Barothermal analysis and structure of the eutectic Al-Ni (2.7 at % Ni) alloy

Phase transformations of the 2.7Ni + 97.3Al eutectic alloy have been characterized by differential barothermal analysis (DBA) at temperatures of up to 750°C and pressures of up to ∼100 MPa. The results demonstrate that the Al matrix of the as-prepared alloy was saturated with micropores. After melting and crystallization in compressed argon, the micropore density increased. As a result of melting and solidification, the fine-grained structure of the as-prepared alloy transformed to a macrocrystalline, dendritic structure. Electron microscopy was used to determine the volume fraction of the intermetallic phase NiAl₃ in the Al matrix. Supersaturated solid solutions of nickel in aluminum decompose at 626°C to give a mixture of Al〈Ni〉 and NiAl₃. At 100 MPa, the nickel-aluminum solid-solution range extends to 2.7–2.8 at % Ni, and the nickel content at the eutectic point may reach 3.1–3.3 at %. The Al and NiAl₃ in the alloy solidified at 100 MPa had reduced lattice parameters. We determined the microhardness of the Al matrix after DBA.     

Study of dilute Al–Cu solidification by cooling curve analysis

The solidification path of a dilute Al–Cu alloy was studied using controlled solidification conditions and thermal analysis. Under equilibrium considerations, below the limit of maximum solubility, a unique α phase is expected, rounded by rich non eutectic composition. However, the precipitation of the second phase θ is present even for dilute compositions, fundamentally favored by segregation in the liquid and instabilities in the front of solidification. This effect has technological and academic implications, related to the precipitation of intermetallic compounds from the melt.     

Wear characteristics of spray formed Al-alloys and their composites

In the present investigation, different Al based alloys such as Al–Si–Pb, Al–Si, Al–Si–Fe and 2014Al + SiC composites have been produced by spray forming process. The microstructural features of monolithic alloys and composite materials have been examined and their wear characteristics have been evaluated at different loads and sliding velocities. The microstructural features invariably showed a significant refinement of the primary phases and also modification of secondary phases in Al-alloys. The Pb particles in Al–Si–Pb alloy were observed to be uniformly distributed in the matrix phase besides decorating the grain boundaries. The spray formed composites showed uniform distribution of SiC particles in the matrix. It was observed that wear resistance of Al–Si alloy increases with increase in Pb content; however, there is not much improvement after addition of Pb more than 20%. The coefficient of friction reduced to 0.2 for the alloy containing 20%Pb. A sliding velocity of 1 ms⁻¹ was observed to be optimum for high wear resistance of these materials. Alloying elements such as Fe and Cu in Al–Si alloy lead to improved wear resistance compared to that of the base alloy. The addition of SiC in 2014Al alloy gave rise to considerable improvement in wear resistance but primarily in the low pressure regime. The wear rate seemed to decrease with increase in sliding velocity. The wear response of the materials has been discussed in light of their microstructural features and topographical observation of worn surfaces.     

Effects of Cu and Al on the crystal structure and composition of η (MgZn2) phase in over-aged Al–Zn–Mg–Cu alloys

Heat treatable Al–Zn–Mg alloys can be strengthened by the precipitation of second phase particles. In this paper, Al–6.57%Zn–2.83%Mg and Al–6.57%Zn–2.83%Mg–3.92%Cu alloys (in wt%) in T7 state (140 °C for 96 h) have been prepared. The effects of Cu and Al on the concentration and structure of equilibrium η (MgZn₂) phase have been investigated by high resolution transmission electron microscopy, aberration-corrected scanning transmission electron microscopy, selected election diffraction pattern simulations, and first-principles calculations. The effects of Cu and Al substitution on the diffraction characteristics of the η phase and the general rule of Cu and Al substitution in the η phase have been discussed.     

