Microstructural study and densification analysis of hot work tool steel matrix composites reinforced with TiB2 particles

Hot work tool steels are characterized by good toughness and high hot hardness but are less wear resistant than other tooling materials, such as high speed steel. Metal matrix composites show improved tribological behavior, but not much work has been done in the field of hot work tool steels. In this paper TiB₂-reinforced hot work tool steel matrix composites were produced by spark plasma sintering (SPS). Mechanical alloying (MA) was proposed as a suited process to improve the composite microstructure. Density measurements and microstructure confirmed that MA promotes sintering and produces a fine and homogeneous dispersion of reinforcing particles. X-ray diffraction patterns of the sintered composites highlighted the formation of equilibrium Fe₂B and TiC, as predicted by thermodynamic calculations using Thermo-Calc® software. Scanning electron microscopy as well as scanning Kelvin probe force microscopy highlighted the reaction of the steel matrix with TiB₂ particles, showing the formation of a reaction layer at the TiB₂-steel interface. Phase investigations pointed out that TiB₂ is not chemically stable in steel matrix because of the presence of carbon even during short time SPS.     

Assessment of reactions occurring during the solid state processing of iron-based titanium diboride composites

The reactions occurring during the solid-state processing of Fe-C/TiB₂ composite materials have been assessed. Optical microscopy and X-ray diffraction have been used to identify the products of reaction after sintering and hot isostatic pressing of such materials in the temperature range 1000–1200 °C. TiC has been seen to form readily at the TiB₂/Fe interface: an apparently continuous layer of TiC forms on the surface of the TiB₂, hindering further reaction. Solid state processing appears to be a potentially viable route for the production of iron-based TiB₂ composite materials.     

Fabrication of TiC and TiB<Subscript>2</Subscript> locally reinforced steel matrix composites using a Fe–Ti–B<Subscript>4</Subscript>C–C system by an SHS-casting route

Steel matrix composites locally reinforced by in situ TiC and TiB₂ particulates were successfully fabricated using self-propagating high-temperature synthesis (SHS) in a Fe–Ti–B₄C–C system during casting. The locally reinforced steel matrix composites consist of three distinct regions: (i) a TiC and TiB₂ particulate-reinforced region, (ii) a transition region, and (iii) a steel matrix region. The TiC and TiB₂ particulates in the locally reinforced regions display a relatively uniform distribution, and their sizes decrease with the increase in Fe content from 10 wt.% to 40 wt.%. The wear resistance of the locally reinforced region of the steel matrix composites is much higher than that of the unreinforced steel matrix.     

Microstructure and mechanical behavior of AlSiCuMgNi piston alloys reinforced with TiB2 particles

AlSiCuMgNi piston composites reinforced with in-situ TiB₂ particles were fabricated by mixing salts reaction process successfully. Microstructures of the composites were observed by mean of scanning electron microscope (SEM) and transmission electron microscope (TEM). X-ray diffraction (XRD) was used to identify the phases in the composites. TiB₂ reinforcement grows in equiaxed or near equiaxed shape and the interfaces between reinforcements and matrix are clear. Compared with the matrix alloys, the composites show an obvious aging peak and an incubation time in the hardness. The aging is accelerated in the composites reinforced with TiB₂. At room temperature, the ultimate tensile strength (UTS) of the composites increases as the percentage of TiB₂ reinforcement increases. When the temperature is beyond 250°C, the ultimate tensile strength of the piston composites decreases sharply. The fracture surfaces of the piston composites are analyzed.     

Laser cladding composite coatings by Ni–Cr–Ti–B4C with different process parameters

To investigate the effect of laser process parameters on microstructure and properties of composite coating, the composite coatings were manufactured by laser cladding Ni–Cr–Ti–B₄C mixed powder on Q235 mild steel with different process parameters. The coatings are bonded with the substrate by remarkable metallurgical binding without cracks and pores. The composite coatings are consisted of in situ synthesized solid solution Ni–Cr–Fe, intermetallic compound (IMC) Ni₃Ti, Cr₂Ti, and ceramic reinforcements TiB₂, TiC. Results of scanning electron microscopy (SEM) revealed that the ceramic reinforcements became coarser with higher specific energy (E_s). There were independent ceramics TiB₂, TiC, eutectic ceramic TiB₂–TiC in coatings, and eutectic alloy–ceramic was detected. Compared with the substrate, the microhardness of coatings was increased significantly, and the maximum microhardness of coatings was approximately five times as high as the substrate. The wear resistance of coatings was improved dramatically than the substrate. Compared to the coatings with lower E_s, higher E_s led to lower microhardness and worse wear resistance ascribing to more Fe diffused into the coating from the substrate.     

