Surface nanocrystallization of Ni3Al by surface mechanical attrition treatment

In this paper, the possibility of surface nanocrystallization of Ni₃Al intermetallic by surface mechanical attrition treatment was explored. The microstructure and hardness of treated sample were characterized through scanning electronic microscope, transmission electronic microscope, X-ray diffraction and nanoindentation examination. The results showed surface nanocrystallization was realized on Ni₃Al intermetallic by surface mechanical attrition method. The nanocrystalline in top surface was about 10 nm and a deformed layer about 10–30 μm formed. Nanoindentation examination showed the nanohardness increased sharply to a maximum and then decreased in a narrow depth till to a stable level with the depth from treated surface. The maximum nanohardness of surface was near 12 GPa.     

Synthesis and structural characterization of nanocrystalline (Ni,Fe)3Al intermetallic compound prepared by mechanical alloying

Synthesis of (Ni, Fe)₃Al intermetallic compound by mechanical alloying (MA) of Ni, Fe and Al elemental powder mixtures with composition Ni₅₀Fe₂₅Al₂₅ was successfully investigated. The effects of Fe-substitution in Ni₃Al alloy on mechanical alloying process and on the final products were investigated. The structural changes of powder particles during mechanical alloying were studied by X-ray diffractometry, scanning electron microscopy and microhardness measurements. At the early stages, mechanical alloying resulted in a Ni (Al, Fe) solid solution with a layered nanocrystalline structure consisting of cold welded Ni, Al and Fe layers. By continued milling, this structure transformed to the disordered (Ni, Fe)₃Al intermetallic compound which increased the degree of L1₂ ordering upon heating. In comparison to Ni–Al system, Ni (Al, Fe) solid solution formed at longer milling times. Meanwhile, the substitution of Fe in Ni₃Al alloy delayed the formation of Ni (Al, Fe) solid solution and (Ni, Fe)₃Al intermetallic compound. The microhardness for (Ni, Fe)₃Al phase produced after 80 h milling was measured to be about 1170HV which is due to formation of nanocrystalline (Ni, Fe)₃Al intermetallic compound.     

Al3Ni2–Al composites with nanocrystalline intermetallic matrix produced by consolidation of milled powders

Mechanically alloyed nanocrystalline Al₆₃Ni₃₇ powder with a metastable structure of NiAl phase was mixed with 20, 30 and 40 vol.% of Al powder. The powder mixtures as well as pure powder of Al₆₃Ni₃₇ alloy were consolidated at 600 °C under the pressure of 7.7 GPa. The bulk materials were characterised by structural investigations (X-ray diffraction, light and scanning electron microscopy, energy dispersive spectroscopy), compression and hardness tests and measurements of density and open porosity. During the consolidation, the metastable NiAl phase transformed into the equilibrium Al₃Ni₂ intermetallic. The mean crystallite size of the Al₃Ni₂ intermetallic in the bulk materials is below 40 nm. The microstructure of the composite samples consists of Al₃Ni₂ intermetallic areas surrounded by lamellae-like Al regions. The hardness of the produced Al₃Ni₂–Al composites is in the range of 5–6.5 GPa (514–663 HV1), while that of the Al₃Ni₂ intermetallic is 9.18 GPa (936 HV1). The compressive strength of the composites increases with the decrease of Al content, ranging from 567 MPa to 876 MPa. The plastic elongation of the composites was increasing with the increase of Al content, while the Al₃Ni₂ intermetallic failed in the elastic region.     

Impression creep behavior of Al–1.9%Ni–1.6%Mn–1%Mg alloy

In the present study, microstructure and creep behavior of an Al–1.9%Ni–1.6%Mn–1%Mg alloy were studied at temperature ranging from 493 to 513 K and under stresses between 420 and 530 MPa. The creep test was carried out by impression creep technique in which a flat ended cylindrical indenter was impressed on the specimens. The results showed that microstructure of the alloy is composed of primary α(Al) phase covered by a mantle of α(Al)+Ni₃Al intermetallic compound. Mn segregated into Al_xMn_yNi_z or Al₆Mn phases distributed inside the matrix phase. It was found that the stress exponent, n, decreases from 5.2 to 3.6 with increasing temperature. Creep activation energies between 115 kJ/mol and 151 kJ/mol were estimated for the alloy and it decreases with rising stress. According to the stress exponent and creep activation energies, the lattice and pipe diffusion- climb controlled dislocation creep were the dominant creep mechanism.     