TLP bonding of SiCp/2618Al composites using mixed Al–Ag–Cu system powders as interlayers

Mixed Al–Ag–Cu and Al–Ag–Cu–Ti powders were used as interlayers for transient liquid phase diffusion bonding (TLP bonding) of SiC particulate reinforced 2618 aluminum alloy matrix composite (SiCp/2618Al MMC). The results show that by using mixed Al–Ag–Cu powder with the eutectic composition as an interlayer, SiCp/2618Al MMC can be TLP bonded at 540 °C, however, the joining layer is porous. Adding a certain amount of titanium into the Al–Ag–Cu interlayer, the TLP bonding quality can be improved. The titanium added into the Al–Ag–Cu interlayer has an effect of shortening the solidification time of the joining layer, thus decreasing SiC particles from the parent materials entering into the joining layer. The joints bonded using Al–Ag–Cu–Ti interlayers have a maximum shear strength of 101 MPa when 2.1% titanium is added.     

The solidification process of Al–Mg–Si alloys

The microstructure and solidification process of three Al–Mg–Si alloys with different magnesium contents have been studied using optical microscopy and the electron probe X-ray microanalysis. The results showed that Al–Mg–Si alloys possessed fairly complicated solidification path: L→α-Al+L1→α-Al+Al15Si2(FeMn)3+L2→α-Al+Al15Si2(FeMn)3+ (α-Al+Mg2Si)+L3→α-Al+Al15Si2(FeMn)3+(α-Al+Mg2Si)+(α-Al+Mg2Si+Al15Si2(FeMn)3), and wide solidification temperature of 75 °C. The magnesium content in the alloys greatly influenced the as-cast microstructure. The higher the magnesium content, the more Mg2Si structure was present. Iron and manganese segregated to the finally solidified zone, which resulted in the formation of ternary eutectic structure. Although their content in the alloys was very low, their effect on solidification behaviour cannot be ignored. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of processing parameters on the microstructure and mechanical properties of Al–steel composite foam

Al–steel composite foams comprise of steel hollow spheres embedded in an aluminum matrix and are processed using a gravity casting technique. The effect of processing parameters such as casting temperature and cooling rate on the microstructure and mechanical behavior was studied to establish structure–property relationships. Results show that the amount and composition of intermetallic phases present in the foam microstructure is directly related to casting temperature and cooling rate. Highest strength and energy absorption were obtained from Al–steel foams with fast solidification rates that minimize the growth of intermetallic phases.     

In situ production of Al–TiB<Subscript>2</Subscript> nanocomposite by double-step mechanical alloying

An in situ Al–TiB₂ nanocomposite was synthesized by mechanical alloying (MA) of pure Ti, B and Al powder mixture in a planetary ball mill. A double-step process was used to prevent the formation of undesirable phases like Al₃Ti intermetallic compound. In the first step, a powder mixture was tailored to obtain nominal Al–90 wt% TiB₂ composition and the second step involved the addition of Al to the mixture in order to achieve Al–20 wt% TiB₂. The structural and thermal characteristics of powder particles were studied by X-ray diffractometry (XRD), scanning electron microscopy (SEM), differential scanning calorimetery (DSC), and transmission electron microscopy (TEM). The results showed that the MA process leads to the in situ formation of nanosized TiB₂ particles in an Al matrix with a uniform distribution. It was also found that the double stage addition of aluminum can prevent the formation of undesirable compounds even after annealing at high temperatures_.     

In situ TEM nanoindentation and dislocation-grain boundary interactions: a tribute to David Brandon

As a tribute to the scientific work of Professor David Brandon, this paper delineates the possibilities of utilizing in situ transmission electron microscopy to unravel dislocation-grain boundary interactions. In particular, we have focused on the deformation characteristics of Al–Mg films. To this end, in situ nanoindentation experiments have been conducted in TEM on ultrafine-grained Al and Al–Mg films with varying Mg contents. The observed propagation of dislocations is markedly different between Al and Al–Mg films, i.e. the presence of solute Mg results in solute drag, evidenced by a jerky-type dislocation motion with a mean jump distance that compares well to earlier theoretical and experimental results. It is proposed that this solute drag accounts for the difference between the load-controlled indentation responses of Al and Al–Mg alloys. In contrast to Al–Mg alloys, several yield excursions are observed during initial indentation of pure Al, which are commonly attributed to the collective motion of dislocations nucleated under the indenter. Displacement-controlled indentation does not result in a qualitative difference between Al and Al–Mg, which can be explained by the specific feedback characteristics providing a more sensitive detection of plastic instabilities and allowing the natural process of load relaxation to occur. The in situ indentation measurements confirm grain boundary motion as an important deformation mechanism in ultrafine-grained Al when it is subjected to a highly inhomogeneous stress field as produced by a Berkovich indenter. It is found that solute Mg effectively pins high-angle grain boundaries during such deformation. The mobility of low-angle boundaries is not affected by the presence of Mg.Special title: Advanced Materials and Characterization: Proceedings of the Brandon Symposium; Guest Editors: Wayne D. Kaplan and Srinivasa Ranganathan     

