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.     

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.     

The difference in the types of intermetallic compound formed between the cathode and anode of an Sn–Ag–Cu solder joint under current stressing

An investigation of microstructural evolution with various current densities in a lead-free Cu/SnAgCu/Au/Cu solder system was conducted in this study. Current stressing induced migration of Cu toward the anode and resulted in the formation of Cu₆Sn₅ at the interface. The consumption rates of Cu were calculated to be 2.24 × 10⁻⁷ μm/s and 5.17 × 10⁻⁷ μm/s at 1.0 × 10³ A/cm² and 2.0 × 10³ A/cm², respectively, while the growth rates of Cu₆Sn₅ were 6.33 × 10⁻⁷ μm/s and 7.72 × 10⁻⁷ μm/s. The atomic fluxes of Cu were found to be 2.50 × 10¹² atom/cm² s and 5.88 × 10¹² atom/cm² s at the above-mentioned current densities. The diffusivities of Cu in Cu₆Sn₅ were 2.02 × 10⁻¹¹ cm²/s and 2.38 × 10⁻¹¹ cm²/s under 1.0 × 10³ A/cm² and 2.0 × 10³ A/cm² of current stressing. Current stressing effectively enhances the migration of Cu in Cu₆Sn₅ and results in a 1000-fold increase of magnitude in diffusivity compared to thermal aging. (Cu_1−x,Au_x)₆Sn₅ compound was formed near the anode after a long period of current stressing.     

Crystallisation of oxygen-stabilised amorphous phase in a Zr50Cu50 alloy

The crystallisation of the oxygen-stabilised amorphous phase in a Zr₅₀Cu₅₀ alloy has been investigated by means of neutron diffraction and electron microscopy. The crystallisation microstructure consists of ZrO₂, Zr₂Cu and Zr₇Cu₁₀. A two-stage crystallisation mechanism is suggested: (i) primary crystallisation of Zr₂Cu and (ii) formation of nanocrystals ZrO₂ and Zr₇Cu₁₀. In (i), it is proposed, Zr₂Cu crystallises from the oxygen-stabilised amorphous phase, leaving an oxygen- and copper-enriched matrix ; Zr₂Cu rapidly grows and eventually attains a grain size of about 100 nm. In (ii), it is suggested, the residual amorphous matrix crystallises into nanocrystals ZrO₂ and Zr₇Cu₁₀ due to the sluggish growth of ZrO₂ and to the already formed ZrO₂ which acts as a growth barrier to Zr₇Cu₁₀. In this case there is no particular orientation relationship between Zr₂Cu and Zr₇Cu₁₀.     

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.     

Irradiation behaviour of α2 and γ phases in He ion implanted titanium aluminide alloy

A Ti–45Al–2Nb–2Mn + 0.8 vol.% TiB₂ (at.%) alloy with fully lamellar microstructure consisting of hexagonal-close-packed (hcp) α₂ and face-centred-tetragonal (fct) γ phases was irradiated by implanting helium ions to different fluences. Microstructural examination showed that helium cavities are formed in both the α₂ and γ phases after He-ion irradiation. However, the helium cavities and their size change with fluence are much larger in the α₂ phase than those in the γ phase, indicating that the γ phase exhibits better tolerance to the He-ion irradiation than the α₂ phase. Since α₂ and γ phases have different crystal structures, they possess differences in helium solubility and interstitial migration. These differences are responsible for the variation in radiation damage behaviour between the two phases.     

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.     

Effect of Mg24Y5 intermetallic particles on grain refinement of Mg-9Li alloy

The microstructures of the as-cast and as-extruded Mg-9Li-xY alloys (x = 0, 0.3; wt%) were observed to investigate the effect of Y on the Mg-9Li alloy, and the crystallographic calculations between Mg₂₄Y₅ and the matrix were examined on the basis of the edge-to-edge matching model. The results indicated that with the addition of 0.3 wt% Y, the average grain size of α-Mg phases in the as-cast Mg-9Li alloy and β-Li phases in the as-extruded Mg-9Li alloy were reduced remarkably, which was caused by the formation of Mg₂₄Y₅ intermetallic compound. Furthermore, crystallographic calculations confirmed that Mg₂₄Y₅ particles were effective grain refiners for both α-Mg and β-Li phases in Mg-9Li alloy.     

