Enhanced thermal stability of amorphous aluminum alloys through microalloying

Amorphous aluminum alloy powders with compositions of Al₈₅Y₇Fe₈, Al₈₃Y₇Fe₈Ti₂, and Al₇₉Y₇Fe₈Ni₃Ti₂Nd₁ were synthesized with crystallization temperatures of 342 °C, 446 °C, and 457 °C, respectively. In-situ high-temperature synchrotron diffraction results were correlated with differential scanning calorimetry studies to investigate the structural evolution during the crystallization. The results show that, through microalloying, the onset-of-crystallization temperature was increased by 115 °C, and the resulting crystalline products changed from a mixture of fcc-Al and an intermetallic phase in the case of Al₈₅Y₇Fe₈ to only intermetallic phases.     

Evaluation of thermal stability of ultrafine grained aluminium matrix composites reinforced with carbon nanotubes

The effect of carbon nanotubes on the thermal stability of ultrafine grained aluminium alloy processed by the consolidation of nano-powders obtained by mechanical alloying was evaluated via measurements of grain size and mechanical property changes upon annealing at various temperatures. It was found that the grain size of the samples containing carbon nanotubes is stable up to high temperatures and even after annealing at 450 °C (0.7T_m) no evident grain growth was observed. The limited grain boundary migration was attributed to the presence of entangled networks of carbon nanotubes located at grain boundaries and to the formation of nanoscale particles of aluminium carbide Al₄C₃. It was also revealed that carbon nanotubes decompose at a relatively low temperature of 450 °C and form fine Al₄C₃ precipitates. This transformation does not significantly affect the mechanical properties due to the nanoscale size of the carbides.     

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.     

Radio frequency plasma nitriding of aluminium at higher power levels

Nitriding of aluminium 2011 using a radio frequency plasma at higher power levels (500 and 700 W) and lower substrate temperature (500 °C) resulted in higher AlN/Al₂O₃ ratios than obtained at 100 W and 575 °C. AlN/Al₂O₃ ratios derived from X-ray photoelectron spectroscopic analysis (and corroborated by heavy ion elastic recoil time of flight spectrometry) for treatments preformed at 100 (575 °C), 500 (500 °C) and 700 W (500 °C) were 1.0, 1.5 and 3.3, respectively. Scanning electron microscopy revealed that plasma nitrided surfaces obtained at higher power levels exhibited much finer nodular morphology than obtained at 100 W.     

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.     

Synthesis of a nanostructured MgH2-Ti alloy composite for hydrogen storage via combined vacuum arc remelting and mechanical alloying

To improve the hydrogen kinetics of magnesium hydride, TiCr_1.2Fe_0.6 alloy was prepared by vacuum arc remelting (VAR) and the alloy was co-milled with MgH₂ to process nanostructured MgH₂-5 at.% TiCr_1.2Fe_0.6 powder. The hydrogen desorption properties of the composite powder were studied and compared with pure magnesium hydride. X-ray diffraction (XRD) analysis showed that the composite powder prepared by VAR/mechanical alloying (MA) procedure consisted of β-MgH₂, γ-MgH₂, bcc Ti-Cr-Fe alloy, and small amount of MgO. The average size of particles and their grain structure after 4 h MA were determined by a laser particle size analyzer and XRD method and found to be 194 nm and 11 nm, respectively. The hydrogen desorption temperature of magnesium hydride decreased from 426 °C to 235 °C by the bcc Ti alloy and the utilized processing method, i.e. combined VAR/MA.     

Characterization of dispersed intermetallic phases in an Al-8.32wt%Fe-3.4wt%Ce alloy

A rapidly solidified Al-8.32Fe-3.4Ce (wt%) alloy was prepared by gas atomization and extrusion. The intermetallic phases present and their thermal stability, at temperatures up to 400°C, have been investigated by means of transmission electron microscopy (TEM). The metastable Al_mFe, Al₈Ce and equilibrium Al₁₃Fe₄ phases were detected in the as-extruded sample and the sample heat-exposed at 230°C, whereas the equilibrium Al₁₃Fe₄ and Al₁₃Fe₃Ce phases existed in the samples heat-exposed at temperatures above 315°C. The Al_mFe and the Al₈Ce phases were firstly observed in this alloy. The Al₁₀Fe₂Ce and Al₂₀Fe₅Ce phases, which were reported by the others in the similar alloys, do not exist in our samples. In addition, various domain structures in Al₁₃Fe₃Ce were also studied.     

