Cr Al Alloys Research Articles

Structure and property transitions of Al-based binary alloy coatings by magnetron sputtering

Al–M (M=Ti, Cr, Mo, W) alloy coatings were subjected to DC magnetron sputtering by a rapid method to analyze the evolution of coating structures with composition. Pure Al and M targets were used to deposit coating arrays across the entire binary range of Al–M alloys. All Al–M alloys exhibited a specific amorphicity range. Al–Ti and Al–Cr alloys were completely amorphous, whereas Al–Mo and Al–W alloys possessed a range of amorphous–crystalline mixtures. Results revealed that Al–Ti alloys had the widest amorphicity range. Nanoindentation tests indicated that the hardness and modulus of amorphous alloys were higher than those of solid solutions for Al–M alloys.

The Role of Cr in H Desorption Kinetics in Rapidly Solidified Al

Hydrogen desorption kinetics for rapidly solidified high purity Al and Al-Cr alloy foils containing 1.0, 1.5 and 3.0 at % Cr were investigated by means of thermal desorption analysis (TDA) at a heating rate of 3.3°C/min. For the first time, it was found that oxide inclusions of Al2O3 are dominant high-temperature hydrogen traps compared with pores and secondary phase precipitates resulted in rapid solidification of Al and its alloys. The correspondent high-temperature evolution rate peak was identified to be positioned at 600°C for high purity Al and shifted to 630°C for Al-Cr alloys. Amount of hydrogen trapped by dislocations increases in the alloys depending on Cr content. Microstructural hydrogen trapping behaviour in low-and intermediate temperature regions observed here was in coincidence with previous data obtained for RS materials using thermal desorption spectroscopy (TDS). The present results on hydrogen thermal desorption evolution indicate that the effect of oxide surface layers becomes remarkable in TDA measurements and show advantages in combinations of both desorption analysis methods to investigate hydrogen desorption kinetics in materials.

Preparation and characterization of Al11Cr4 phase by diffusion of Al/Cr composite film

Al–Cr alloys were prepared by making Al/Cr composite films go through a low temperature heat treatment. Al/Cr composite films were electrodeposited from aqueous solution and ionic liquid successively. The effects of the composition of Al/Cr composite film on the phase constitutions of Al–Cr alloy were then investigated. The samples were characterized by metallographic microscope (OM), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and spherical aberration corrected transmission electron microscope (Cs-TEM). The decomposition temperature of Al–Cr alloy was studied using differential thermal analysis. The results showed that single Al11Cr4 phase can be obtained from the Al/Cr composite film at the composition range of 75.0–76.0at.% Al. The Al11Cr4 would decompose into Al4Cr and Al8Cr5 at 829°C.

Icosahedral quasicrystal-enhanced nucleation of the fcc phase in liquid gold alloys

Although the grain refinement of yellow gold alloys with Ir has been used in industry and known for nearly half a century, the basic mechanism is still unknown. The present contribution shows that the mechanism is the same as that evidenced recently in Al–Zn alloys, when small amounts of Cr (1000ppm) are added to the melt (Kurtuldu et al., 2013). The reduced face-centered cubic (fcc) crystal size, the abnormal fraction of twin, or near-twin, grain boundaries and the 5-fold symmetry crystallographic orientation of multiple nearest-neighbor grains reproduce the symmetry of icosahedral quasicrystals (iQCs) with the following heteroepitaxial relationships: {111}fcc/〈110〉fcc⊥ 3-fold/2-fold symmetry axes of iQCs. While iQCs and the approximant stable Al45Cr7 phase, which contains several 5-fold symmetry building blocks in its unit cell, are known to exist in Al–Cr alloys, no such phases have been reported for yellow gold +Ir. Nevertheless, when minute amounts of Ir (≲200ppm) are added to the gold alloy melt, it is shown that the grain refinement from 248 to 30μm is accompanied by a spectacular increase in the fraction of twinned grain boundaries, i.e. from less than 1% without Ir to 11% with 200ppm Ir. Furthermore, up to 9 grains have been shown to reproduce the six 5-fold symmetry axes of the icosahedron, while many other grain configurations exhibit this heteroepitaxial relationship with the icosahedron or interlocked icosahedron. This confirms that fcc crystals can form in a supercooled liquid by heteroepitaxial growth from an iQC template.