In situ reacted TiB2-reinforced alumina

In situ formation of TiB₂ in Al₂O₃ matrix through the reaction of TiO₂, boron and carbon has been studied. In hot-pressed samples, in addition to TiB₂, TiC and Al₂TiO₅ were also found to be dispersed phases in Al₂O₃ matrix. However, in the case of pressureless-sintered samples, pure Al₂O₃/TiB₂ composite with > 99% relative density can be obtained through a preheating step held at 1300°C for longer than 30 min and then sintering at a temperature above 1500°C. Pressureless-sintered composite containing 20vol% TiB₂ gives a flexural strength of 580 MPa and a fracture toughness of 7.2 MPa m^1/2.     

Impact behavior of in situ TiB2/Al composite at various temperatures

The impact behavior of the in situ TiB₂/Al composite was investigated at temperatures varying from −50 to 200 °C. The effects of the reinforcement, heat treatment as well as temperature on the impact toughness and failure mechanism were discussed. Results showed that the impact toughness of the composite decreases significantly due to the presence of the stiff TiB₂ reinforcements. The precipitations caused by aging play the same role as TiB₂ reinforcements, which constrain the deformation of the matrix and reduce the impact toughness. The TiB₂/Al composite is more endurable in suffering the impact load at subzero and high temperatures compared to that at room temperature. The fractography of the TiB₂/Al composite is a cleavage-and-dimple morphology. The eutectic silicon is the preferred site for catastrophic cracking. There is no cracking in the in situ TiB₂ reinforcement because of the small size and near spherical shape. However, the “pulled-out” failure occurs for the TiB₂ reinforcement, which is due to the relative weaker interfacial strength than the strength of reinforcement.     

Microstructure and wear properties of Fe–TiC surface composite coating by laser cladding

AISI 1045 steel surface was alloyed with pre-placed ferrotitanium and graphite powders by using a 5-kW CO₂ laser. In situ TiC particles reinforced Fe-based surface composite coating was fabricated. The microstructure and wear properties were investigated by means of scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, as well as dry sliding wear test. The results showed that TiC carbides with cubic or flower-like dendritic form were synthesized via in situ reaction between ferrotitanium and graphite in the molten pool during laser cladding process. The TiC carbides were distributed uniformly in the composite coating. The TiC/matrix interface was found to be free from cracks and deleterious phase. The coatings reinforced by TiC particles revealed higher wear resistance than that of the substrate.     

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.     

Effect of different reinforcements on composite-strengthening in aluminium

In the development of metal-matrix composites, reinforcements of aluminium and its alloys with ceramic materials has been pursued with keen interest for quite sometime now. However, a systematic comparison of the effect of different reinforcements in powder-processed aluminium and its alloys is not freely available in the published literature. This study examines the influence of SiC, TiC, TiB₂ and B₄C on the modulus and strength of pure aluminium. B₄C appears slightly superior as a reinforcement when comparing the effect of SiC, TiC, B₄C and TiB₂ on specific modulus and specific strength values of composites. However, TiC appears to be a more effective reinforcement, yielding the best modulus and strength values among those considered in this study. The differences in thermal expansion characteristics between aluminium and the reinforcements do not seem to explain this observation. The other advantage of TiC is that it is economically a more viable candidate as compared to B₄C and TiB₂ for reinforcing aluminium alloys. It is suggested that the superior effect of TiC as a reinforcement is probably related to the high integrity of the bond at the Al-TiC interface.     

Influence of matrix alloying elements on reactive synthesis of 2124 aluminium alloy metal matrix composites

Abstract

In situ TiB₂ particle reinforced Al alloys are produced by reactive synthesis from elemental and prealloyed powders. The influence of 2124 alloying elements on the reactive synthesis is evaluated with a comparison of elemental AI, elemental AI-Cu mixture, and 2124 Al prealloyed powders as matrix materials. Experimental investigations by differential scanning calorimetry and dilatometry showed that the presence of Cu leads to an increase in the reaction rate during the formation of intermediate reaction products in comparison with the elemental Al matrix. X-ray diffraction of the reaction products showed a more complete conversion of the intermediate Al₃ Ti as a result of Cu addition. The Cu has no influence on the TiB₂ particle size, but the TiB₂ morphology changed from pure hexagonal to a more rounded morphology.     