Microstructure,tensile properties,and creep behavior of high-pressure die-cast Mg–4Al–4RE–0.4Mn (RE = La,Ce) alloys

High-pressure die-cast (HPDC) Mg–4Al–4RE–0.4Mn (RE = La, Ce) magnesium alloys were prepared and their microstructures, tensile properties, and creep behavior have been investigated in detail. The results show that two binary Al–Ce phases, Al₁₁Ce₃ and Al₂Ce, are formed mainly along grain boundaries in Mg–4Al–4Ce–0.4Mn alloy, while the phase composition of Mg–4Al–4La–0.4Mn alloy contains only α-Mg and Al₁₁La₃. The Al₁₁La₃ phase comprises large coverage of the grain boundary region and complicated morphologies. Compared with Al₁₁Ce₃ phase, the higher volume fraction and better thermal stability of Al₁₁La₃ have resulted in better-fortified grain boundaries of the Mg–4Al–4La–0.4Mn alloy. Thus higher tensile strength and creep resistance could be obtained in Mg–4Al–4La–0.4Mn alloy in comparison with that of Mg–4Al–4Ce–0.4Mn. Results of the theoretical calculation that the stability of Al₁₁La₃ is the highest among four Al–RE intermetallic compounds supports the experimental results further.     

Optimization of the Al–Cr–Zn system at 460 °C

The Al–Cr–Zn ternary system is assessed by the CALPHAD method. The solution phases are modelled using the Redlich–Kister formalism. The ternary intermetallic compounds are described by using the sublattice model. The main intermetallic compound τ₁, of the Al–Cr–Zn system, was treated as δ₁-FeZn₉ solid phase in accordance with its isotype structure. A comparison with experimental phase diagram is also presented.     

Control of solidified structures in aluminum–silicon alloys by high magnetic fields

In order to investigate the effects of high magnetic fields on the as-solidified structures of Al alloys, solidification experiments of hypoeutectic and hypereutectic Al–Si alloys under various high magnetic field conditions (up to 12 T) have been conducted. It was found that uniform magnetic fields and gradient magnetic fields affect the solidification process by Lorentz force and magnetization force, respectively. The primary silicon crystals of hypereutectic Al–Si alloys are distributed, relatively, homogeneously under uniform magnetic fields, whereas they congregate near the top surface or bottom of samples by the combined action of buoyancy and magnetization force under gradient magnetic fields. The results indicate that it is possible to control the behaviors of reinforced particles in the metal matrix and improve the material performances by using high magnetic fields in the solidification process of metal matrix composites. The experiments also showed that high magnetic fields decrease the interlamellar spacing of the eutectic structure, while there exists a certain optimum value of magnetic intensity corresponding to the minimum value of interlamellar spacing, and magnetic energy is capable of influencing thermodynamic equilibrium of solidifying system and makes the content of eutectic aluminum in eutectic structures increased.     

Influence of zirconium on grain refining efficiency of Al–Ti–C master alloys

The influence of Zirconium on the grain refinement performance of Al–Ti–C master alloys and the effect mechanism has been studied in this paper. The experimental results show that Zr not only results in poisoning the Al–Ti–B master alloy, but also poisons the Al–Ti–C master alloys. The poisoning effect is more obvious at higher melting temperature. When 0.12%Zr is added into the melt, the grain refinement performance of Al–5Ti–0.4C refiner with 0.2% addition level absolutely disappears at 800 °C. The experimental results also show that it is difficult to refine the commercial purity Al containing 0.15%Zr by Al–5Ti–0.4C master alloy. Further experiments show that the Zr element can interact with both TiAl₃ and TiC phases. If both of them are present, Zr preferentially reacts with TiAl₃ phase.     