Oxidation behaviour of reaction-bonded alumina compacts using an Al88Si12 alloy precursor

The oxidation behaviour of attrition-milled Al₈₈Si₁₂/Al₂O₃ powder mixtures was investigated for the formation of mullite/Al₂O₃ composites by the reaction bonded alumina (RBAO) process. Cylindrical powder compacts were heated at 5°C min^–1 to temperatures between 450 and 1400°C. Oxidation occurred rapidly between ca. 400 and 750°C. Dense, outer reaction layers which formed at the lower temperatures inhibited complete oxidation and led to fracture of the body during continued heating to higher temperatures (above ca. 850°C) While the incorporation of ZrO₂ improved the oxidation of the samples, X-ray analysis indicated that the Si in the alloy reacted with the ZrO₂ to form phases which prevented the formation of mullite at the temperatures used in the experiments.     

Thermal stability in the Al-Al3Ti system

Directionally solidified samples of an Al-2 wt% Ti alloy were annealed at temperatures between 435° C and 660° C to investigate the thermal stability of phases formed during an incomplete peritectic transformation. The proportion of Al₃Ti present in the assolidified alloy is less than equilibrium up to about 480° C, and more than equilibrium at higher temperatures. Hence, Al₃Ti particles will be stable up to about 500° C and will tend to dissolve at higher temperatures. Diffusion due to non-equilibrium composition of the phase continues at all temperatures but is sluggish up to about 600° C. The diffusion coefficient of Ti in Al at 635° C is estimated to be 2×10^–11 cm² sec^–1.     

Effects of liquid quenching and subsequent heating on the structure of Co-Al Alloys

The phase composition and structure of Co-Al alloys after liquid-quenching (LQ) and subsequent heating were determined by x-ray diffraction, differential thermal analysis, and differential scanning calorimetry. After LQ, the Co-19.5 at % Al alloy consisted of two phases: CoAl (B2 structure, a=0.2852 nm) and an fcc Co_{1 – x} Al_x solid solution (a=0.3573 nm). The phase composition of the LQ alloy and the lattice parameters of the intermetallic phase CoAl and solid solution corresponded to a solidification temperature of 1100°, rather than to the eutectic temperature (1400°C). Heating to 1000°C brought the alloy to the equilibrium state. In the range 25–28 at % Al, the LQ alloys were single-phase and consisted of Co-enriched CoAl (B2). Decomposition of this intermetallic phase during heating in the calorimeter gave rise to an exothermic peak at 680°C and led to precipitation of hcp Co, which converted to an fcc Co_{1 – x} Al_x solid solution on heating to 1000°C. The activation energy of the decomposition process, evaluated by the Kissinger method, was E _a=125 ± 10 kJ/mol. The LQ Co-71.4 at % Al alloy consisted of three phases: metastable quasicrystalline phase (similar in structure to Al₁₄Co₃Ni₃), Co₂Al₅, and CoAl. Heating brought the alloy to the equilibrium state (Co₂Al₅), with an exotherm near 690°C. The activation energy of this phase transformation was E _a=195 ± 15 kJ/mol. The LQ alloys containing 76.5 and 82 at % Al were single-phase and consisted of monoclinic Co₄Al₁₃ or Co₂Al₉ with reduced lattice parameters.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 4, 2005, pp. 420–426.Original Russian Text Copyright © 2005 by Portnoi, Tretyakov, Filipova, Latuch.     

Study of interfacial reactions in ionized metal plasma (IMP) deposited Al-0.5%wtCu/Ti/SiO2/Si structure

Interfacial reactions in Al-0.5%wtCu/Ti/SiO₂/Si structure have been investigated up to the annealing temperature of 600°C for 30 min in Argon ambient. Annealing temperature at above 500°C, Al alloy and Ti start to react and produce Al₃Ti, which was already reported. Annealing at higher temperatures (550°C, and 600°C) made Al₃Ti transformed into Al₅Ti₂, which is thermodynamically more stable than Al₃Ti. The unreacted 52 nm thick Ti which existed underneath of Al₅Ti₂ might lead to retardation of the reaction between Al₅Ti₂ and the underlying SiO₂. Hence, the formation of ternary compound (Al _x Ti _y Si _z ) which is believed to be detrimental to the contact metallization layers was protected.     

The effect of Ca and RE elements on the precipitation kinetics of Mg17Al12 phase during artificial aging of magnesium alloy AZ91

The effect of simultaneous alloying with Ca and rare earth (RE) elements on the age hardening kinetics of AZ91 was studied through the fitting of the Johnson-Mehl-Avrami (JMA) equation. The results showed that the addition of both Ca and RE elements not only suppress discontinuous precipitation of the Mg₁₇Al₁₂ phase during the age hardening process, but also decrease the alloy hardness. Fitting the JMA equation to the experimental data indicated that the phase transformation during age hardening of an alloy variant containing both Ca and RE (at 170 °C and 190 °C) and standard AZ91 (at 170 °C) takes place by the nucleation of precipitates on dislocations. In contrast, the precipitation during age hardening of AZ91 at 190 °C occurs via nucleation at grain boundaries. Although it was observed that the creep strength of age hardened specimens are lower than that of the as cast specimens, but age hardening treatment has lower deleterious influence on the creep resistance of the alloy containing Ca and RE in comparison with conventional AZ91. This may be ascribed to the decreased precipitation rate resulting from the addition of both Ca and RE elements.     