Grain boundary diffusion and precipitates in B2 Ti−50.2 at.% Ni alloy

Grain boundary diffusion of ⁴⁴Ti and ⁶³Ni in the B2 Ti−50.2 at.% Ni polycrystalline alloy was measured in Harrison's B regime (573−923 K) using the radiotracer technique. The triple product P = sδD_gb (s is the segregation factor, δ the grain boundary width, and D_gb the corresponding grain diffusion coefficient) for Ti and Ni was determined. Although the absolute values of the triple products P are typical for the B2-ordered alloys, both Ti and Ni GB diffusion in NiTi reveals a unique behavior with significant deviations from a linear Arrhenius-type temperature dependence. Transmission electron microscopy analysis of GB structures at 673 K and 923 K substantiated the occurrence of different interface types which may provide the slower and faster grain boundary diffusion paths in agreement with the experimental data. The influence of different types of precipitates on grain boundary diffusion and possible diffusion mechanisms in the different regimes are discussed.     

Influence of Sc addition on microstructure and transformation behaviour of Ni24.7Ti50.3Pd25.0 high temperature shape memory alloy

Vacuum-arc melted Ni_24.7Ti_50.3Pd_25.0 and Ni_24.7Ti_49.3Pd_25.0Sc_1.0 (at.%) alloys were investigated to study effect of Sc micro-addition on microstructure and transformation behaviour of NiTiPd alloy. Study showed that microstructure of homogenized NiTiPd alloy consisted of NiTiPd matrix interspersed with Ti₂(Ni,Pd) precipitates. In contrast, NiTiPdSc alloy showed a single phase NiTiPdSc matrix with a few scandium oxide particles at isolated places. TEM and X-ray diffraction studies confirmed matrix phase of the alloys to be of orthorhombic B19 structure. TEM observations showed that NiTiPdSc alloy had relatively larger martensite plates with a smaller twin ratio compared to that of NiTiPd alloy. Also, APB (anti-phase boundary) like regions with twinless martensites was observed in both the alloys, area fraction of APB-like regions being more in NiTiPdSc alloy. Thermal analysis showed that transformation temperatures (TTs) of NiTiPd alloy decreased significantly with addition of Sc. The martensite finish temperature (M_f) of 181 °C for NiTiPd alloy lowered to 139 °C upon 1.0 at.% Sc addition. The transformation hysteresis of Ni_24.7Ti_49.3Pd_25.0Sc_1.0 (at.%) alloy was measured to be 7 °C, significantly lower than that of 15 °C for Ni_24.5Ti_50.0Pd_25.0Sc_0.5 alloy, reported in literature. Alloy purity, lower volume fraction of second phase and presence of twinless/small twin ratio martensite in microstructure is believed to be the reasons for such low transformation hysteresis. The transformation behaviour of the alloys upon stress-free thermal cycling was found stable, variation in TTs being within 1–2 °C.     

Allotropic transformation induced stacking faults and discontinuous coarsening in a γ-γ′ Co-base alloy

The stability of a Co-based alloy designed to possess a microstructure comprising of L1₂, γ′ Co₃Ti-type precipitates embedded in an A1, γ Co solid solution matrix has been investigated. The alloy showed acute microstructural instabilities upon ageing at 700 °C, resulting in the degeneration of the γ-γ′ aggregate into i) a faulted Co-based martensite and Co₃Ti and ii) a lamellar aggregate of A3-Co and Co₃Ti. The faulted Co-based phase was formed by isothermal diffusionless transformation of the metastable A1-phase, whilst the lamellar aggregate was a discontinuous reaction product.     