Improving the wear resistance of AA 6061 by laser surface alloying with NiTi

To improve the wear resistance of a popular aluminum alloy AA 6061, a 1.5 mm thick hard surface layer consisting of Ni–Al and Ti–Al intermetallic compounds was synthesized on the alloy by laser surface alloying technique. NiTi powder was preplaced on the aluminum alloy substrate and irradiated with a high-power CW Nd:YAG laser in an argon atmosphere. With optimized processing parameters, a modified surface layer free of cracks and pores was formed by reaction synthesis of Al with Ni and Ti. X-ray diffractometry (XRD) confirmed the main phases in the layer to be TiAl₃ and Ni₃Al. The surface hardness increased from below 100 HV for untreated AA 6061 to more than 350 HV for the laser-treated sample. Accompanying the increase in hardness, the wear resistance of the modified layer reached about 5.5 times that of the substrate.     

Evolution of manufacturing parameters in Al/Ni3Al composite powder formation using blending and mechanical milling processes

Aluminu–matrix composites produced by Ni₃Al intermetallic particles are increasingly used in aerospace and structural applications because of their outstanding properties. In manufacturing of metal–matrix composites using powder metallurgy blending and milling are important factors. They control the final distribution of reinforcement particles and porosity in green compacts which in turn, strongly affect the mechanical properties of the produced PM materials. This paper studies different conditions for producing composite powders with uniform dispersion of Ni₃Al particles in aluminum powders and improved physical and mechanical properties. The results indicated that an intermediate milling time for fabrication of composite powder, better than prolonged and shortened ones, causes better microstructure and properties. It was shown that addition of 5 vol.% Ni₃Al particles, produced by 15 h mechanical alloying to aluminum powders, and then 12 h blending operation provides an appropriate condition for producing Al–Ni₃Al composite powder.     

Effect of Fe and Mn additions on microstructure and mechanical properties of spray-deposited Al–20Si–3Cu–1 Mg alloy

Spray deposition is a novel process which is used to manufacture rapidly solidified bulk and near-net-shape preforms. In this paper, Al–20Si–3Cu–1 Mg alloy was prepared by spray deposition technique. The effect of Fe and Mn additions on microstructure and mechanical properties of spray-deposited Al–20Si–3Cu–1 Mg alloy was investigated. The results show that two kinds of intermetallics, i.e. δ-Al₄FeSi₂ and β-Al₅FeSi, is formed in the microstructure of spray-deposited Al–20Si–5Fe–3Cu–1 Mg alloy. With additions of 5% Fe and 3% Mn to Al–20Si–3Cu–1 Mg alloy, the needle shape of Al–Si–Fe intermetallic phases is substituted by the particle shape of Al₁₅(FeMn)₃Si₂ phases. The presence of the intermetallic phases (δ-Al₄FeSi₂, β-Al₅FeSi and Al₁₅(FeMn)₃Si₂) improves the tensile strengths of the alloy efficiently at both the room and elevated temperatures(300 °C).     

Synthesis,microstructure and mechanical properties of Al2O3 reinforced Ni3Al matrix composite

A new method to synthesize alumina reinforced Ni₃Al intermetallic matrix composites has been described. The powder mixture of nickel and aluminium was mechanically alloyed. The powder mixture was excessively heated during mechanical alloying and then exposed to atmosphere for oxidation. The oxidized powder mixture was transformed into alumina reinforced nickel aluminide matrix composite on subsequent pulse current processing. Alumina reinforcements were generated in the nickel aluminide matrix by in situ precipitation. The microstructure of the composite showed that the alumina reinforcements were 50–150 nm in size. The fine alumina reinforcements were homogeneously distributed in the matrix phase. The mechanical properties of the alumina reinforced nickel aluminide matrix composite fairly exceeded the nickel aluminide alloys. This novel synthesis approach allowed the rapid and facile production of high strength alumina reinforced Ni₃Al matrix composites.     

