Development of Al-Ti-C grain refiners containing TiC

Cast Al-Ti-C grain refiners were synthesized by reacting up to 2 pct graphite particles of 20 micron average size with stirred Al-(5 to 10) pct Ti alloy melts, which generated submicron-sized TiC particles within the melts, and their solidified structures showed preferential segregation of the carbide phase in the grain or cell boundary regions and occasional presence of free carbon whose amount exceeded equilibrium values. At the usual melt temperatures of below 1273 K, though, TiC formed first, but was subsequently found to react with the melt forming a sheathing of A1₄C₃ and Ti₃AlC which resulted into poisoning of the TiC particles. However, it was possible to reverse these reactions in order to regain the virgin TiC particles by superheating the melts in the temperature region where TiC particles are thermodynamically stable. Grain refining tests using the TiC master alloys produced fine equiaxed grains of cast aluminum whose sizes were comparable to that obtainable with the standard TiB₂ commercial grain refiner. TiC particles introducedvia the master alloys were found to occur in the grain centers, thereby confirming that they nucleated aluminum crystals. On leave from Regional Research Laboratory (CSIR), Bhopal, is Research Associate.     

Coating of 6028 Aluminum Alloy Using Aluminum Piston Alloy and Al-Si Alloy-Based Nanocomposites Produced by the Addition of Al-Ti5-B1 to the Matrix Melt

The Al-12 pctSi alloy and aluminum-based composites reinforced with TiB₂ and Al₃Ti intermetallics exhibit good wear resistance, strength-to-weight ratio, and strength-to-cost ratio when compared to equivalent other commercial Al alloys, which make them good candidates as coating materials. In this study, structural AA 6028 alloy is used as the base material. Four different coating materials were used. The first one is Al-Si alloy that has Si content near eutectic composition. The second, third, and fourth ones are Al-6 pctSi-based reinforced with TiB₂ and Al₃Ti nano-particles produced by addition of Al-Ti5-B1 master alloy with different weight percentages (1, 2, and 3 pct). The coating treatment was carried out with the aid of GTAW process. The microstructures of the base and coated materials were investigated using optical microscope and scanning electron microscope equipped with EDX analyzer. Microhardness of the base material and the coated layer were evaluated using a microhardness tester. GTAW process results in almost sound coated layer on 6028 aluminum alloy with the used four coating materials. The coating materials of Al-12 pct Si alloy resulted in very fine dendritic Al-Si eutectic structure. The interface between the coated layer and the base metal was very clean. The coated layer was almost free from porosities or other defects. The coating materials of Al-6 pct Si-based mixed with Al-Ti5-B1 master alloy with different percentages (1, 2, and 3 pct), results in coated layer consisted of matrix of fine dendrite eutectic morphology structure inside α-Al grains. Many fine in situ TiAl₃ and TiB₂ intermetallics were precipitated almost at the grain boundary of α-Al grains. The amounts of these precipitates are increased by increasing the addition of Al-Ti5-B1 master alloy. The surface hardness of the 6028 aluminum alloy base metal was improved with the entire four used surface coating materials. The improvement reached to about 85 pct by the first type of coating material (Al-12 pctSi alloy), while it reached to 77, 83, and 89 pct by the coating materials of Al-6 pct Si-based mixed with Al-Ti5-B1 master alloy with different percentages 1, 2, and 3 pct, respectively.     

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.     

Correlation of microstructure with hardness and wear resistance in (TiC,SiC)/stainless steel surface composites fabricated by high-energy electron-beam irradiation

Stainless-steel-based surface composites reinforced with TiC and SiC carbides were fabricated by high-energy electron beam irradiation. Four types of powder/flux mixtures, i.e., TiC, (Ti + C), SiC, and (Ti + SiC) powders with 40 wt. pct of CaF₂ flux, were deposited evenly on an AISI 304 stainless steel substrate, which was then irradiated with an electron beam. TiC agglomerates and pores were found in the surface composite layer fabricated with TiC powders because of insufficient melting of TiC powders. In the composite layer fabricated with Ti and C powders having lower melting points than TiC powders, a number of primary TiC carbides were precipitated while very few TiC agglomerates or pores were formed. This indicated that more effective TiC precipitation was obtained from the melting of Ti and C powders than of TiC powders. A large amount of precipitates such as TiC and Cr₇C₃ improved the hardness, high-temperature hardness, and wear resistance of the surface composite layer two to three times greater than that of the stainless steel substrate. In particular, the surface composite fabricated with SiC powders had the highest volume fraction of Cr₇C₃ distributed along solidification cell boundaries, and thus showed the best hardness, high-temperature hardness, and wear resistance.     

