Aluminium-chromium Alloys Research Articles

Phase transformations in rapidly quenched Al–Cr–Zr alloys during heat treatment

Results from studying the effect zirconium has on solid-phase processes in aluminum–chromium alloys are presented. Rapidly quenched alloys are prepared via melt spinning. The quenching rate is ~106 K/s. By means of physicochemical analysis, it is shown that doping Al–Cr alloys with zirconium improves the thermal stability of supersaturated solid solutions and stabilizes their microcrystalline structure; this hinders the coagulation of intermetallic phases and thus improves the hardness of the alloys. It is found that supersaturated solid solutions of Cr and Zr in aluminum undergo stepwise decomposition; the temperature and time parameters of each step are shown in TTT diagrams.

Improvement of resistance of TiAl alloy against high temperature oxidation by electroplating in AlCl 3–NaCl–KCl–CrCl 2 molten salt

For improvement of the oxidation resistance of a TiAl alloy under high temperature conditions, aluminum chromium alloys were electroplated on the TiAl alloy in an AlCl 3–NaCl–KCl molten salt containing CrCl 2 at 423 K. The deposit electroplated at −0.1 V vs. Al/Al 3+ consists of γ-Al 8Cr 5 single phase, including chromium content at 41 at.%. A mixture phase of γ-Al 8Cr 5 and Al is, however, formed at potential from −0.2 to −0.4 V. The oxidation resistance of the electrodeposit layer was evaluated by a high temperature oxidation test at 1173 K for 24 h. On the TiAl specimen covered with γ-Al 8Cr 5 which was deposited at −0.1 V, a uniform AlCr 2 layer was formed during the oxidation at 1173 K. The AlCr 2 layer was covered by a thin dense Al 2O 3. Both AlCr 2 and Al 2O 3 layers play a role of superior protection from the high temperature oxidation of the TiAl alloy; therefore, the plating of the γ-Al 8Cr 5 single layer deposited at −0.1 V reduced oxide thickness by 1/40 during the oxidation at 1173 K for 24 h from that on the TiAl without any plating.

Effect of Cold Work and Trace Rare-Earth Additions on the Aging Behavior of Al–Cr Alloys Containing Zirconium

Precipitation behavior during aging of a cold-worked aluminium chromium alloy containing zirconium has been investigated by electron microscopy transmission. The effect of trace rare-earth additions on the aging behavior of Al–Cr–Zr alloys has been studied using microhardeness testing and metallographic analysis. A detailed microstructural analysis revealed that the age hardening in these alloys is mainly due to the formation of precipitates on dislocations. It was further noted that cold working prior to aging greatly accelerated the process of age hardening, and a two-stage hardening has been observed in most of the alloys with or without cold work. Two types of precipitates, viz CrAl 7 and Cr 2A 11, formed at different aging temperatures, have been identified.

Mechanical properties of aluminium-chromium alloys extruded from atomized powder : A.Inoue et al. (Tohoku University, Sendai, Japan.) J. Jpn Soc. Powder Powder Metall., vol 44, no 9, 1997, 858–863. (In Japanese.)

Mechanical properties of aluminium-chromium alloys extruded from atomized powder : A.Inoue et al. (Tohoku University, Sendai, Japan.) J. Jpn Soc. Powder Powder Metall., vol 44, no 9, 1997, 858–863. (In Japanese.)

The effect of heat treatment on the corrosion behavior of sputter-deposited aluminum–chromium alloys

The effect of heat treatment on the corrosion resistance of sputter-deposited aluminum–chromium alloys containing 16–51 at% chromium has been studied in 0.1 and 0.5 M HCl. Structural relaxation decreases the corrosion rate of Al–16Cr alloy on which the passive film cannot be formed, since the corrosion rate is controlled by the reactivity of the alloy surface. The corrosion rate of spontaneously passive Al–35Cr alloy increases by crystallization. The chromium enrichment of the matrix, as a result of precipitation of a nanocrystalline aluminum-rich phase, results in enhancement of the corrosion resistance. However, if the size of the less corrosion-resistant aluminum-rich grains exceeds a critical limit of ∼20 nm, the corrosion resistance decreases.

Age-hardening characteristics of aluminium–chromium alloys

The age-hardening behaviour of Al–Cr alloys containing varying amounts of chromium has been studied, and the effect of silicon on the ageing response of the alloy system was also investigated. Hardness measurement and tensile properties evaluation were used to describe the ageing response of the alloys. With the aid of microstructural observation, the ageing mechanism has been explained. Results of isothermal ageing were used to describe the kinetics of precipitation in both the binary and ternary alloys. It was noticed that the age hardening in aluminium–chromium alloys was mainly due to the precipitation of intermetallics on to the dislocations. The ageing response of the binary alloy containing chromium in excess of its solubility limit was rather poor. However, addition of silicon helped to improve the ageing response of the alloys. Silicon was found to stimulate nucleation of ageing precipitates in the investigated alloys.