Quantitative analysis of the effect of heat treatment on microstructural evolution and microhardness of an isothermally forged Ti–22Al–25Nb (at.%) orthorhombic alloy

The microstructural features of the 980 °C isothermally forged Ti–22Al–25Nb (at.%) orthorhombic alloy during heat treatment were quantitatively investigated. The volume fraction of the O phase precipitates, the width and length of the lath O phase, and the diameter of equiaxed grains at different heat treatment temperatures were measured using an image analysis software. Quantitative relationships among heat treatment temperature, microstructure parameters, and microhardness were established. The relationship between microstructure parameters and microhardness was analyzed with a multiple regression analysis technique. The results indicate that the microstructure of this alloy is mainly depended on the heat treatment schedule. Only equiaxed O/α₂ grains and B2 matrix existed when the samples were solution-treated above 980 °C, while equiaxed α₂ grains, rim O around α₂, and equiaxed/lath O could be obtained after the samples were solution treated below 980 °C. The width of lath and acicular O phases, and volume fraction of total precipitates could be controlled in the range of 0.37–0.88 μm, 0.09–0.48 μm and 10.91–60.18%, respectively. Experimental and statistical analysis showed a linear relationship between the microstructure parameters and microhardness.     

Response of the solidification microstructure of a high Nb containing TiAl alloy to an isothermal high-temperature heat treatment

Vacuum induction melting device combined with temperature control system was employed to investigate the effect of isothermal heat treatment (IHT) during solidification process on the microstructure evolution of a high Nb containing TiAl alloy. The microstructures of the alloy in as-cast condition and after IHT were studied. The results show that the as-cast microstructures exhibit a significant microstructural inhomogeneity with fine grains in dendrite core and coarse grains in interdendritic region. A new treatment approach by means of a short-term IHT within the β phase field during solidification process is proposed to obtain a uniform and refined microstructure. Compared with the as-cast alloy, IHT can reduce the tendency for crack. This phenomenon is attributed to the improvement of microstructural homogeneity by the elimination of peritectic α phase and the microstructure refinement by β → α transformation.     

The effects of stoichiometry on the dry sliding wear of FeAl

The effects of alloy stoichiometry on the dry sliding wear behavior of B2-structured FeAl were investigated using four different FeAl alloys containing 40, 43, 48, and 50 at.% Al. Room temperature pin-on-disk tribotests were performed against an yttria-stabilized zirconia counterface in four different environments: air, oxygen, 4% hydrogen in nitrogen, and argon. It was found that the alloys had lower wear rates in both oxygen-free and water vapor-free environments, where the wear loss was roughly inversely related to the hardness. Interestingly, the wear rates were little affected by the presence of molecular hydrogen. In oxygen- or water vapor-containing environments, the effects of the environment predominated over any effects from the mechanical properties of the alloys. The tips of the worn pins were examined using both scanning electron microscopy and transmission electron microscopy, the latter using specimens produced by focused ion beam milling. Zirconia particles were found to be embedded in a tribolayer on the worn tips of the pins. The results indicate that both two-body and three-body abrasive wear, as well as plastic deformation and delamination were the main wear mechanisms. The abrasive particles largely consisted of the counterface material.     

Investigation of the influence of Ni powder size on microstructural evolution and the thermal explosion combustion synthesis of NiTi

A key microstructural feature that controls the sintering behavior of Ni + Ti powders was determined to be the transformation of alpha-Ti to beta-Ti during heating. The use of very fine Ni powders causes this transformation to occur at the eutectoid temperature (i.e., 765 °C). The use of coarse Ni powders causes a gradual beta-Ti transformation from 765 to 882 °C. At 950 °C a large volume fraction of beta-Ti remains in coarse Ni/Ti mixtures whereas in fine Ni/Ti mixtures this phase is almost eliminated. Further heating above 950 °C causes the beta-Ti to melt, initiating a large exothermic reaction in the coarse Ni/Ti mixtures (i.e., 158 J/g) at 980 °C. The use of fine Ni significantly reduces this reaction (i.e., 3 J/g). Consequently, Ni powder size, and its influence over beta-Ti content can be used to control the reactive sintering behavior of Ni + Ti mixtures.     

