FeAl and Mo–Si–B intermetallic coatings prepared by thermal spraying

FeAl and Mo–Si–B intermetallic coatings for elevated temperature environmental resistance were prepared using high-velocity oxy-fuel (HVOF) and air plasma spray (APS) techniques. For both coating types, the effect of coating parameters (spray particle velocity and temperature) on the microstructure and physical properties of the coatings was assessed. Fe–24Al (wt%) coatings were prepared using HVOF thermal spraying at spray particle velocities varying from 540 to 700 m/s. Mo–13.4Si–2.6B coatings were prepared using APS at particle velocities of 180 and 350 m/s. Residual stresses in the HVOF FeAl coatings were compressive, while stresses in the APS Mo–Si–B coatings were tensile. In both cases, residual stresses became more compressive with increasing spray particle velocity due to increased peening imparted by the spray particles. The hardness and elastic moduli of FeAl coatings also increased with increasing particle velocity. For Mo–Si–B coatings, plasma spraying at 180 m/s resulted in significant oxidation of the spray particles and conversion of the T1 phase into amorphous silica and -Mo. The T1 phase was retained after spraying at 350 m/s.     

The effects of ceria on the mechanical properties and thermal shock resistance of thermal sprayed NiAl intermetallic coatings

The effects of ceria addition to thermal sprayed NiAl intermetallic coatings were investigated through micro-indentation, thermal shock testing and microstructural analysis techniques (scanning electron microscopy with energy-dispersive X-ray spectrometry and X-ray diffraction analysis). It has been found that the addition of CeO₂ to NiAl coatings reduces the tendency of brittle peeling during thermal spraying. This reduction in peeling is presumably due to the improved wetting of the substrate by the molten coating material, which leads to better coating adhesion. The addition of CeO₂ resulted in higher coating hardness and elastic modulus. The coatings containing CeO₂ also exhibited significant increases in thermal shock resistance compared with that of the pure NiAl coating. The NiAl coating containing 2 wt.%CeO₂ had the highest hardness, elastic modulus and thermal shock resistance of the four NiAl-based coatings tested. The possible mechanisms responsible for the improvement of the properties upon addition of CeO₂ are addressed.     

Effect of directional solidification and heat treatments on the microstructure and mechanical properties of multiphase intermetallic Zr-doped Ni–Al–Cr–Ta–Mo alloy

The effect of directional solidification and heat treatments on the microstructure and mechanical properties of intermetallic Ni–21.29Al–7.04Cr–1.46Ta–0.64Mo–0.57Zr (at.%) alloy was studied. Increasing growth rate is found to decrease primary dendrite arm spacing and to increase volume fraction of β(NiAl)-based dendrites and low melting point γ′(Ni₃Al)/Ni₅Zr eutectic. Room-temperature tensile yield strength and ultimate tensile strength increase and plastic elongation to fracture decreases with the increasing growth rate. Two types of heat treatments of directionally solidified (DS) specimens including two-step ageing at temperatures of 1273 and 1123 K and two-step solution annealing at 1373 and 1493 K were performed. Ageing at 1273 and 1123 K decreases volume fractions of the dendrites and eutectic regions and leads to a coarsening of spherical -Cr and needle-like γ′ precipitates within the β-phase. Annealing at 1373 K for 100 h is shown to be sufficiently long to completely dissolve the eutectic regions. Compressive yield strength increases with increasing temperature reaching a peak value at about 1023 K and then decreases at higher temperatures. Minimum creep rate is found to depend strongly on the applied stress and temperature according to a power law. The power law stress exponent n is determined to be 5.1 and apparent activation for creep Q_a is measured to be 326 kJ/mol.     

Energetic intermetallic materials formed by cold spray

Cold spray was used to synthesize three intermetallic energetic materials from powders composed of mixed Ni/Al, mixed Ni/Al/MoO₃, and Ni-clad Al. After bulk samples were produced, characterization was performed to determine their composition and reactivity. Ignition was achieved with a 30 W CO₂ laser and propagation rates were measured using a high speed digital video camera. Differential scanning calorimetry was used to obtain energetic properties of the composites at slow heating rates. The energetic properties of cold sprayed samples were compared with lower density axially pressed pellets of identical composition. Samples composed of a mechanical blend of Ni and Al powders had higher reaction propagation rates at lower densities; with the near fully-dense cold spray samples having the lowest propagation rates. The opposite was found in samples composed of Ni-clad Al powders, with propagation rate increasing with increasing density for pellets, and reaching a maximum in the cold sprayed samples. The samples containing MoO₃ had mixed results, with pellets experiencing higher propagation rates for all MoO₃ contents' with increasing density, but cold sprayed samples having lower propagation rates as the MoO₃ content increased.     