Electrodeposition of aluminum-chromium alloys from AlCl 3-BPC melt and its corrosion and high temperature oxidation behaviors

The binary aluminum-chromium alloys were electrodeposited from 2:1 AlCl 3- N-( n-butyl) pyridinium chloride room temperature molten salt. The chromium content of the electrodeposit varies from 0 to 94 at. % Cr) and is dependent upon the deposition parameters, applied current density, applied potential, and concentration of Cr(II) ion in the bath. Depending upon the deposition parameters, the deposit microstructure consisted of both Cr rich and Al rich solid solutions, and intermetallic compounds. The rate determining step for deposition process of Cr(II) ion is suggested as the charge transfer step from monovalent ion to metallic state. The electrochemical response of the deposited AlCr alloys containing various concentration of Cr (4 to 40 at. % Cr) showed a low passive current and suppressed transpassive dissolution current in a neutral buffer solution at pH 6 and 9. Excellent high temperature oxidation resistance of a series of AlCr alloys containing 20 to 50 at. % Cr was revealed between 600 and 800 °C.

Structures of mechanically alloyed titaniumaluminiumchromium alloy after sintering

Structures of mechanically alloyed titaniumaluminiumchromium alloy after sintering

Formation of microstructure of the icosahedral phase in rapidly solidified aluminium-chromium alloys

Rapid solidification of AlCr alloys resulted in the nucleation and growth of the icosahedral phase in the 14–36 wt% Cr range. The highest volume fraction of this phase was obtained between 21–26 wt% Cr. It was found that the different alloy compositions contain a variety of morphologies and microstructures which also depend on location regarding the cross section of the ribbon. The icosahedral phase is not stoichiometric and its maximum Cr content is between 26–30 wt% Cr. In the higher alloy compositions the stable intermetallic ϵ phase grows epitaxial on the icosahedral phase with a distinctive orientation relationship.

Deposition of metastable aluminium-chromium alloys by r.f. magnetron sputtering from mixed-powder targets

Aluminium-chromium alloys over the full binary range have been prepared at low pressure (0.7 Pa) and temperature (400 K) by r.f. magnetron sputtering from mixed-powder targets. For glass or metallic substrates, the deposits are primary metastable supersaturated single-phase solid solutions over the approximate ranges 0–5 at.%Cr and 0–82 at.%Al. At intermediate compositions, they consist of a mixture of these two solid solutions. The as-sputtered deposits are much harder than conventional alloys (hardness up to 10 000 MPa). By immersing in neutral aerated 30 g 1 −1 NaCl solution, the deposits containing less than 18 at.%Cr are anodic with respect to iron and can thus realize a galvanic protection of steel substrates.

Rotating water layer process for atomization of an aluminium-chromium alloy powder: J.J. Ramon et al, J of Metals, Vol 44, No 8, 1992, 15–19

Rotating water layer process for atomization of an aluminium-chromium alloy powder: J.J. Ramon et al, J of Metals, Vol 44, No 8, 1992, 15–19

Distribution of heterogeneous nucleants in aluminium-chromium alloy powders : N.J.E. Adkins, P. Tsakiropoulos. (University of Surrey, Guildford, England.) J. Materials Science and Engineering A, Vol A133, 1991, 767–770

Distribution of heterogeneous nucleants in aluminium-chromium alloy powders : N.J.E. Adkins, P. Tsakiropoulos. (University of Surrey, Guildford, England.) J. Materials Science and Engineering A, Vol A133, 1991, 767–770

Influence of mechanical grinding on the structure and mechanical properties of PM aluminium-chromium alloys : T. Morooka et al, J. Japan Inst. of Light Metals, Vol 41, No 1, 1991, 25–31. In Japanese

Influence of mechanical grinding on the structure and mechanical properties of PM aluminium-chromium alloys : T. Morooka et al, J. Japan Inst. of Light Metals, Vol 41, No 1, 1991, 25–31. In Japanese

Distribution of intermetallic compounds in high pressure gas atomized aluminium-chromium alloy powders: N.J.E. Adkins and P. Tsakiropoulos, (Univ of Surrey, Guildford, UK, Int J Rapid Solidification, Vol 6, 1991, 101–114

An aluminum–lithium–copper alloy was rapidly solidified via the electrospark deposition process. High-resolution scanning electron microscopy (HR-SEM), time-of-flight secondary-ion-mass-spectroscopy (TOF-SIMS) and atom probe tomography (APT) were employed to investigate the distribution of solute within the deposited materials. The TOF-SIMS data revealed evidence that solute trapping of lithium occurred during solidification, while SEM and APT revealed the presence of fine copper-rich cells within the microstructure (∼30–60 nm in width). This morphology correlated directly with the microstructural morphology predicted by the Kurz–Giovanola–Trivedi (KGT) model for microstructural development during rapid solidification. The KGT model, which can be used to describe the planar–cellular transition within a microstructure, then predicted a solidification front velocity of ∼1 m s−1 being realized during electrospark deposition solidification. This SFV corroborated the chemical mapping data, and therefore supported the solute trapping hypothesis, as the continuous growth model for solute trapping as developed by Aziz and Kaplan (Acta Metallurgica 1988; 36:2335) predicts significant trapping of lithium at a SFV of 1 m s−1. Finally APT revealed the presence of Al3Li phase upon the copper-rich cell walls. It was then determined that the Al3Li was not formed during solidification, as predicted by a time-dependent nucleation model for phase prediction during rapid solidification, and therefore is the result of a subsequent aging process.