Fabrication of Ni-Ti Alloy by Self-Propagating High-Temperature Synthesis and Spark Plasma Sintering Technique

This work is focused on the possibilities of preparing Ni-Ti46 wt pct alloy by powder metallurgy methods. The self-propagating high-temperature synthesis (SHS) and combination of SHS reaction, milling, and spark plasma sintering consolidation (SPS) are explored. The aim of this work is the development of preparation method with the lowest amount of undesirable phases (mainly Ti₂Ni phase). The SHS with high heating rate (approx. 200 and 300 K min^?1) was applied. Because the SHS product is very porous, it was milled in vibratory disk milling and consolidated by SPS technique at temperatures of 1173 K, 1273 K, and 1373 K (900 °C, 1000 °C, and 1100 °C). The microstructures of samples prepared by SHS reaction and combination of SHS reaction, milling, and SPS consolidation are compared. The changes in microstructure with increasing temperature of SPS consolidation are observed. Mechanical properties are tested by hardness measurement. The way to reduce the amount of Ti₂Ni phase in structure is leaching of powder in 35 pct hydrochloric acid before SPS consolidation.     

The effect of molybdenum addition on properties of iron aluminides

The effect of the addition of up to 10 pct molybdenum on several metallurgical properties of Fe-28Al (at. pct) to which 1 pct TiB₂ was added for grain refinement has been studied. It was determined that the addition of molybdenum results in a decrease in grain size, an increase in the recrystallization temperature, and an increase in the DO₃ to B2 ordering transformation temperature. The solubility limit of molybdenum in the matrix of the base alloy was found to be about 6 pct. At this concentration, another 1 pct is dissolved in the TiB₂ precipitates. Tensile strengths were increased slightly by adding up to 2 pct Mo, but ductility decreased, even though grain sizes were reduced. The fracture mode in tension did not change with addition of molybdenum up to 2 pct.     

Improvement of hardness and wear resistance of (TiC,TiB)/Ti-6Al-4V surface-alloyed materials fabricated by high-energy electron-beam irradiation

The objective of this study is to investigate microstructure, hardness, and wear properties of three kinds of (TiC,TiB)/Ti-6Al-4V surface-alloyed materials fabricated by high-energy electron-beam irradiation. The mixtures of Ti+C, TiC+TiB₂, and Ti+B₄C powders and CaF₂ flux were deposited on a Ti-6A1-4V substrate, and then high-energy electron beam was irradiated on these mixtures. The surface-alloyed layers of 0.9 to 1.6 mm in thickness were homogeneously formed, and contained a large amount (30 to 44 vol. pct) of hard precipitates such as TiC and TiB in the martensitic matrix. This microstructural modification improved the hardness and wear resistance of the surface-alloyed layer 2 times and 6 to 9 times, respectively, greater than that of the substrate. Particularly, the surface-alloyed material fabricated with Ti+B₄C powders had a larger volume fraction of TiB and TiC homogeneously distributed in the martensitic matrix, and thus showed the best hardness and wear resistance. These findings suggested that the surface-alloying using high-energy electron-beam irradiation was economical and useful for the development of titanium-base surface-alloyed materials with improved hardness and wear properties.     

Functionally Graded Al Alloy Matrix <Emphasis Type="Italic">In-Situ</Emphasis> Composites

In the present work, functionally graded (FG) aluminum alloy matrix in-situ composites (FG-AMCs) with TiB₂ and TiC reinforcements were synthesized using the horizontal centrifugal casting process. A commercial Al-Si alloy (A356) and an Al-Cu alloy were used as matrices in the present study. The material parameters (such as matrix and reinforcement type) and process parameters (such as mold temperature, mold speed, and melt stirring) were found to influence the gradient in the FG-AMCs. Detailed microstructural analysis of the composites in different processing conditions revealed that the gradients in the reinforcement modify the microstructure and hardness of the Al alloy. The segregated in-situ formed TiB₂ and TiC particles change the morphology of Si particles during the solidification of Al-Si alloy. A maximum of 20 vol pct of reinforcement at the surface was achieved by this process in the Al-4Cu-TiB₂ system. The stirring of the melt before pouring causes the reinforcement particles to segregate at the periphery of the casting, while in the absence of such stirring, the particles are segregated at the interior of the casting.     