Interface electrochemical corrosion behaviour of alclad Al–Zn–Mg–Cu–Cr Al alloy adhesively bonded with carbon fibre reinforced plastic stiffener

Stiffening of Al alloy aircraft structures with carbon fibre reinforced plastic laminate called ‘patches’ is considered as a viable option to enhance fatigue life. Interface so created between the reinforcement and Al alloy structure is potential site for crevice corrosion attack. Therefore, studies were conducted to investigate the interface corrosion behaviour of peak and two-step aged Al–Zn–Mg–Cu–Cr alclad alloy with and without the patch and examine the possibility to control the interfacial attack with the addition of cerium chloride inhibitor exposed to 3·5 wt-% NaCl solution for the durations of 2 and 168 h. Electrochemical impedance spectroscopy revealed that the interface attack was significantly reduced by the addition of 1000 ppm cerium chloride. The response of the peak and two-step aged alloy for such an addition was further investigated and found that the alloy under step aged condition behaves better than that of the peak aged condition.

Phase-field study the effects of elastic strain energy on the occupation probability of Cr atom in Ni–Al–Cr alloy

The occupation probability (OP) of Cr atom at α sublattices and β sublattices of DO22–Ni3Cr and L12–Ni3Al in Ni–Al–Cr alloy was studied by using phase-field microelasticity model. The elastic strain energy (ESE) arised by a coherent misfit has little effect on the precipitation sequence of alloy while it changes the behavior of the temporal evolution OP of Cr atom in DO22 and L12 phases. With the ESE increasing, the OP of Cr atom at both α sublattices and β sublattices in DO22 phase increases, the OP of Cr atom at β sublattices in L12 phase decreases, and the OP of Cr atom at α sublattices in L12 phase increases. Eventually, the ESE leads to directional coarsening of coherent microstructure.

Trans-interface-diffusion-controlled coarsening of γ′ precipitates in ternary Ni–Al–Cr alloys

Published data on the coarsening behavior of γ′ precipitates in three ternary Ni–Al–Cr alloys are examined in light of the theory of trans-interface-diffusion-controlled (TIDC) coarsening, in which the kinetics is controlled by diffusion through the coherent precipitate–matrix interface. The experimental data are independent of the equilibrium γ′ volume fraction, as expected for TIDC coarsening. Kinetics of the type 〈r〉n∝t for the growth of precipitates of average radius 〈r〉, and X∞Al and X∞Cr∝t–1/n for the variations of the far-field matrix solute concentrations, X∞Al,Cr, with aging time, t, are characteristic of TIDC coarsening. The temporal exponent n≈2.4 was obtained from the fitting of published particle size distributions. Based on correlation coefficients, the dependencies of 〈r〉n on t and X∞Al,Cr on t−1/n were comparable for n=2.4 and n=3 (the temporal exponent for matrix-diffusion-controlled coarsening). The dependencies of volume fraction, f, and number density, Nv, on t are also compared with theoretical predictions. Using a thermodynamic model of the Ni–Al–Cr phase diagram, the interfacial free energy, σ, was estimated from analysis of the data; σ varies from ∼14.5 to 19mJm–2 in the three alloys. Interfacial diffusion coefficients, also obtained from analysis of the data, are greater than those in the γ′ phase but smaller than those in the γ phase, which is consistent with the demands of the TIDC theory. Comparisons with the results from previously published work are noted and all discrepancies are discussed.

Effect of nanoscale precipitation on strengthening of ferritic ODS Fe–Cr–Al alloy

The kinetics of Fe rich α and Cr rich α′ phase separation during aging of Fe–Cr–Al oxide dispersion strengthened alloys have been analysed with a combination of atom probe tomography and thermoelectric power measurements. The slight lattice parameter decrease measured by X-ray diffraction after thermal aging was associated with a lattice misfit between the emerging α and α′ domains. The elastic strain associated with this misfit and the different elastic moduli for the α and α′ phases would produce an increase in internal stresses that would retard the motion of dislocations and thus would be the main cause for the strengthening.

Effects of duty cycle and pulse frequency on the fabrication of AlCrN thin films deposited by high power impulse magnetron sputtering

High power impulse magnetron sputtering (HIPIMS) is a newly developed coating technology, characterized by its ultra-high peak current and peak power density to achieve unique thin film properties, such as high hardness, good adhesion and tribological performance. In this study, an AlCr alloy target was used to deposit AlxCr1−xN coatings on various substrates by the HIPIMS technique. The aim of this work was to systematically study the microstructure evaluation and mechanical properties of AlxCr1−xN coatings as a function of duty cycle and pulse frequency. The experimental results showed that the peak power density increased linearly as the duty cycle decreased from 5% to 2% at constant frequency of 1000Hz. A maximum peak power density of 2.76kWcm−2 was achieved at the duty cycle of 2% and repetition frequency of 833Hz. A mixture of cubic and hexagonal AlxCr1−xN phases was found for each coating. The x value of AlxCr1−xN thin films changed from 0.72 to 0.75 and showed no direct relationships with duty cycle and repetition frequency. The peak power density had great influence on the microstructure evolution and altered the microstructure from coarse columnar to fine columnar structure. Re-nucleation grains were also found between columnar grains. The hardness increased with increasing frequency at constant duty cycle of 2% and reached the optimum hardness of 39.9GPa at x=0.75. Good adhesion and wear resistance was observed for each coating. The critical load increased as the repetition frequency increased from 500 to 1250Hz at the same duty cycle of 2%. Since the wear resistance for each coating was excellent, the duty cycle and pulse frequency showed limited influence on the wear resistance of thin films.