Physical properties of Sn58Bi–xNi lead-free solder and its interfacial reaction with copper substrate

The aims of this research are to investigate the effects of Ni on the physical properties of Sn58Bi–xNi lead-free solder, and to examine its interfacial reaction with the copper substrate. In the experiments, four concentrations of Ni (i.e. 0.05, 0.1, 0.5 and 1.0 wt.%) were individually added into Sn58Bi and their respective microstructure, tensile strength, elongation, melting temperature, wettability and electrical resistivity of Sn58Bi–xNi were subsequently measured. The results indicated that Ni refined the microstructure of the solder matrix and induced the formation of Ni₃Sn₄ intermetallic phase, and that the size and volume fraction of Ni₃Sn₄ were positively correlated to the Ni content. The optimal concentration of Ni to enhance the tensile strength of the alloy was 0.1 wt.%, but the elongation of the alloy was inversely correlated to the Ni content. The addition of Ni contributed positively to the melting temperature and wetting behavior of the alloy, whereas no significant change in the electrical resistivity of Sn58Bi–xNi was detected. In addition, Ni increased the thickness of the intermetallic layer at the interface, and only monoclinic η′-Cu₆Sn₅ phase was present at the intermetallic layer. Nevertheless, the intermetallic phase of this research was dissimilar from the findings of existing literature.     

The effect of Ti addition on alloying and formation of nanocrystalline structure in Fe–Al system

In this work, the effect of Ti addition on alloying and formation of nanocrystalline structure in Fe–Al system was studied by utilizing mechanical alloying (MA) process. Structural and morphological evolutions of powder particles were studied by X-ray diffractometry, microhardness measurements, and scanning electron microscopy. In both Fe₇₅Al₂₅ and Fe₅₀Al₂₅Ti₂₅ systems MA led to the formation of Fe-based solid solution which transformed to the corresponding intermetallic compounds after longer milling times. The results indicated that the Ti addition in Fe–Al system affects the phase transition during mechanical alloying, the final crystallite size, the mean powder particle size, the hardness value and ordering of DO₃ structure after annealing. The crystallite size of Fe₃Al and (Fe,Ti)₃Al phases after 100 h of milling time were 35 and 12 nm, respectively. The Fe₃Al intermetallic compound exhibited the hardness value of 700 Hv which is significantly smaller than 1050 Hv obtained for (Fe,Ti)₃Al intermetallic compound.     

Effect of friction time on mechanical and metallurgical properties of continuous drive friction welded Ti6Al4V/SUS321 joints

Dissimilar joint of Ti6Al4V titanium alloy and SUS321 stainless steel was fabricated by continuous drive friction welding. The effect of friction time on the mechanical properties was evaluated by hardness measurement and tensile test, while the interfacial microstructure and fracture morphologies were analyzed by scanning electron microscope, energy dispersive spectroscope and X-ray Diffraction. The results show that the tensile strength increases with friction time under the experimental conditions. And the maximum average strength 560 MPa, which is 90.3% of the SUS321 base metal, is achieved at a friction time of 4 s. For all samples, studied fracture occurred along the joint interface, where intermetallic compounds like FeTi, Fe₂Ti, Ni₃(Al, Ti) and Fe₃Ti₃O and many other phases were formed among elements from the two base metals. The width of intermetallic compounds zone increases with friction time up to 3 μm, below which it is beneficial to make a strong metallurgical bond. However, the longer friction time leads to oversized flash on the Ti6Al4V side and overgrown intermetallic compounds. Finally the optimized friction time was discussed to be in the range of 2–4 s, under which the sound joint with good reproducibility can be expected.     

Microstructure stability and mechanical properties of an age-hardenable Ni–Mo–Cr alloy subjected to long-term exposure to elevated temperature

This paper characterizes the microstructure and mechanical properties of a nickel-based superalloy with a nominal composition of Ni–25Mo–8Cr (wt.%) after long-term exposures to elevated temperatures. The alloy is strengthened by long-range-ordered precipitates of an oI6 metastable phase with the Ni₂(Mo,Cr) stoichiometry. The alloy was annealed at 650 °C for 1000, 2000 and 4000 h, after it had been plastically deformed in order to accelerate diffusion processes occurring at elevated temperature and consequently to ease the formation of stable phases. The microstructure was characterized using TEM, SEM and X-ray phase analyses; mechanical properties were measured in tensile tests.It has been determined that the alloy loses its phase stability upon plastic deformation and subsequent long-term annealing at 650 °C. The microstructure, composed initially of a dispersed Ni₂(Mo,Cr) strengthening phase in a Ni-based solid solution, decomposes during annealing into a mixture of Ni₃Mo- and Ni₄Mo-type phases, Mo-lean Ni-based solid solution and a complex intermetallic P phase. The dominant new phase is a plate-shaped Ni₃Mo-type phase while the P phase appears as singular small precipitates. The Ni₃Mo phase is formed mainly in regions of highly localized deformation, e.g., in shear bands, and only occasionally nucleates in regions where the deformation was relatively uniform (dislocations or twins in one system). Regions adjacent to the plates of the Ni₃Mo phase are recrystallized and free from an Ni₂(Mo,Cr) strengthening phase. Changes in microstructure of the deformed alloy during long time annealing at 650 °C result in the decrease in the yield strength as well as tensile elongation at both room temperature and 650 °C. A significant decrease in elongation at 650 °C occurs only in specimens tested in air but not those tested in vacuum.     

