni made using coarser powders. Porosity was almost eliminated from Ti–7Ni–xB alloys made by adding 1–3%TiB₂ to the coarser Ti and Ni powders. The action of TiB₂ as a sintering aid is possibly owing to a combination of the formation of a small amount of liquid at the sintering temperature and the restriction of grain growth owing to the presence of TiB particles.     

Co-deformation processing and modeling of <Emphasis Type="Italic">In-Situ</Emphasis> multiphase composites

A series of in-situ, deformation-processed metal matrix composites were produced by direct powder extrusion of blended constituents. The resulting composites are comprised of a metallic Ti-6Al-4V matrix containing dispersed and co-deformed discontinuously reinforced-intermetallic matrix composite (DR-IMC) reinforcements. The DR-IMCs are comprised of discontinuous TiB₂ particulate within a titanium trialuminide or near-γ Ti-47Al matrix. Thus, an example of a resulting composite would be Ti-6Al-4V+40 vol pct (Al₃Ti+30 vol pct TiB₂) or Ti-6Al-4V+40 vol pct (Ti-47Al+40 vol pct TiB₂), with the DR-IMCs having an aligned, high aspect ratio morphology as a consequence of deformation processing. The degree to which both constituents deform during extrusion has been examined using systematic variations in the percentage of TiB₂ within the DR-IMC, and by varying the percentage of DR-IMC within the metal matrix. In the former instance, variation of the TiB₂ percentage effects variations in relative flow behavior; while in the latter, varying the percentage of DR-IMC within the metallic matrix effects changes in strain distribution among components. The results indicate that successful co-deformation processing can occur within certain ranges of relative flow stress; however, the extent of commensurate flow will be limited by the constituents’ inherent capacity to plastically deform.     

Reciprocating wear response of Ti(C,N)–Ni3Al cermets

Abstract

Ti(C,N) based cermets have received significant attention due to their enhanced mechanical properties at elevated temperatures, low mass and increased chemical stability, when compared to WC–Co hardmetals. These properties have been found to be superior in many cases to traditional hardmetal replacements, such as TiC–Ni. The current study focuses on Ti(C,N)–Ni₃Al cermets formed through melt infiltration (with binder contents ranging from 20 to 40 vol.-%Ni₃Al). Comparison is made with TiC–Ni₃Al cermets using an identical binder alloy. The reciprocating ball on flat wear properties of the cermets were evaluated as a function of applied load (using a WC–Co counterface), together with the composite hardness and indentation fracture resistance. It is shown that nitrogen content negatively affects infiltration, resulting in non-infiltrated areas within low binder content cermets, which decreases the indentation fracture resistance and hardness. This problem can be largely mitigated by increasing binder content. When comparing fully infiltrated cermets, increasing nitrogen content decreases hardness and increases toughness, while all Ti(C,N) cermets outperform TiC (at 40 vol.-% binder). Reciprocating wear increased with increasing load, and typically was the most severe for the lowest binder contents. A combination of wear mechanisms were apparent, including both abrasive and adhesive wear, with the formation of an oxide tribolayer containing components from both the tested cermets and the WC–Co counterface material.

Les cermets à base de Ti(C,N) ont reçu une attention importante grâce à leurs propriétés mécaniques améliorées aux températures élevées, à leur faible masse et à leur stabilité chimique augmentée, lorsque comparés aux carbures métalliques de WC–Co. Dans plusieurs cas, on a trouvé que ces propriétés étaient supérieures à celles des remplacements traditionnels de carbures métalliques, comme le TiC–Ni. La présente étude se concentre sur les cermets de Ti(C,N) –Ni₃Al formés par infiltration du bain (la teneur du liant variant de 20 à 40% en volume de Ni₃Al). On les compare aux cermets de TiC–Ni₃Al en utilisant un alliage de liaison identique. On a évalué les propriétés d’usure par déplacement alternatif de bille sur disque des cermets en fonction de la charge appliquée (utilisant un antagoniste en WC–Co), ainsi que de la dureté du composite et de la résistance à la fracture d’indentation. On montre que la teneur en azote affecte négativement l’infiltration, ayant pour résultat des régions non infiltrées à l’intérieur des cermets à faible teneur en liant, ce qui diminue la résistance à la fracture d’indentation et la dureté. On peut largement atténuer ce problème en augmentant la teneur en liant. Lorsque l’on compare des cermets entièrement infiltrés, l’augmentation de la teneur en azote diminue la dureté et augmente la ténacité, alors que tous les cermets de Ti(C,N) surpassent le TiC (à 40% en volume de liant). L’usure par déplacement alternatif augmentait avec une augmentation de la charge, et était typiquement plus sévère pour les plus faibles teneurs en liant. Une combinaison de mécanismes d’usure était apparente, incluant tant l’usure par abrasion que l’usure d’adhérence, avec formation d’une couche tribologique oxyde contenant des composantes tant des cermets évalués que du matériau antagoniste en WC–Co.     