Tribological properties of aluminum alloy matrix TiB2 composite prepared by in situ processing

An investigation of the wear behavior, in lubricated sliding and rolling of in situ prepared TiB₂ particle-reinforced 2024 T4 Al alloy matrix composites against 52100 steel and hardened pearlitic nodular cast iron, respectively, was undertaken. In sliding contact, the 10 vol pct 0.3-μm TiB₂-metal matrix composite (MMC) showed slightly less wear than the 10 vol pct 1.3-μm TiB₂-MMC. Transmission electron microscopy of cross sections, taken normal to the wear track and parallel to the sliding direction, revealed that the TiB₂ particles on the wear track were polished and particle pullout was largely absent. This was attributed to the strong interfacial bonding between the Al-alloy matrix and the TiB₂ reinforcing phase. The TiB₂ particles on the wear track inhibited spalling. Subsurface damage of the MMC did not occur. The wear of the steel mating surfaces worn against the TiB₂-MMCs was minor and caused by the cutting action of the TiB₂ particles that resided on the MMC wear track. In rolling contact, the 0.3-μm-size TiB₂-MMC showed 5 times higher weight loss than the 1.3-μm TiB₂-MMC for the same content of reinforcement, but the weight loss of the cast iron mating surface was less for the former. For the smaller particle size, the wear of 5 and 10 vol pct TiB₂-MMCs was the same. A high density of surface cracks was present on the wear track of the 0.3-μm TiB₂-MMC but not on the 1.3-μm MMC. The significance of strong particle/matrix interfacial bonding and particle size effect on the wear behavior of ceramic particulate-reinforced MMCs in lubricated sliding and rolling wear is discussed.     

Microstructures and Properties of Nanocrystalline NiFe Alloy With and Without Particulate TiC Reinforcement by Mechanical Alloying

In the current study, Ni₅₀Fe₅₀ alloy powders were prepared using a high-energy planetary ball mill. The effects of TiC addition (0, 5, 10, 20, and 30 wt pct) and milling time on the sequence of alloy formation, the microstructure, and microhardness of the product were studied. The structure of solid solution phase, the lattice parameter, lattice strain, and grain size were identified by X-ray diffraction analysis. The correlation between the apparent densities and the milling time is explained by the morphologic evolution of the powder particles occurring during the high-energy milling process. The powder morphology was examined using scanning electron microscopy. It was found that FCC γ (Fe–Ni) solid solution was formed after 10 hours of milling, and this time was reduced to 7 hours when TiC was added. Therefore, brittle particles (TiC) accelerate the milling process by increasing crystal defects leading to a shorter diffusion path. Observations of polished cross section showed uniform distribution of the reinforcement particles. The apparent density increases with the increasing TiC content. It was also found that the higher TiC amount leads to larger lattice parameter, higher internal strain, and lower grain size of the alloy.     

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.     

Microstructure of TiB2/carbon steel surface-alloyed materials fabricated by high-energy electron beam irradiation

The processing and the microstructural analysis of TiB₂/carbon steel surface-alloyed materials using the irradiation of a high-energy electron beam were investigated in this study. The mixtures of TiB₂ powders and flux were deposited on a plain carbon steel substrate, and then electron beam was irradiated on these mixtures using an electron beam accelerator. The microstructure of the irradiated surface layer was composed of a melted region, an interfacial region, a coarse-grained heat-affected zone (HAZ), and a fine-grained HAZ. A few residual micropores were found in the melted region of the specimen processed without flux because of irregular thermal transfer, but their number was decreased in the specimens processed with a considerable amount of flux. As a result of irradiation, the Ti content was homogeneously maintained throughout the melted region, whose hardness was greatly improved. This was associated with the microstructural modification including the segregation of Ti and B along solidification cell boundaries and the formation of fine Ti(C, N) particles. The proper flux mix ratio was 15 to 30 pct to obtain excellent surface alloying and a homogeneous microstructure.     