Chemical and thermodynamic properties of several Al–Ni–R systems

The standard enthalpies of formation at 300 K of the RNiAl phases (R=rare earth) have been obtained by using a high temperature direct reaction drop calorimeter and an aneroid isoperibol calorimeter. State and composition of the samples were checked by X-ray diffraction analysis. Metallographic examination was performed and the phases were further identified by electron microscopy and electron probe microanalysis. The results obtained are discussed and compared with those available for the binary RNi₂ and RAl₂ compounds.     

Strong bonding of infrared brazed α2-Ti3Al and Ti–6Al–4V using Ti–Cu–Ni fillers

Infrared dissimilar brazing of α₂-Ti₃Al and Ti–6Al–4V using Ti–15Cu–25Ni and Ti–15Cu–15Ni filler metals has been performed in this study. The brazed joint consists primarily of Ti-rich and Ti₂Ni phases, and there is no interfacial phase among the braze alloy, α₂-Ti₃Al and Ti–6Al–4V substrates. The existence of the Ti₂Ni intermetallic compound is detrimental to the bonding strength of the joint. The amount of Ti₂Ni decreases with increasing brazing temperature and/or time due to the depletion of Ni content from the braze alloy into the Ti–6Al–4V substrate during brazing. The shear strength of the brazed joint free of the blocky Ti₂Ni phase is comparable with that of the α₂-Ti₃Al substrate, and strong bonding can thus be obtained.     

Microstructure and chemical characterization of the intermetallic phases in Cu/(Sn,Ni) diffusion couples with various Ni additions

The paper presents new results concerning the influence of nickel addition (1 and 5 at.%) into tin on the development of the Cu/(Sn,Ni) interface area in diffusion couple experiment. The morphology and chemical composition of the intermetallic phases growing in the Cu/(Sn,Ni) diffusion couples were examined by means of the scanning (SEM) and transmission (TEM) electron microscopy after annealing at 215 °C in vacuum for different period time.It was shown that even 1 at.% of nickel addition into tin resulted in formation of intermetallics of complex microstructure. The presence of (Cu_1−xNi_x)₆Sn₅ in two morphological and compositional variants was noted. The discontinuous layer consisting up to 7.2 at.% of Ni closer to copper end-member coexisted with needle-like and faceted precipitates with even 22.3 at.% of Ni, which intensively detached from the interface. At the Cu/(Cu_1−xNi_x)₆Sn₅ interface the formation of Cu₃Sn wavy layer compound was observed in all examined diffusion couples which became thicker with time. The porosity within the both formed intermetallic phases existed irrespective of the amount of added nickel.     

Rapid solidification effect on the Ni–25Al–xFe intermetallics

The melt-spun Ni–25Al–xFe ribbons were used to investigate the rapidly solidified effect on hardness and microstructures. The results revealed that the phases existing in these ribbons were the same as the ingot-casting specimens, but the interdendritic regions were suppressed by rapid solidification, and the volume fraction of γ phase decreased. In addition, the rapidly solidified technique was found to increase the hardness significantly. The maximum increment of 250 DPH (from HV400 to HV650) was found in the Ni–25Al–30Fe because of the effects of the suppression of the second phase formation and the size refinement.     

The strengthening effect of (Ni,Fe)Al precipitates on the mechanical properties at high temperatures of ferritic Fe–Al–Ni–Cr alloys

Strengthening through a homogeneous distribution of a second phase is a concept that is widely employed in high-temperature materials. The most prominent among this group are nickel-based superalloys which owe their high-temperature strength to finely dispersed Ni₃Al particles. Similar microstructures can be obtained in the Fe–Al–Ni–Cr system with B2-ordered (Ni,Fe)Al precipitates in a ferritic matrix. These precipitates lead to an increase of high-temperature strength compared to conventional iron-base high-temperature alloys. However, secondary precipitates form during air cooling from high temperatures and affect the ductility. The results show that the ductility can be improved by a two-step aging treatment. Within the stress and temperature range investigated, the dependence of the secondary creep rate on the applied stress of aged alloys can be described by a power law if a threshold stress is introduced.     