Development and characterisation of SiAlON composites with TiB2, TiN, TiC and TiCN

30 vol% of TiB₂, TiCN, TiN or TiC was added to a sialon matrix with an X-phase sialon (Si₁₂Al₁₈O₃₉N₈) and an Al₂O₃–Si₃N₄ (77/23 wt%) starting powder composition and hot pressed at 1650°C in vacuum. The microstructures of the obtained composites were characterised by means of X-ray diffraction and electron microscopy, and the mechanical properties; E-modulus, hardness, bending strength and fracture toughness were measured and evaluated.Fully dense composites with an X-phase sialon or a polyphase Al₂O₃–-sialon–X-sialon matrix with 30 vol% of TiB₂, TiN and TiCN were obtained. TiC, added as a dispersed phase, however reacts with the nitrogen from the Si₃N₄ during liquid phase sintering, with the formation of TiC_1–x N _x , SiC and a changed sialon matrix composition. In the case of the X-phase sialon starting composition, a mullite matrix is obtained after sintering. The microstructural observations with respect to the sialon-TiC composites are found to be in agreement with the thermodynamic calculations.     

Fabrication of TiC/TiB/Ti3AlC2 phases reinforced coatings on Ti-6Al-4V substrate

Intermetallic matrix composite coatings reinforced by TiC, TiB₂, and Ti₃AlC₂ were fabricated by laser cladding the mixed power Ti, Al, and B₄C on the Ti-6Al-4V alloy. X-ray diffraction, scanning electron microscope, and energy dispersive spectroscopy were chosen to investigate the structures and morphologies of the coatings. Results showed that the coatings mainly consisted of the reinforcements of TiC, TiB₂, and Ti₃AlC₂ and the matrix of Ti₃Al, TiAl, TiAl₃, and α-Ti. The hardness and wear-resisting property of the prepared specimens of Ti-45Al-10B₄C and Ti-45Al-20B₄C were studied contrastively. It was found that the coating was metallurgical bonded to the Ti-6Al-4V substrate. The micro-hardness and dry sliding wear-resisting properties of the specimen of Ti-45Al-20B₄C were enhanced further. And the micro-hardness of Ti-45Al-20B₄C was from 900 HV_0.2 to 1225 HV_0.2. The wear-resisting property of Ti-45Al-20B₄C was four times as large as that of the Ti-6Al-4V alloys.     

In situ reacted TiB2-reinforced mullite

In situ formation of TiB₂ in mullite matrix through the reaction of TiO₂, boron and carbon has been studied. In hot-pressed and pressureless-sintered samples, in addition to TiB₂, TiC was also found to be dispersed phases in mullite matrix. However, in the case of pressurelesssintered samples, mullite/TiB₂ composite with 98% relative density can be obtained through a preheating step held at 1300 °C for longer than 3 h and then sintering at a temperature above 1600 °C. Hot-pressed composite containing 30 vol% TiB₂ gives a flexural strength of 427 MPa and a fracture toughness of 4.3 MPam^1/2. Pressureless-sintered composite containing 20 vol% TiB₂ gives a flexural strength of 384 MPa and a fracture toughness of 3.87 MPam^1/2.     

An experimental study on synthesizing TiC-TiB2-Ni composite coating using electro-thermal explosion ultra-high speed spraying method

The synthesis of TiC-TiB₂-Ni composite coating on steel substrate using electro-thermal explosion ultra-high speed spraying (EEUSS) method has been investigated. The coating exhibits a compact microstructure and good interface bonding. TiC-TiB₂-Ni coating consists of TiC, TiB₂, Ni, as well as the residual C. Microstructure characterization reveals micro-sized clubbed TiB₂ particles with the length of 5-10 µm and submicron-sized spherical TiC particles with the average diameter of 1 µm and these different morphologies are due to their respective crystal structure. The average hardness value for TiC-10TiB₂-Ni and TiC-20TiB₂-Ni are HV0.3 1800 and HV0.3 2000.     