Discussion of “On the free energy of formation of TiC and Al4C3”

Several years ago, Banerji and Reif^[1] reported some very interesting studies on a process to react Ti and C in molten Al to form particles of TiC. The process was used to prepare a master alloy with a fine dispersion of TiC to inoculate Al for grain refinement. Approximately 2 wt pct of preheated graphite particles were stirred into the Al-5 to 10 pct Ti melts. The authors explained that the melts needed to be superheated above 1000 °C to avoid the undesirable formation of A1₄C₃ and Ti₃AlC at the TiC/melt interface. Their explanation for this phenomenon was based on thermodynamics. They observed that the standard free energy of formation curves for AI4C3 and TiC cross near 1175 °C, with A1₄C₃ having the lower free energy of formation below this temperature. There are several aspects of this work which merit further discussion.     

Effect of phosphorous surface segregation on iron-zinc reaction kinetics during hot-dip galvanizing

Phosphorous was ion implanted on one surface of a large grain (10 to 20 mm) low-carbon steel sheet in order to study the effect of surface segregation on the formation of Fe-Zn phases during galvanizing. Both an Al-free and a 0.20 wt pct Al-Zn bath at 450 °C were used in this investigation. It was found that P surface segregation did not affect the kinetics of Fe-Zn phase growth for the total alloy layer or the individual Fe-Zn gamma, delta, and zeta phase alloy layers in the 0.00 wt pct Al-Zn baths. In the 0.20 wt pct Al-Zn bath, the Fe₂Al₅ inhibition layer formed with kinetics, showing linear growth on both the P-ion implanted and non-P-ion implanted surfaces. Fe-Zn phase growth only occurred after extended reaction times on both surfaces and was found to directly correspond to the location of substrate grain boundary sites. These results indicate that P surface segregation does not affect the growth of Fe-Zn phases or the Fe₂Al₅ inhibition layer. It was shown that in the 0.20 wt pct Al-Zn bath, substrate grain boundaries are the dominant steel substrate structural feature that controls the kinetics of Fe-Zn alloy phase growth.     

Effects of Cr and B Contents on Volume Fraction of (Cr,Fe)2B and Hardness in Fe-Based Alloys Used for Powder Injection Molding

In the current study, Fe-based alloys were used for powder injection molding (PIM) parts with various qualities and hardness ranges by varying chemical compositions according to thermodynamically calculated phase diagrams. Their microstructure and hardness values were analyzed and compared with those of the PIM specimens made from conventional Fe-based alloy powders or stainless steel powders. The Cr-to-B ratio (X _Cr/X _B) and the sum of Fe, Cr, and B content (X _Fe+X _Cr+X _B) were varied to design nine Fe-based alloy compositions based on the composition of Armacor ??M?? alloy powders (Liquidmetal Technologies, Lake Forest, CA). According to the microstructural analysis results of the cast and heat-treated Fe-based alloys, large amounts of (Cr,Fe)₂B were formed in the tempered martensite matrix. The volume fraction of (Cr,Fe)₂B was varied from 42?pct to 91?pct with alloy compositions, and these results were well matched with the thermodynamically calculated volume fractions of (Cr,Fe)₂B. The hardness of the fabricated alloys was varied from 300?VHN to 1600?VHN with alloy compositions, and this value increased linearly with the increasing volume fraction of (Cr,Fe)₂B. From the correlation data between the volume fraction of (Cr,Fe)₂B and hardness, the high-temperature equilibrium phase diagram, which could be used for the design of Fe-based alloys with various fractions and hardness values of (Cr,Fe)₂B, was made.     