Beneficial effects of O-phase on the hydrogen absorption of Ti–Al–Nb alloys

The beneficial effects of O-phase on hydrogen absorption/desorption were demonstrated in three Ti–Al–Nb alloys with compositions in the vicinity of Ti₂NbAl. The alloys were first quenched from high temperature and then aged for a certain period of time at a lower temperature to get O-phase precipitation in the matrix before pressure–composition (P–C) isotherm measurements. All three alloys absorb hydrogen to β hydride at a very low equilibrium pressure. Further absorption to γ hydride is difficult for the quenched O-phase free specimens. β->γ hydride transformation occurs in all the aged specimens and the reversible absorption/desorption between β and γ hydrides is observed in some aged specimens with relatively high volume fraction of O-phase in the matrix. The hydrogen absorption/desorption abilities of these alloys become poor when the Nb content is decreased. The determined formation heat for β->γ hydride transformation is in the range of −30–45 kJ/mol H₂, which is larger than that of binary Ti₃Al. It is found that the value of formation heat becomes more negative with the increasing amount of O-phase and is relatively independent of the interface between O-phase and matrix. This indicates the beneficial effect is related to the O-phase structure itself.     

Characterization of silicide phases formed during pack siliconizing coating on the Nb-1Zr-0.1C alloy

The present paper describes the morphology, chemistry and crystallography of the phases observed in the silicide coatings produced by pack cementation technique on Nb based alloys. Cross-sectional microstructures examined by transmission electron microscopy and scanning electron microscopy techniques have shown that the coating has two silicide layers: NbSi₂ and Nb₅Si₃. NbSi₂ formed at the surface of the sample and Nb₅Si₃ formed in between the substrate (Nb alloy) and NbSi₂ coating layer. Electron diffraction analyses revealed that NbSi₂ has hexagonal crystal structure with lattice parameters as a = 0.48 nm and c = 0.66 nm and Nb₅Si₃ has tetragonal crystal structure with lattice parameters as a = 0.65 nm and c = 1.19 nm. Nb₅Si₃ showed fine equiaxed grains, whereas, NbSi₂ exhibited duplex morphology having columnar grain morphology near to the Nb₅Si₃ layer and large equiaxed grains at the surface of the coating sample. The presence of duplex morphology was explained by estimating diffusion of various species and it was shown that columnar morphology of grains could be attributed to outward diffusion of Nb and equiaxed grains to inward diffusion of Si. In the case of Nb₅Si₃, growth takes place due to single element Si diffusion, leading to development of single equiaxed grain morphology of the Nb₅Si₃ phase.     

Deformation behavior of an ordered B2 phase in Ti–25Al–25Zr alloy

The mechanical behavior of the B2 phase in alloy Ti–25Al–25Zr has been studied under compression. True stress–strain curve exhibits similar behavior to those of typical B2 intermetallics such as NiAl and FeAl. The alloy exhibits highest yield strength in comparison to those reported in other titanium based B2 alloys with around 2% plastic strain. The microstructural characterization of specimen after compression reveals that the B2 phase transforms to an orthorhombic martensitic phase during compression.     

Si-modified aluminide coatings deposited on Ti46Al7Nb alloy by slurry method

Ti46Al7Nb alloy has been used as the research substrate material for the deposition of water-based slurries containing Al and Si powders. The diffusion treatment has been carried out at 950 °C for 4 h in Ar atmosphere. The structure of the silicon-modified aluminide coatings 40 μm thick is as follows: (a) an outer zone consisting of TiAl₃ phase and titanium silicides formed on the matrix grain boundaries composed of TiAl₃–type Ti₅Si₃; (b) a middle zone containing the same phase components with the matrix TiAl₃ and the silicides Ti₅Si₃, which formed columnar grains; (c) an inner zone, 2 μm thick, consisting of TiAl₂ phase. Cyclic oxidation tests were conducted in 30 cycles (690 h at high temperature) and showed a remarkably higher oxidation resistance of the Ti46Al7Nb alloy with the protective coating in comparison with the uncoated sample.     

Influence of non-reactive particles on the microstructure of NiAl and NiAl–ZrO2 processed by thermal explosion

Nickel aluminide intermetallic alloy and NiAl–ZrO₂ composites were synthesized in a hot press by sintering reaction and thermal explosion (or Self-propagating High temperature Synthesis, SHS). The addition of a post-SHS heat treatment allows a control of the microstructure and an enhancement of the mechanical characteristics. Thus, NiAl properties processed by self-combustion are above those obtained by reactive sintering (RS). For all these syntheses, the role played by the non-reactive particles is determining. Indeed, the product granulometry is a function of the diluent size distribution since this latter acts as nucleation sites during the reactive processes. These particles can also enhance mechanical properties by specific reinforcement mechanisms and exercise an influence on SHS reaction parameters by controlling its reactivity and the thermal exchanges during self-combustion.     