Microstructure and tensile properties of in situ synthesized (TiBw + TiCp)/Ti6242 composites

In the present work, (TiB_w+ TiC_p)/Ti6242 composites were fabricated via common casting and hot-forging technology utilizing the SHS reaction between titanium and B₄C. The XRD technique was used to identify the phases of composites. The microstructures were characterized by means of OM and TEM. Results from DSC and analysis of phase diagram determine solidification paths of in situsynthesized Ti6242 composites as following stages: -Ti primary phase, monovariant binary eutectic -Ti + TiB, invariant ternary eutectic -Ti + TiB + TiC and phase transformation from -Ti to -Ti. In situsynthesized reinforcements are distributed uniformly in titanium matrix alloy. Reinforcement TiB grows in whisker shape whereas TiC grows in globular or near-globular shape. TiB whiskers were made to align the hot-forging direction after hot-forging. The interfaces between reinforcements and Ti matrix alloy are very clean. There is no any interfacial reaction. Moreover, the mechanical properties improved with the addition of TiB whiskers and TiC particles although some reduction in ductility was observed. Fractographic analysis indicated that the composites failed in tension due to reinforcements cracking. The improvements in the composite properties were rationalized using simple micromechanics principles. The strengthening mechanisms are attributed to the following factors: undertaking load of TiB whiskers and TiC particles, high-density dislocations and refinement of titanium matrix alloy's grain size.     

Synthesis and characterization of in situ formed titanium diboride particulate reinforced AA7075 aluminum alloy cast composites

In situ fabrication of aluminum matrix composites (AMCs) has gathered widespread attention of researchers due to inherent advantages over ex situ methods. Aluminum alloy AA7075 reinforced with various amounts (0, 3, 6 and 9 wt.%) of TiB₂ particles were prepared by the in situ reaction of inorganic salts such as K₂TiF₆ and KBF₄ to molten aluminum. X-ray diffraction patterns of the prepared AMCs clearly revealed the formation of TiB₂ particles without the presence of any other intermetallic compounds. The microstructures of the AMCs were studied using optical and scanning electron microscopy. The in situ formed TiB₂ particles were characterized with uniform distribution, clear interface, good bonding and various shapes such as cubic, spherical and hexagonal. The formation of TiB₂ particles enhanced the microhardness and ultimate tensile strength (UTS) of the AMCs.     

SiC-TiC and SiC-TiB2 composites densified by liquid-phase sintering

Particle-reinforced SiC composites with the addition of TiC or TiB₂ were fabricated at 1850 °C by hot-pressing. Densification was accomplished by utilizing a liquid phase formed with added Al₂O₃, Y₂O₃, and surface SiO₂ on SiC. Their mechanical and electrical properties were measured as a function of TiC or TiB₂ content. Adding TiC or TiB₂ to the SiC matrix increased the toughness, and decreased the strength and electrical resistivity. The fracture toughnesses of SiC-50 wt% TiC and SiC-50 wt% TiB₂ composites were approximately 60% and 50%, respectively, higher than that of monolithic SiC ceramics. Microstructural analysis showed that the toughening was due to crack deflection, with some possible contribution from microcracking in the vicinity of TiC or TiB₂ particles.     

Effect of in situ reaction conditions on the microstructure changes of Cu-TiB2 alloys by combining in situ reaction and rapid solidification

A new short flow technique combining in situ reaction and rapid solidification has been developed and used to prepare Cu-TiB₂ (0.45, 1.6 and 2.5 wt% TiB₂) alloys. The effects of in situ reaction conditions, cooling rate and solute concentration on the microstructure change of Cu-TiB₂ alloys were systematically investigated and analyzed by modeling. It is shown that the size and distribution of TiB₂ particles are strongly dependent on the choice of reactor shape, in situ reaction conditions and solute concentration, specifically, the size and aggregation level of TiB₂ particles tend to increase as increasing normal volume percent of TiB₂ particles when the same in situ reaction condition is used. Some different in situ reaction mechanisms, based on the microstructure change and TiB₂ particle distribution under different conditions, were also established and analyzed, which can be used to quantitatively predict the size of melt micelles needed for synthesizing uniformly distributed TiB₂ particles with different sizes in the copper matrix.     

Thermite Synthesis of TiC/FeNiCr Cermet with Double-Layer Structure

TiC/FeNiCr cermet with TiC particles as hard phases and FeNiCr alloy as binder phase was in situ synthesized by thermite reactions under high gravity. A double-layer structure was obtained, including an upper layer enriched with TiC particles and an under layer with few TiC particles. Between the two layers, no interfacial line, pores, or defaults existed. A large amount of needle-like Cr₇C₃ phases were homogeneously dispersed in the FeNiCr binder phase as multiple reinforcements. A braiding structure was formed between the precipitated NiAl phases and the matrix, where the two phases kept a coherent or semi-coherent relationship. The hardness and wear resistance were evaluated, and the upper layer possessed high hardness and excellent wear resistance.