Microstructure of Al-Ti-B-Er refiner and its grain refining performance

Al-Ti-B-Er refiner was successfully prepared by CR (contact reaction process), a process based on SHS (self propagating high-temperature synthesis). The microstructure of the alloy was studied by optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy-dispersive spectrometry. The results showed that Al-Ti-B-Er alloy was composed of a-Al, block-like TiAl₃ and flocked TiB₂. Compared with Al-Ti-B refiner, formation of TiAlEr compounds, Er modified the morphology of TiAl₃ phase, and dispersed the TiB₂ and TiAl₃. An excellent grain refining performance was obtained when adding 1 wt.% Al-Ti-B-Er in Al-10Zn-1.9Mg-1.6Cu-0.12Zr alloy, the average grain size was about 40 µm. The refinement mechanism of Al-Ti-B-Er was also discussed. Er changed the morphology of TiAl₃, TiB₂ phase, the refiner would be more efficient. The decomposition of TiAlEr compounds which released Er refrained the growth of TiAl₃ and made TiB₂ difficult to aggregate or deposit, therefore resulted in more particles being efficient nucleation substrate.     

Fabrication and characterisation of bulk Al2O3/Mo nanocomposite by mechanical milling and sintering

Abstract

In this study, fabrication and mechanical properties of alumina based ceramic matrix nanocomposite reinforced with 15 and 26·6 vol.-%Mo particles were investigated. Alumina–molybdenum nanocomposite powders were prepared by ball milling of Al and MoO₃ in an SPEX8000 type ball mill. The powder particles were consolidated by cold uniaxial pressing followed by sintering in vacuum atmosphere at 1300 and 1400°C. The structural evaluation of as milled and sintered samples was studied by X-ray diffraction, differential scanning calorimetry and scanning electron microscopy. Sintered samples were examined by hardness measurements and three-point flexural strength. Results show a significant improvement in flexural strength of Al₂O₃–Mo nanocomposites in comparison to monolithic alumina and increases by Mo content. During sintering, grain growth and α-Al₂O₃ to γ-Al₂O₃ transformation occurred. In addition, an increase in temperature of sintering resulted in higher density and hardness of consolidated nanocomposites.     

Wear behaviour of Fe3 Al intermetallic particle reinforced PM based iron metal matrix composites

Abstract

The wear behaviour of unreinforced and reinforced PM based iron metal matrix composite, the latter containing 10 and 20 vol.-% nano sized Fe₃Al intermetallic particles, was studied as a function of sliding distance under two different loads and dry lubricated conditions. The intermetallic Fe₃Al nanoparticles were prepared by mechanical alloying and used as particle reinforcement with 10 and 20 vol.-% in the matrix. The processing of the composites included mixing and cold compaction followed by sintering at 1120°C. The influence of Fe₃Al additions on the dry sliding wear behaviour was studied at loads 20 and 40 N over sliding distances 2160, 3240, 4320 and 6480 m. The study showed that the composite exhibited a lower wear rate than that of the unreinforced matrix and the wear rate was influenced by the volume percentage of Fe₃Al particles. It is understood that iron aluminide reinforcement has a beneficial effect on the wear properties. Delamination and microcutting were the chief mechanisms of wear for the composites.     

Microstructural and Phase Composition Differences Across the Interfaces in Al/Ti/Al Explosively Welded Clads

The microstructure and phase composition of Al/Ti/Al interfaces with respect to their localization were investigated. An aluminum-flyer plate exhibited finer grains located close to the upper interface than those present within the aluminum-base plate. The same tendency, but with a higher number of twins, was observed for titanium. Good quality bonding with a wavy shape and four intermetallic phases, namely, TiAl₃, TiAl, TiAl₂, and Ti₃Al, was only obtained at the interface closer to the explosive material. The other interface was planar with three intermetallic compounds, excluding the metastable TiAl₂ phase. As a result of a 100-hour annealing at 903 K (630 °C), an Al/TiAl₃/Ti/TiAl₃/Al sandwich was manufactured, formed with single crystalline Al layers. A substantial difference between the intermetallic layer thicknesses was measured, with 235.3 and 167.4 µm obtained for the layers corresponding to the upper and lower interfaces, respectively. An examination by transmission electron microscopy of a thin foil taken from the interface area after a 1-hour annealing at 825 K (552 °C) showed a mixture of randomly located TiAl₃ grains within the aluminum. Finally, the hardness results were correlated with the microstructural changes across the samples.