Precipitation behaviors in a quenched high Nb-containing TiAl alloy during annealing

The precipitation of γ phase and heterogeneous nucleation of ω_o phase within β_o phase areas are common phenomena in TiAl alloys. However, detailed explanation on the corresponding phase transformation mechanisms is still lacking. In this study, the precipitation behaviors of γ and ω_o phases in a quenched Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy are investigated. The results show that large γ grains form after quenching whereas small γ particles can directly nucleate within the remaining β_o phase during annealing. Semi-coherent interfaces are observed between γ and β_o phases and the average distance between dislocations is evaluated. The heterogeneous nucleation of ω_o phase at the lamellar colony boundary is imaged by HRTEM. Edge-to-edge method is used to calculate the orientation relationship between γ and ω_o phases. The γ phase grows up faster than ω_o phase within the β_o phase areas during annealing at 800 °C.     

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.     

Microstructure of pulsed laser deposited FePd thin films on amorphous and crystalline substrates

Equiatomic FePd thin films have been deposited at room temperature by pulsed laser deposition (PLD) on amorphous, Si₃N₄ and SiO₂, and crystalline, (100)-NaCl, substrates. The resulting FePd film microstructures have been characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). We observed a complex orientation relationship between grains in nanocrystalline thin films of FCC FePd and the single crystal NaCl substrates. FePd films obtained under identical deposition conditions using the amorphous substrates also exhibited nanocrystalline morphology but with a fiber texture and consisted of a phase mixture of FCC FePd and the tetragonal ordered L1₀-FePd phase.     

Thermal stability of an intermediate strength fully lamellar Ti–45Al–2Mn–2Nb-0.8 vol.% TiB2 alloy

A fully lamellar γ-TiAl alloy Ti–45Al–2Mn–2Nb + 0.8 vol.% TiB₂ was exposed at 700 °C for 10,000 h in air. No perpendicular decomposition of α₂ into α₂ + β/α₂ + γ sections and no recrystallisation of β equiaxed grains on decomposed α₂ + γ lamellae were observed all through the prolonged exposure. The dominant change in microstructure was that the α₂ lamellae thinned gradually and became increasingly discontinuous with increasing exposure time. The average thickness of α₂ lamellae reduced to a half and the volume fraction reduced by 1/4 after 10,000 h exposure. Correspondingly, the tensile properties were hardly changed, while fatigue strengthening was increased by 30%. The reasons for the relatively high degree of stable microstructure and mechanical properties are discussed based on production processing and alloy composition. The “exposure-induced fatigue strengthening” observed after 10,000 h exposure is assumed to be caused by structure relaxation and stress dissipation throughout the specimen.     

The effect of sliding velocity on the dry sliding wear of nanophase Fe30Ni20Mn25Al25 against yttria-stabilized zirconia

In order to investigate the influence of the temperature increase due to frictional heating on the wear behavior of the nanostructured alloy Fe₃₀Ni₂₀Mn₂₅Al₂₅, which consists of alternating b.c.c. and B2 phases with interfaces aligned along 〈100〉, dry sliding pin-on-disk tests were conducted in air at several sliding velocities (0.1 m s⁻¹, 0.25 m s⁻¹, 0.5 m s⁻¹, 0.75 m s⁻¹ and 1 m s⁻¹) for 1 km. Disks of yttria-stabilized zirconia were used as the counterface. A combination of state-of-the-art techniques was used to characterize the pins, disks and debris after wear tests. It was found that the pins had much higher wear rates at a lower sliding velocity compared to a higher sliding velocity, although the wear rate of the zirconia counterface was higher at the higher sliding velocities. It was concluded, based on X-ray diffraction and transmission electron microscopy, that the increase in zirconia wear at higher sliding velocities was due to localized phase transformation of the zirconia caused by high frictional contact temperatures. The zirconia debris removed from the disk because of the phase transformation also contributed to the lower mass loss of the pins in wear tests at sliding velocities of 0.5 m s⁻¹ or higher because zirconia embedded in the surface of the pins provided added protection against pin wear.