Preparation of Self-Healing α-Al2O3 Films by Low Temperature Thermal Oxidation

This paper reports a new approach to lowering the temperature necessary for the preparation of α-Al2O3. Oxidation of Al–Cr alloys, with Cr contents of 18, 23 and 27 %, was performed at temperatures ranging from 620 to 720 °C in air for 100 h. The resulting oxide films were analyzed by SEM, EDS, XRD and XPS. The results showed that α-Al2O3 films were obtained following oxidation of the 18 and 23 wt% Cr alloy samples at 720 °C and that rough surfaces were conducive to the formation of α-Al2O3 such that peened surface samples showed significant α-Al2O3 growth while polished samples showed no oxide by XRD. A 23 wt% Cr sample with a roughened surface exhibited the formation of α-Al2O3 at a temperature of 670 °C. Conversely, only a very thin oxide film was observed on a 27 wt% Cr sample after oxidation at 720 °C.

Effect of annealing on microstructure and mechanical properties of large dimensional bulk nanocrystalline Fe–Al–Cr alloy prepared by aluminothermic reaction

Microstructure and mechanical properties of large dimension bulk nanocrystalline Fe–Al–Cr alloy after being annealed at 1073K for 16h and at 1273K for 8h, 16h, and 24h were investigated. It was shown that microstructure of the alloy after the annealing consisted of a nanocrystalline phase with a small amount of micron-sized Cr7C3 phase. The crystal structure of the alloy did not change and still kept the disorder bcc structure after annealing. With the increase of annealing temperature and time, average grain size of the nanocrystalline phase increased, and shape of the Cr7C3 phase transformed from needles to particles. The alloy after annealing had more stable deformation capacity and high strength in the compression at room temperature, and yield strength of the alloy after annealing was about triple the time of the Fe3Al based alloy with microcrystalline structure. The strength of alloy decreased after annealing at 1273K for 24h.

Microstructure and mechanical properties of a large-dimensional bulk nanocrystalline-based Fe–Al–Cr alloy prepared by an aluminothermic reaction casting and followed annealing at 1000 °C

A large-dimensional bulk nanocrystalline phase-based Fe–Al–Cr alloy with 10 wt.% Cr, which was about 200 mm in diameter and 10 mm in thickness, was prepared by an aluminothermic reaction casting and followed annealing at 1000 °C. Microstructures of the alloy were investigated by optical microscope, electron probe microscope, scanning electron microscope attached with electron backscattered diffraction, X-ray diffraction and transmission electron microscope. The magnetization curves of the alloy were tested by Lake Shore 7410 vibrating sample magnetometer. Compressive properties of the alloy were tested. The results show the alloy was consisted of a Fe–Al–Cr nanocrystalline matrix, Cr7C3 phase and contaminants in micrometre. Average grain size of the nanocrystalline matrix was 19 nm. Volume fraction of the Cr7C3 phase in the alloy was about 4.5%. After annealing, the saturated intensity of magnetization and the specific magnetic susceptibility of the alloy increased slightly from 99 emu/g and 0.083 emu/g Oe to 104 emu/g and 0.113 emu/g Oe, respectively. Compressive strength of the alloy was 1200 MPa and much higher than that of the small-scale nanocrystalline alloy and alloy with grains in micrometre.

Effects of Yttrium on the Oxidation Behavior of Low Sulfur Content Fe–Cr–Al Alloys for Heat Resistant Wires

The melting of FeCrAl–Y alloy in an electric furnace allows for a very low sulfur content to be obtained, of around 5 ppm or less. So, the question of the necessary quantity of yttrium to add in this ferritic alloy to reach the application requirements is once again of interest. Laboratory heats of FeCrAl with yttrium additions from 0 to 1,510 ppm and ≤6 ppm of sulfur content were melted, processed and studied. Oxidation kinetics in dry air were measured, using thermogravimetric measurements. High resolution scanning electron micrographic examinations provided a good description of the alumina oxide scales. These investigations were completed with a quantification of secondary phases according to the yttrium content of each melt. The results were analyzed using the oxidation mechanisms in relation to the yttrium content. Finally, a moderate level of yttrium was defined to reach the longest lifetime during the cyclic oxidation test of the commercial Resistohm® Y grade.