Solidification of high pressure gas atomized aluminium-chromium alloy powders : N.J.E.Adkins and P.Tsakiropoulos, (Univ of Surrey, Guildford, UK). Int, J, Rapid Solidification, Vol 6, 10991, 87–100

Solidification of high pressure gas atomized aluminium-chromium alloy powders : N.J.E.Adkins and P.Tsakiropoulos, (Univ of Surrey, Guildford, UK). Int, J, Rapid Solidification, Vol 6, 10991, 87–100

Substitution du chrome par de l'aluminium et du silicium dans des aciers a 13 % de chrome et 0,1 % de carbone

Aluminium and silicon were substituted to chromium in iron chromium containing initially 13 wt % of chromium and 0.1 wt % of carbon. The structures were studied after different thermal treatments: in every case the precipitation of carbides was observed. The main result is the decrease in the hardness of aluminium-chromium alloys when maintained at 1300°C, whereas it seems that alloys containing silicon showed a hardness maximum.

Peritectic transformations in crystallization of alloys of the Al-Cr system. I

Established laws of the nonequilibrium crystallization of aluminum-chromium alloys permit the conclusion that each peritectic horizontal on the diagram of the Al-Cr system (at 725, 930, and 1000°C) corresponds to a region of structural transformation to a liquid extending from aluminum is the vertical of an intermediate compound. Retention of the stability of the high-temperature structural state of the liquid during its cooling predetermines the possibility of a delay in the peritectic transformation, crystallization according to metastable diagrams and, in individual cases, the formation of supersaturated solid solutions based on aluminum.

Oxidation of tantalum coated with aluminum and aluminum-chromium alloys

The oxidation of Ta and Ta-10%W hot-dipped in aluminum and Ta hotdipped in Al-Cr alloys has been studied in the temperature range 800–1600°C and at oxygen pressures ranging from 0.1 torr to 1 atm O 2. The experimental studies comprised thermogravimetric measurements of oxidation, and X-ray diffraction studies, metallographic investigations, and electron-microprobe analyses on reacted specimens. The unalloyed coatings on Ta and Ta-10%W gave at 0.1 torr O 2 excellent oxidation resistance at temperatures above 1100°C, but at 800–1000°C breakaway oxidation occurred. The protectivity is provided by an α-Al 2O 3 layer. At 1 atm O 2 the unalloyed coatings failed at all temperatures. The interdiffusion of aluminum and tantalum leads to the formation of TaAl 3, Ta 3Al, the Ta-Al σ-phase with approximately 7–9 wt.%Al, and a Ta-Al phase with approximately 20 wt.%Al. The Al-Cr coatings exhibited significantly improved performance compared with the pure aluminide coatings. At 0.1 torr O 2, Al-20% Cr and Al-30%Cr coatings on Ta provided excellent oxidation protection above 1000°C and at 800–1000°C breakaway oxidation was significantly retarded. Increasing concentrations of Cr also reduced failures at 1 atm O 2, and good protection could be achieved with Al-30%Cr coatings in 1 atm O 2 at 1500°C. The protectivity is provided by a layer of the α-Al 2O 3 phase (Al 2O 3-Cr 2O 3 solid solution).

New method for electroplating aluminum: Brass World, vii, 202.

For improvement of the oxidation resistance of a TiAl alloy under high temperature conditions, aluminum chromium alloys were electroplated on the TiAl alloy in an AlCl3–NaCl–KCl molten salt containing CrCl2 at 423 K. The deposit electroplated at −0.1 V vs. Al/Al3+ consists of γ-Al8Cr5 single phase, including chromium content at 41 at.%. A mixture phase of γ-Al8Cr5 and Al is, however, formed at potential from −0.2 to −0.4 V. The oxidation resistance of the electrodeposit layer was evaluated by a high temperature oxidation test at 1173 K for 24 h. On the TiAl specimen covered with γ-Al8Cr5 which was deposited at −0.1 V, a uniform AlCr2 layer was formed during the oxidation at 1173 K. The AlCr2 layer was covered by a thin dense Al2O3. Both AlCr2 and Al2O3 layers play a role of superior protection from the high temperature oxidation of the TiAl alloy; therefore, the plating of the γ-Al8Cr5 single layer deposited at −0.1 V reduced oxide thickness by 1/40 during the oxidation at 1173 K for 24 h from that on the TiAl without any plating.