Interfacial diffusion effect on phase transitions in Al/Mn multilayered thin films

Thin films of Al and Mn multilayers were synthesized using thermal evaporation under high vacuum conditions. The whole film thickness containing three bilayers of Al and Mn is about 120 nm. The global concentration of the samples was varied between 10 and 46.5 at.% Mn, by changing the thickness of the bilayer. The as-evaporated samples were heat treated at different temperatures (473, 623, 823 and 873 K) for 2 and 8 h to investigate the interfacial diffusion induced phase transformations in the multilayered thin films. Transmission electron microscopy (TEM) has been mainly used to characterize the crystalline structure of a variety of phases revealed on annealing, such as μ, λ and φ phases up to 823 K, δ phase at 823 K and T6 phase at 873 K. The occurrence of a variety of structures on annealing has been attributed to the interfacial reactions at the Al–Mn bilayers, and, therefore, the global composition of the composite films is not significant during the process of phase transformations. The crystallographic relationships of Al–Mn approximant structures of the decagonal quasicrystal are discussed to understand the evolution and stability of the T6 phase at high temperature.     

Multiscale characterization of nanostructured Al–Si–Zr alloys obtained by rapid solidification method

Properties of engineering metallic alloys (e.g., fracture toughness, corrosion resistance) are often limited by the presence of primary intermetallic particles which form during conventional solidification. Rapid solidification brings about much more homogenous amorphous and/or nanocrystalline structure with reduced density of primary particles. Rapidly solidified thin ribbons obtained by melt spinning are usually considered as intrinsically homogenous. However, due to different cooling conditions at the wheel surface and on the side exposed to the ambient environment, structure of such ribbons may vary significantly across its thickness. The materials studied in this study were 30–40 μm thickness ribbons of nanocrystalline hyper- and hypo-eutectic Al–Si–Zr alloys produced by melt-spinning method. Transmission electron microscopy and high resolution scanning transmission electron microscopy were used to characterize the structure homogeneity across the ribbons. Thin foils for transmission observations were prepared by focused ion beam system. Microstructural observations confirmed nanocrystalline character of Al–Si–Zr alloys. However, these observations revealed inhomogeneity of the structure across the ribbon width.     

Micro-structural and tensile strength analyses on the magnesium matrix composites reinforced with coated carbon fiber

Magnesium matrix composites reinforced with SiO₂ coated carbon fibers have been investigated, with an emphasis given on the relation between the material strength and interfacial microstructure. The composites were studied as a function of aluminium (Al) content that is varied between 0 and 9 wt%. The obtained results indicate that the reactivity at the C/Mg–Al interface of the composite can be controlled by varying the Al content. The low Al content in C/Mg–1Al has been completely dissolved in the matrix with no segregation even after solidification, leading to the best mechanical performance. If the Al content is increased to ≥3 wt% (composites such as C/AZ31 and C/AZ91), the SiO₂ coatings are fully depleted due to an extensive formation of carbides at the interface. The precipitates are further identified as Al₂MgC₂ phase that is similar to binary carbide Al₄C₃. SiO₂ coating on the fiber layer prior to fabrication of composite is found to be a promising way to suppress the carbide formation and enable the use of Mg–Al matrix with appropriate Al content.     

Effect of microstructural features on the ageing behaviour of Al–Cu/SiC metal matrix composites processed using casting and rheocasting routes

In the present study, microstructural variation in Al–2 wt% Cu/SiC composites was accomplished by synthesizing them using conventional casting and partial liquid phase casting (rheocasting) routes. Microstructural characterization studies conducted on the rheocast composite samples revealed a finer grain size, minimal porosity, uniform distribution of SiC particulates, and a superior matrix – particulate interfacial integrity when compared to the conventionally cast composite samples. Furthermore, the results of interfacial characterization studies revealed that the presence of porosity associated with either individual SiC particulate or SiC clusters significantly influence the constitutional characteristics of the interfacial region. Results of ageing studies revealed an accelerated ageing kinetics in case of rheocast samples when compared to the conventionally cast composite samples. The results of ageing studies were finally rationalized in terms of the difference in microstructural characteristics of the rheocast and conventionally cast composite samples. This revised version was published online in November 2006 with corrections to the Cover Date.     