     

Development of TiB<Subscript>2</Subscript> reinforced in-situ Ti aluminide matrix composite through reaction synthesis

TiB₂ reinforced in-situ titanium aluminide matrix composite was made through reaction synthesis process using high purity elemental powders of Ti, Al, Cr, Nb and B. XRD of the synthesized block showed presence of mainly Al₃Ti and TiB₂ phases. To obtain γ Ti aluminide based matrix, the material was homogenized in two phase region (α₂+γ). Presence of γ phase matrix alongwith α₂ was confirmed through XRD, SEM and TEM. Uniform distribution of TiB₂ phase was confirmed through elemental mapping and by analyzing specimens of different locations. Differential scanning calorimetry of powder mixture showed presence of endothermic peak for Al melting and exothermic peak of Ti aluminide and TiB₂ formation.     

Fabrication of in situ TiB2–TiC reinforced steel matrix composites by spark plasma sintering

Abstract

In situ TiB₂ and TiC particulates reinforced steel matrix composites have been fabricated using cheap ferrotitanium and boron carbide powders by spark plasma sintering (SPS) technique. The sintering behaviour and the formation mechanism of the composite were studied. The results show that when the composite was sintered at 1050°C for 5 min, the maximum relative density and hardness of the composite are 99·2% and 83·8 HRA respectively. The phase evolution of the composite during sintering indicates that the TiB₂ and TiC reinforcements were formed in situ as follows: first, the solid/solid interface reaction between Fe₂Ti and B₄C, resulting in the formation of a small amount of TiB₂ and TiC below 950°C; second, the solid–liquid solution precipitation reaction in the Fe–Ti–B–C system, resulting in the formation of the main TiB₂ and TiC reinforcements at ～1000°C.     

Influence of Nb and Cu Elements on the Intermetallic Particles Morphology in Ti/Al Multilayer Composite Processed Through Hot Pressing and Rolling

Ti–Al–Nb composites were produced by solid state diffusion bonding through hot pressing and rolling followed by annealing at 700 °C for 0.5, 1, 1.5 and 2 h. The morphologies of TiAl₃ intermetallics were investigated by Scanning Electron Microscopy combined with Energy-dispersive X-ray spectroscopy. Titanium tri-aluminide (TiAl₃) particles with blocky morphology were dispersed into Aluminum matrix. In the presence of niobium and copper, TiAl₃ particles were produced in different sizes and morphologies. The presence of Nb in the composite led to the formation of irregular angular morphology, while the copper resulted in cubic morphology of the intermetallic particles. The EDS results indicated that TiAl₃, (Ti, Nb)Al₃ and (Ti, Nb, Cu)Al₃ intermetallic compounds appeared near Ti zone, Nb Zone and in the presence of Cu, respectively.     

Effect of C/Ti ratio on the compressive properties and wear properties of the 50 vol.-% submicron-sized TiCx/2014Al composites fabricated by combustion synthesis and hot press consolidation

In this study, in situ 50?vol.-% TiC_x/2014Al composites with different C/Ti molar ratios (0.6, 0.7, 0.8, 0.9 and 1) were successfully produced by the method of combustion synthesis and hot press consolidation. The microstructure and the mechanical properties of the composites were investigated. Microstructure characterisation of the TiC_x/2014Al composites showed relatively uniform distribution of the TiC_x particles with the particle size in the range of 200–900?nm. With the increase of the C/Ti molar ratio, the yield strength (σ_0.2) and the ultimate compression strength (σ_UCS) increased first then decreased, and the fracture strain (ε_f) increased. The σ_0.2, σ_UCS and the abrasive wear resistance of the 50?vol.-% TiC_x/2014Al composites reached the highest value when the value of the C/Ti molar ratio comes to 0.8. The σ_0.2, σ_UCS and ε_f of the 50?vol.-% TiC_x/2014Al composites with the C/Ti molar ratio of 0.8 are 1094?MPa, 1454 and 6.13%, respectively.     

Stiffness variation of porous titanium developed using space holder method

Abstract

The excellent properties of Ti have resulted in its generalised use for bone implants. However, Ti is very stiff in comparison with human cortical bone, and this creates problems of bone weakening and loosening of the implant. This article discusses the mechanical properties (flexural and compressive strength, and stiffness) of porous Ti–6Al–4V specimens developed using the space holder method. These properties are examined relative to the production process parameters: compacting pressure and sintering time, as well as temperature, and the addition of spacer and its particle size. It is seen that when spacer is added, compressive strength decreases with the application of compacting pressure and that these are the most influential parameters. The developed pieces show a closed and unconnected porosity. Small additions of spacer (25 vol.-%) reduce stiffness to around half of that shown by the solid material, and the resulting pieces are strong enough to be used as bone substitute.