Unlubricated fretting wear of TiB2-containing composites against bearing steel

Fretting tests under dry unlubricated conditions (22 °C to 25 °C, 50 to 55 pct RH) were performed on monolithic TiB₂, TiB₂-based cermet with a Ni₃(Al,Ti) binder, sialon-TiB₂, and ZrO₂-TiB₂ composites to assess their relative wear performance against bearing-grade steel. Based on the measured friction and wear data, the relative ranking of the investigated fretting couples is established. The fretting wear of all investigated tribocouples was found to fall in the tribochemical wear regime. The extent of the tribochemical reaction was observed to be strongly dependent on the chemical solubility of TiB₂ and the binder phases in steel at the tribocontacts and was accompanied by tribo-oxidation. For the monolithic TiB₂ and TiB₂-based cermet materials, however, abrasion and adhesive wear, in addition to the tribochemical reactions, are found to be the cause for the high volumetric wear loss of these tribosystems. On the other hand, mild abrasion coupled with reduced tribochemical reactions is observed to play a major role in the low wear loss of the ZrO₂-TiB₂/steel fretting couple. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis of the tribochemical layer formed in the monolithic TiB₂/steel tribocouple revealed the formation of mixed oxides containing predominantly TiO₂ (anatase), B₂O₃, and Fe₂O₃. Based on the thermodynamic calculations and the experimental observations, a tribochemical wear model is proposed to explain the observed tribological behavior of the investigated tribosystems.     

Microstructure evolution in tial alloys with b additions: Conventional solidification

Solidification microstructures of arc-melted, near-equiatomic TiAl alloys containing boron additions are analyzed and compared with those of binary Ti-Al and Ti-B alloys processed in a similar fashion. With the exception of the boride phase, the matrix of the ternary alloy consists of the same α₂ (DO₁₉) and γ (Ll₀) intermetallic phases found in the binary Ti-50 at. pct Al alloy. On the other hand, the boride phase, which is TiB (B27) in the binary Ti-B alloys, changes to TiB₂ (C32) with the addition of Al. The solidification path of the ternary alloys starts with the formation of primary α (A3) for an alloy lean in boron (∼1 at. pct) and with primary TiB₂ for a higher boron concentration (∼5 at. pct). In both cases, the system follows the liquidus surface down to a monovariant line, where both α and TiB₂ are solidified concurrently. In the final stage, the α phase gives way to γ, presumably by a peritectic-type reaction similar to the one in the binary Ti-Al system. Upon cooling, the α dendrites order to α₂ and later decompose to a lath structure consisting of alternating layers of γ and α₂.     

Microstructure,crystallization, and coercivity of rare earth-iron-boron amorphous alloy ribbons

Amorphous alloys of rare earth-iron-boron develop high coercivity when crystallized around 650 °C to 700 °C. These alloys are located in the iron-rich corner of the ternary system, and the alloys contain 10 to 15 at. pct rare earth (R). In the amorphous state, isomorphous substitution of different rare earth atoms occurs. The short-range Fe-Fe and R-Fe environments in amorphous ribbons are similar to those in Fe₃B and R₆Fe₂₃ (within 6 Å), respectively. Beyond 6 Å, the R-Fe environment appears similar to R₂Fe₁₄B. On nonisothermal heating of these alloy ribbons, a stress-relieving process occurs around 460 °C. The crystallization of α-Fe and R₂Fe₁₄B occurs at around 520 °C and near 600 °C, respectively. The Fe₃B and R₆Fe₂₃ phases crystallize next. Depending on the alloy composition, the Fe₃B crystallizes between 600 °C and 650 °C, and R₆Fe₂₃ crystallizes between 650 °C and 680 °C. The presence of rare earth atoms, 10 to 15 at. pct, significantly raises the crystallization temperatures of a-Fe and Fe₃B. The favorable short-range Fe-Fe and R-Fe environments may be responsible for the nucleation and growth (crystallization) of Fe₃B and R₆Fe₂₃ in the ternary alloys. The high coercivity of the annealed ribbons containing 10 at. pct Tb, Dy, and Ho is related to the single magnetic domain nature of the small crystallized grains of Fe₃B and R₆Fe₂₃. For the annealed alloy ribbons with 12 to 15 at. pct rare earth, the high coercivity is related to the ternary hard phase, R₂Fe₁₄B.     

Aging behavior of Fe-30 Ni alloys containing niobium

A study has been made of the precipitation reactions in Fe-30 wt pct alloys containing up to 5 wt pct Nb. The as-quenched structures of these alloys consist, of austenite, martensite in twinned as well as in massive form, and Ni₃Nb and Fe₂Nb precipitates. On aging at 700° and 800°C the main precipitation reaction results in the formation of hexagonal Laves phase Fe₂Nb, but Ni₃Nb in both bct and orthorhombic structures also precipitates. The precipitation of Fe₂Nb is a heterogeneous process and results in a considerable increase in the hardness of the alloy.