Effects of samarium on microstructures and tensile properties of Mg-5Al-0.3Mn alloy

Microstructures and tensile properties of Mg-5Al-0.3Mn-xSm (x = 0, 1, 2 and 3 wt.%) alloys prepared by metal mould casting method were investigated. It was demonstrated that Mg-5Al-0.3Mn alloy was mainly composed of α-Mg and β-Mg₁₇Al₁₂ phases. However, the other two precipitates (Al₁₁Sm₃ and Al₂Sm) were observed along grain boundaries in the alloys containing Sm. The amount of Al₁₁Sm₃ and Al₂Sm precipitates was increased with the increment of Sm content. Meanwhile, volume fraction of β-Mg₁₇Al₁₂ phase was decreased. Moreover, the morphology of β-Mg₁₇Al₁₂ was altered from bulk bone-like shape to spherical one. Tensile results showed that Mg-5Al-0.3Mn-2Sm alloy exhibited the highest tensile properties both at room temperature and 150 °C. Compared with ultimate tensile strength (UTS), yield strength (YS) and elongation (?) of Mg-5Al-0.3Mn alloy, UTS, YS and ? of Mg-5Al-0.3Mn-2Sm alloy were enhanced by 30%, 45% and 35% at room temperature, and by 17%, 48% and 96% at 150 °C, respectively. The improvement of tensile properties was attributed to the decreased amount of β-Mg₁₇Al₁₂ and its refined morphology, and high thermal stable Al₁₁Sm₃ and Al₂Sm precipitates which effectively prohibited dislocation movement and grain boundary sliding during deformation process.     

Initial stages of metal-organic chemical-vapor deposition of ZrO2 on a FeCrAl alloy

The initial stages of metal-organic chemical-vapor deposition of ZrO₂ on a model FeCrAl alloy was investigated using synchrotron radiation photoelectron spectroscopy, X-ray absorption spectroscopy, scanning Auger microprobe, and time of flight secondary mass spectrometry. The coatings were grown in ultra-high vacuum at 400 °C and 800 °C using the single source precursor zirconium tetra-tert-butoxide. At 400 °C the coatings mainly consist of tetragonal ZrO₂ and at 800 °C a mixed ZrO₂/Al₂O₃ layer is formed. The Al metal diffuses from the FeCrAl bulk to the metal/coating interface at 400 °C and to the surface of the coating at 800 °C. The result indicates that the reaction mechanism of the growth process is different at the two investigated temperatures.     

Oxidation of Au4Al in gold ballbonds

Two different 4N (99.99% purity) gold wires were ballbonded on 1 μm thick Al-1 wt.% Si-0.5 wt.% Cu bondpad metallisation and subjected to high temperature storage (HTS) at 175 °C in air. Each wire type showed ball lift failures, Type A after 500 h and Type B after 1500 h, which in both cases was a result of Au₄Al oxidation. With wire Type A the dominant compound underneath the ball was Au₈Al₃. A thin layer of Au₄Al (≈ 1 μm thick) was observed between the Au₈Al₃ and the gold ball. Ball lift failures occurred in the Au₄Al layer, which appeared to disintegrate due to oxidation and the resulting by products of oxidation were deposited on the underlying and unoxidised Au₈Al₃. With wire Type B, a double layer Au₄Al was dominant after long term ageing and Au₈Al₃ was confined to the ball periphery. Consequently, because of the much greater volume of Au₄Al, compound oxidation resulted in the formation of a large amount of a completely new microstructure consisting of gold precipitates embedded in a dark oxide matrix. The Au₈Al₃ compound remained unoxidised. It is speculated that internal stress and contamination may accelerate the oxidation reaction.     

Formation of Na-A and -X zeolites from waste solutions in conversion of coal fly ash to zeolites

Na-A and -X zeolites were synthesized from waste solutions in conversion of coal fly ash (Fa) to zeolite. The amorphous SiO₂ and Al₂O₃ of Fa were completely dissolved to form Po, Pt, and Pc type zeolites in NaOH solutions at 85°C. Only 24% of Si⁴⁺ eluted from Fa were converted to the zeolites and the remaining waste solutions contained high Si⁴⁺ concentrations. When molar ratio SiO₂/Al₂O₃ of the waste solutions was modified at 1.0≤SiO₂/Al₂O₃≤2.0 by addition of NaOH-NaAlO₂ solutions and the solutions were agied at 85°C, a single phase of Na-A zeolite was formed. The Na-X zeolite was formed at SiO₂/Al₂O₃≥2.5 and its crystallinity was increased with increasing the SiO₂/Al₂O₃ ratio, whereas the crystallinity of Na-A zeolite was decreased. At SiO₂/Al₂O₃=7.3, a single phase of Na-X zeolite was produced.     