Interface microstructure of diffusion bonded Ni3Al intermetallic alloy and austenitic stainless steel

The diffusion bonding of a Ni₃Al intermetallic alloy to an austenitic stainless steel has been carried out at temperatures 950, 1000 and 1050 °C. The influence of bonding temperature on the microstructural development and hardness across the joint region has been determined. The microvoids in the interface have been found to decrease with increasing bonding temperature. The intermetallic phase Al₃Ni has been detected at the Ni₃Al side of the diffusion couple. Diffusion of Cr and Fe from the stainless steel to the Ni₃Al alloy has been observed.     

Effect of holding time on microstructure and mechanical properties of Si3N4/Si3N4 joints brazed with Au58.7Ni36.5V4.8 filler alloy

Si₃N₄ ceramics were brazed using Au–Ni–V metal foils at 1423 K for different holding times. Effect of holding time on microstructure and mechanical properties of the joints was investigated. The results indicate that a reaction layer of VN exists at the interface between Si₃N₄ ceramic and filler alloy. With increasing holding time from 0 to 90 min, thickness of the VN reaction layer increases from 0.4 to 2.8 μm, obeying a linear relation. Mechanism of the interfacial reaction was discussed by calculating the formation of free energy of VN. No specific orientation relationship exists between VN reaction layer and Si₃N₄ ceramic. In addition, Ni₃Si intermetallic compound appears in the joint when the holding time increases to 90 min, resulting in the deterioration of the joint strength.     

Effect of heat treatment on the microstructure and property of cold-sprayed nanostructured FeAl/Al2O3 intermetallic composite coating

It is difficult to deposit dense intermetallic compound coatings by cold spraying directly using compound feedstock powders due to their intrinsic low temperature brittleness. A method to prepare intermetallic compound coatings in-situ employing cold spraying was developed using a metastable alloy powder assisted with post heat treatment. In this study, a nanostructured Fe(Al)/Al₂O₃ composite alloy coating was prepared by cold spraying of ball-milled powder. The cold-sprayed Fe(Al)/Al₂O₃ composite alloy coating was evolved in-situ to FeAl/Al₂O₃ intermetallic composite coating through a post heat treatment. The effect of heat treatment on the phase formation, microstructure and microhardness of cold-sprayed Fe(Al)/Al₂O₃ composite coating was investigated. The results showed that annealing at a temperature of 600 °C results in the complete transformation of the Fe(Al) solid solution to a FeAl intermetallic compound. Annealing temperature significantly influenced the microstructure and microhardness of the cold-sprayed FeAl/Al₂O₃ coating. On raising the temperature to over 950 °C, diffusion occurred not only in the coating but also at the interface between the coating and substrate. The microhardness of the FeAl/Al₂O₃ coating was maintained at about 600HV_0.1 at an annealing temperature below 500 °C, and gradually decreased to 400HV_0.1 at 1100 °C.     

Reactive sintering of boron-doped Ni76Al24 intermetallic

A preliminary investigation into the formation of boron-doped nickel-rich Ni₃Al with boron additions up to 2 wt% (i.e. to levels above the equilibrium solid solubility limit of boron in Ni₃Al) from elemental powders by reaction synthesis was carried out. The application of reaction synthesis was seen as a low-energy alternative to the production of Ni₃Al/boride composite suitable for wear applications. X-ray diffraction, Neutron diffraction, SEM/EDS,WDS, Image analysis, Archimedes principle and Rockwell hardness measurements; were used to study the effect of boron addition on the final microstructure, average grain size, bulk density and hardness of as-prepared Ni₇₆Al₂₄. Up to 0.3 wt% boron content, the microstructure consisted of single-phase Ni₃Al, however, at a boron content of 0.5 wt% an apparent transition from a single phase microstructure to a two-phase intermetallic/boride composite microstructure was observed, which dominated when the boron content increased, up to 2 wt%. The two-phase microstructure was identified as Ni₃Al (particles) within an Ni₄₁Al₅B₁₂ boride matrix, with no remaining un-reacted boron. The boron addition was found to increase the Rockwell hardness of Ni₃Al via two mechanisms. Below the solubility limit, the increase in hardness was due to solution hardening. Above 0.5 wt%B, solution hardening in addition to the formation of the harder boride phase, were found to amount to up to 50% increase in the hardness compared with boron free Ni₃Al. The extrusion of semi-molten beads at the surface of the compact at high B-content may be a limiting factor, in the formation of Ni₃Al/boride composites via this route.     