Influence of ultrasonic melt treatment on the formation of primary intermetallics and related grain refinement in aluminum alloys

Ultrasonic melt treatment (UST) is known to induce grain refining in aluminum alloys. Previous studies have clearly shown that in Al–Zr–Ti alloys, the primary Al₃Zr intermetallics were dramatically refined by cavitation-assisted fragmentation, and a good refinement effect was achieved. In this article, Al–Ti, Al–Ti–Zr alloys, and some commercial aluminum alloys are used to analyze the effect of UST on primary intermetallics and grain refinement. The addition of a small amount of Al–3Ti–B master alloy is also studied in order to compare with the addition of Ti and Zr in commercial aluminum alloys. Experimental results show that the ultrasonic grain refining effect is not only related to the size of particles which are refined and/or dispersed by UST, but also related to an undercooling available for activation of these particles in the solidification process. Athermal heterogeneous nucleation theory is considered to explain the effect of size and distribution of substrate particles on the grain structure with different undercoolings. The distribution of primary particle sizes results in the distribution of required undercoolings. Grain refining occurs when the undercooling is large enough to activate the refined primary intermetallics or dispersed inoculants.     

Effect of Al content on the formation of intermetallic compounds in Sn–Ag–Zn lead-free solder

The effect of addition of Al, up to 1 wt.%, on the formation of intermetallic compounds in the microstructure of Sn–3.7%Ag–0.9%Zn lead-free solder was investigated. The typical microstructure of Sn–Ag–Zn solder is composed of the β-Sn phase and mixed granules which contain the intermetallic compounds (IMCs) of Ag₃Sn and AgZn. After alloying with 0.5 wt.% Al, the microstructure of the explored solder evolves into a mixture of the bulk Agl IMCs and β-Sn phase, and most of the bulk Ag₂Al IMCs distribute on and around the grain boundaries. The addition of 1 wt.% Al into the Sn–Ag–Zn solder brings many granules and bars of the Ag₂Al IMCs, while the amount of the bulk Ag₂Al IMCs decrease greatly. The above observation suggests that the bulk Ag₂Al IMCs is replaced by the granule-like Ag₂Al IMCs appearing along the grain boundaries. Since the grain size of the solder alloyed with 0.5%Al is relatively small as compared to the one alloyed with 1 wt.% Al, the growth of the Ag₂Al IMCs was prompted through the feasible diffusion channels along the grain boundaries. Thus, the addition of Al plays an important role on the morphology of the Ag₂Al IMCs in the final microstructure of the explored Sn–Ag–Zn solder.     

Glass formation and phase selection of melt-spun Al–Zn–Ce alloys

The glass-formation characteristics and phase-selection behavior of Al–Zn–Ce alloys have been studied by X-ray diffraction (XRD) and different scanning calorimetry (DSC). As the concentration of Ce increases, an intermetallic compound Al₂Zn₂Ce appears, which prevents the occurrence of phase separation, and improves the forming ability of the single amorphous phase. In Al₈₃Zn₁₀Ce₇ alloys, the precipitation of fcc-Al was accompanied with the Al₂Zn₂Ce phase and Al₄Ce phase. Moreover, the presence of fcc-Al appears to favor the nucleation and growth of the Al₂Zn₂Ce and Al₄Ce phase. However, it seems that the Al₂Zn₂Ce and Al₄Ce nucleate competitively with the fcc-Al phase and the growth of fcc-Al and Al₂Zn₂Ce prefers to that of Al₄Ce. The competitive nucleation and growth limitation of the various phases are favorable to the formation of Al–Zn–Ce amorphous alloys. For the amorphous Al–Zn–Ce alloys, the glass formation is not controlled by nucleation restrictions but largely by the suppression of growth of nuclei formed during rapid melt quenching.