Sol-gel synthesis and characterization of sub-microsized lanthanide (Ho, Tm, Yb, Lu) aluminium garnets

Sub-microsized and nanosized holmium aluminium garnet (Ho₃Al₅O₁₂, HoAG), thulium aluminium garnet (Tm₃Al₅O₁₂, TmAG), ytterbium aluminium garnet (Yb₃Al₅O₁₂, YbAG) and lutetium aluminium garnet (Lu₃Al₅O₁₂, LuAG) powders were prepared by a simple aqueous sol-gel method using aluminium nitrate nonahydrate, lutetium oxide, thulium oxide, holmium oxide and ytterbium oxide as starting materials. Ethane-1,2-diol was used as complexing agent. The powder X-ray diffraction (XRD) patterns of the specimens sintered at 1000 °C revealed the formation of monophasic HoAG, TmAG, YbAG, and LuAG. The phase composition of the samples was also characterized by infrared (IR) spectroscopy. Microstructural features of the polycrystalline garnets were studied by scanning electron microscopy (SEM).     

Thermal stability and mechanical properties of mechanically alloyed Al-10Ti alloy

The mechanical properties of Al-10Ti alloy prepared by mechanical alloying and subsequent hot hydrostatic extrusion were evaluated at room and elevated temperatures. Transmission electron microscopy was used to characterize the microstructural changes of this alloy on heat treatment at 500 °C for various times. The results show that the mechanically alloyed Al-10Ti has high strength and high thermal stability at elevated temperature. The strength and stability of this alloy are attributed to its fine grain size and to the high volume fraction of small Al₃Ti intermetallic compounds dispersed in the aluminium matrix. After 50 h annealing at 500 °C, no serious coarsening of either the Al₃Ti dispersoids or the grains was observed.     

Analysis of fatigue crack initiation and S-N response of model cast aluminium piston alloys

Fatigue crack initiation and S-N fatigue behaviour of hipped model Al7Si-Sr and Al0.7Si piston alloys have been investigated after overaging at 260 °C for 100 h to provide a practical simulation of in-service conditions. The results show that hipping did not affect the S-N behaviour of Al7Si-Sr. This is attributed to the lack of significant change in porosity distribution in this alloy because of its low porosity levels even in the unhipped state. However, hipping profoundly improved the fatigue performance of alloy Al0.7Si due to the significant reduction in porosity. In this investigation, it was observed that porosity was rendered impotent as a fatigue crack initiator in both hipped alloys. Instead, fatigue cracks were observed to originate mainly from intermetallic particles (particularly the Al₉FeNi phase) in both alloys and sometimes from oxide particles in Al0.7Si alloy. Fatigue cracking was also frequently observed at intermetallic clusters in hipped Al0.7Si. The observed scatter in fatigue life is discussed in terms of the size of fatigue crack initiating particles and the overall particle size distribution which follows a power law distribution function.     

Preparation and study of YSTZ-Al2O3 nanocomposites

Yttria stabilized zirconia-alumina (YSTZ-Al₂O₃) nanocomposite system with various Al₂O₃ concentrations has been synthesized by sol-gel route. The experimental techniques XRD, DTA, TGA, FT-Raman, FT-IR, SEM, Vickers hardness measurements, density measurements and Impedance spectroscopy were used to characterize the synthesized specimens. DTA result shows two exothermic reactions: one around 760°C and another around 960°C. XRD results confirm that the specimen starts to crystallize on heating above 750°C. Well resolved XRD reflections corresponding to tetragonal (t) ZrO₂ were obtained after the specimens were heated at 1000°C. FT-Raman results confirmed that the crystallites developed above 750°C was t-ZrO₂. It was observed from the XRD and DTA results that the bulk and grain boundary region crystallize independently in two different temperatures with a difference in temperature of about 200°C. The crystallization temperatures increase with Al₂O₃ contents. At 1300°C, the pure YSTZ and 5 and 10 wt % Al₂O₃ added YSTZ specimens underwent structural transformation from tetragonal to monoclinic ZrO₂. But, the tetragonal symmetry remains stable at 1300°C with an addition of 15 wt % Al₂O₃. The system which retain its tetragonal symmetry at its processing temperature (1300°C) gives high hardness and maximum density values. Almost 100% theoretical density value was obtained at 1300°C with an addition of 15 wt % of Al₂O₃.