Effect of TiN nanoparticles on the grain size,wear resistance,and strength of the intermetallic compound Ni<Subscript>3</Subscript>Al

We have studied the effect of TiN nanoparticles as crystallization centers for the intermetallic compound Ni₃Al on the grain size of Ni₃Al synthesized under pressure using a stoichiometric elemental powder mixture. The results demonstrate that the addition of 0.3–0.7 wt % TiN nanoparticles to a starting mixture of nickel and aluminum powders reduces the average grain size of the synthesized intermetallic compound and raises its wear resistance and tensile strength in the temperature range from 20 to 1000°C.     

Effects of Al content on structure and mechanical properties of hot-rolled ZrTiAlV alloys

Zirconium alloys show attractive properties for astronautic applications where the most important factors are anti-irradiation, corrosion resistance, anti-oxidant, very good strength-to-weight ratio. The effects of Al content (2.2–6.9 wt%) on structure and mechanical properties of the hot-rolled ZrTiAlV alloy samples were investigated in this study. Each sample of the hot-rolled ZrTiAlV alloys with Al contents from 2.2 wt% to 5.6 wt% is composed of the α phase and β phase, meanwhile, the relative content of the α phase increased with the Al content. However, the (ZrTi)₃Al intermetallic compound was observed as the Al content increased to 6.9 wt%. Changes of phase compositions and structure with Al content distinctly affected mechanical properties of ZrTiAlV alloys. Yield strength of the alloy with 2.2 wt% Al is below 200 MPa. As Al content increased to 5.6 wt%, the yield strength, tensile strength and elongation of the examined alloy are 1088 MPa, 1256 MPa and 8%, respectively. As Al content further increased to 6.9 wt%, a rapid decrease in ductility was observed as soon as the (ZrTi)₃Al intermetallic compound precipitated. Results show that the ZrTiAlV alloys with Al contents between 3.3 wt% and 5.6 wt% have excellent mechanical properties.     

Analysis of process parameters effects on friction stir welding of dissimilar aluminum alloy to advanced high strength steel

Thin sheets of aluminum alloy 6061-T6 and one type of Advanced high strength steel, transformation induced plasticity (TRIP) steel have been successfully butt joined using friction stir welding (FSW) technique. The maximum ultimate tensile strength can reach 85% of the base aluminum alloy. Intermetallic compound (IMC) layer of FeAl or Fe₃Al with thickness of less than 1 μm was formed at the Al–Fe interface in the advancing side, which can actually contribute to the joint strength. Tensile tests and scanning electron microscopy (SEM) results indicate that the weld nugget can be considered as aluminum matrix composite, which is enhanced by dispersed sheared-off steel fragments encompassed by a thin intermetallic layer or simply intermetallic particles. Effects of process parameters on the joint microstructure evolution were analyzed based on mechanical welding force and temperature that have been measured during the welding process.     

Microstructure characteristics and mechanical properties of cold metal transfer welding Mg/Al dissimilar metals

AZ31B Mg alloy and 6061 Al alloy were joined by using cold metal transfer (CMT) welding with pure copper (HS201) as the filler metal. The microstructure of Mg/Al CMT weld joint was studied by means of Optical Microscopy, Scanning Electron Microscope (SEM), Energy Dispersive X-ray (EDX), X-ray Diffraction (XRD). Results showed that dissimilar metals of Mg/Al could be successfully joined by CMT under proper processing parameters. The bonding strength of the joint was 34.7 MPa. A variety of Al–Cu intermetallic compounds, i.e. AlCu, CuAl₂, Cu₉Al₄, presented in the fusion zone of Al side, and Cu based solid solution was generated in weld zone, while Cu₂Mg and Al–Cu–Mg ternary eutectic structure was formed in the fusion zone of Mg side. The micro-hardness in the both sides of fusion zones increased sharply, which were 362 HV in Mg side and 260 HV in Al side. The joint was brittle fractured in the intermetallic compound layer of the fusion zone of Mg side, where plenty of Cu₂Mg intermetallic compounds were distributed continuously.