Microstructural changes and estimated strengthening contributions in a gamma alloy Ti–45Al–5Nb pack-rolled sheet

Thin sheets made of a gamma-titanium aluminide alloy, Ti–45Al–5Nb, produced by a pack-rolling process, were evaluated for microstructure variation and evolution taking place during aging and annealing treatments. The as-received sheet material was characterized by remarkably high yield strength, 810 MPa, and a complex bimodal microstructure. The microstructure consisted of a matrix of twinned gamma-phase grains and fine-lath lamellar grain microconstituent, together with a dispersed ultra-fine-grained gamma + alpha-2 mixture microconstituent. High-temperature isothermal aging treatments changed the microstructure to a stable mixture of gamma-phase grains (matrix) and coarse alpha-2-phase particles, having size distributions and volume fractions that were specific to the aging temperature. A concurrent strength loss reflects this trend and results in a stable strength level of 550 MPa upon aging at 1000 °C for 144 h. Using composition estimates from the phase-boundary shifts that occur from the Nb addition to a Ti–45Al base alloy and, the rule of mixtures, an analysis was made to show that the gamma-phase matrix has an intrinsic strength of 178 MPa. This is a significant intrinsic strength level, well over that of 70 MPa for the Ti–45Al binary alloy. This is rationalized as the solid-solution strengthening effect from shifts of the Ti and Nb levels in the gamma phase and, by an added effect due to increased oxygen solubility in the gamma phase. The overall strength of Ti–45Al–5Nb, however, is roughly the same as that of Ti–45Al, and this is explained by a drastic reduction in the volume fraction of alpha-2-phase in Ti–45Al–5Nb alloy, which is a result of the Nb-induced phase-boundary shifts.     

Features of fracture surface in a fully lamellar TiAl-base alloy

The deformation mechanisms associated with different fracture surface appearances of a fatigue tested lamellar TiAl-based alloy have been studied in detail by focussed ion beam and transmission electron microscopy. The results show that linear markings within translamellar plates correspond to twins and/or slip bands. The markings in interlamellar region are associated with the crack propagation from a lamellar boundary to another. The fan-like region with linear markings belongs to a γ grain and the markings are related to twins. Intralamellar crack propagation is associated with twin–twin interaction within a γ lamella.     

In-situ alloying of Sn–3.5Ag solder during reflow through Zn nanoparticle addition and its effects on interfacial intermetallic layers

Driven by the necessity to improve the reliability of lead free electronic products and by the trend towards miniaturization, researchers are putting intense efforts to improve the properties of Sn based solders. The present work investigates the effects of Zn nanoparticle addition to Sn-3.5Ag (SA) alloy through paste mixing on the interfacial structure between solder and copper substrate during reflow. Results show that the addition of Zn nanoparticles does not alter the morphology of the interfacial intermetallic compounds although they substantially suppress their growth. Zn nanoparticles are seen to be most efficient compared with Co and Ni nanoparticles in suppressing the growth of Cu₃Sn layers. It is suggested that Zn nanoparticles exert their influence through an in-situ dissolution and alloying effect.     

Direct synthesis of Nb–Al intermetallic nanoparticles by sodiothermic homogeneous reduction in molten salts

Nb–Al intermetallic nanoparticles were directly synthesized via sodiothermic reduction process in molten salts using NbCl₅ and AlCl₃ as the raw materials. The as-prepared samples were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The NbCl₅ and AlCl₃ were dissolved in LiCl–KCl–CaCl₂ or LiCl–KCl–NaCl–CaCl₂ molten salts forming a homogeneous system. It was found that a series of intermetallic nanoparticles, such as Nb₃Al, Nb₂Al, NbAl₃ and Nb₂Al/NbAl₃, were successfully synthesized at low temperature of 350–500 °C using the homogeneous molten salt systems. The phase transformations of Nb₃Al, Nb₂Al and NbAl₃, were achieved via the controllable variation of molar ratio of Nb to Al. Furthermore, the influence of the reaction temperature on the particle size of the intermetallic nanoparticles was also investigated.     

Formation of interfacial voids in cast and micro-grained γ′-Ni3Al during high temperature oxidation

Specimens of cast and micro-grained γ′-Ni₃Al, which were produced with vacuum casting and unbalanced magnetron sputter deposition, respectively, were isothermally oxidised in air at 1473 K for different periods of time. The formation of interfacial voids at the alloy/oxide interface was observed with SEM, which indicated that there were more interfacial voids formed in the cast Ni₃Al than in the micro-grained alloy under the same oxidation conditions. A phenomenological equation describing the fraction of the void projected areas was established, in which the impingement and coalescence between voids during their growth was taken into consideration. It was elucidated that low vacancy density in the micro-grained Ni₃Al due to the high creep, re-crystallisation and the enhanced Al diffusion reduced the void percentage. Also, it was confirmed that aluminium evaporation, perhaps supplemented by surface diffusion, supplied most Al to the oxide scales formed above the interfacial voids.