Effect of initial transpassive treatment on properties of passive film formed on Fe–Mn–Al–Cr alloy

The present study explores an electrochemical surface modification process to improve the corrosion resistance of Fe–24Mn–4Al–5Cr alloy. The effects of transpassive aging potential and time on the corrosion resistance, composition and stability of the passive film formed on an Fe–24Mn–4Al–5Cr alloy in 1M Na2SO4 solution were studied using combined electrochemical measurements, Auger electron spectroscopic and X-ray photoelectron spectroscopic analysis. The passivation of Fe–24Mn–4Al–5Cr alloy in 1M Na2SO4 solution was performed under the optimal parameters of critical potential of 1100 mV and for a passivation time of 30 min. The corrosion resistance in 1M Na2SO4 solution of Fe–24Mn–4Al–5Cr alloy modified by this treatment is superior to that of Fe–13Cr–0·1C stainless steel. Auger electron spectroscopic and X-ray photoelectron spectroscopic depth profiles of the passive film formed on the alloy using this treatment show Al2O3 and Cr2O3 enrichment together with Fe and Mn depletion in the passive film. This compositional change provides the increase in stability and in corrosion resistance of the alloy.

Castability and Microstructural Development of Iron-based Alloys under Conditions Pertinent to Strip Casting – Specialty Fe–Cr–Al Alloys

Castability and Microstructural Development of Iron-based Alloys under Conditions Pertinent to Strip Casting – Specialty Fe–Cr–Al Alloys

Effect of chromium content on microstructure and mechanical properties of large dimensional bulk nanocrystalline based Fe–Al–Cr alloys prepared by aluminothermic reaction

Large dimensional bulk nanocrystalline based Fe–Al–Cr alloys with 5, 10 and 15wt.% Cr which were about 200mm in diameter and 10mm in thickness was prepared by an aluminothermic reaction casting method. Microstructures of the alloys were investigated by scanning electron microscope (SEM) attached with energy dispersive spectrometer (EDS) and electron backscattered diffraction (EBSD), X-ray diffraction (XRD) and transmission electron microscope (TEM). Compressive properties of the alloys were tested. The results show that the alloys were consisted of a Fe–Al–Cr nanocrystalline matrix with the grain size in a range of 15–18nm and a precipitated Cr7C3 phase which dispersed homogeneously in the matrix. The content of Cr dissolved in the nanocrystalline matrix, volume fraction and morphology of the Cr7C3 phase varied with the increasing of Cr element added in the alloys. Compressive strength of the alloys was in a range of 1100–1200MPa. The alloy with 10wt.% Cr had the highest yield strength of about 1048MPa, while the alloy with 15wt.% Cr had the lowest yield strength of about 911MPa. The alloys showed considerable ductility and best plastic deformation capability for that of 15wt.% Cr.

Influence of composition on monomodal versus multimodal γ′ precipitation in Ni–Al–Cr alloys

This study investigates the influence of alloy composition on γ′ precipitation in Ni–8Al–8Cr and Ni–10Al–10Cr at.% during continuous cooling from a supersolvus temperature. When subjected to the same cooling rate, Ni–8Al–8Cr develops a monomodal population, whereas Ni–10Al–10Cr develops a multimodal (primarily bimodal) population of γ′ precipitates. The bimodal γ′ precipitate size distribution in Ni–10Al–10Cr alloy can be attributed to two successive nucleation bursts during continuous cooling while the monomodal γ′ size distribution in Ni–8Al–8Cr results from a single nucleation burst followed by a longer time—wider temperature window for nucleation resulting in a larger number density of precipitates. Three-dimensional atom probe investigations reveal that while local equilibrium is achieved across the γ/γ′ interface, far-field γ compositions still retain an excess of Al and a depletion of Cr with respect to equilibrium, thus giving direct experimental evidence of the driving force for subsequent nucleation bursts during continuous cooling in case of Ni–10Al–10Cr. Contrastingly, in case of the Ni–8Al–8Cr alloy, while the γ phase retains an excess of Al and a depletion of Cr with respect to equilibrium after continuous cooling to room temperature, the additional driving force is consumed during subsequent isothermal annealing by growth of the large number of γ′ precipitates without any new nucleation.

Fabrication of Ni–Al–Cr alloy anode for molten carbonate fuel cells

Ni-(3,5) wt.% Al-(3–10) wt.% Cr alloys suitable for use as an anode material for molten carbonate fuel cells (MCFCs) were prepared by a solid-gas-solid diffusion method. The creep resistances and sintering resistances of the prepared Ni–Al–Cr alloy samples evaluated in the creep test system were significantly improved with increasing Al and Cr concentration. The Ni-5 wt.% Al-10 wt.% Cr alloy showed the best creep resistance with a creep strain of 1.7% among as-prepared samples. To determine the electrochemical performance of the Ni–Al–Cr alloy anode, a single cell test was carried out. The cell performances of all the Ni–Al–Cr alloy anodes were above 0.8 V at 150 mA cm−2 for 1000 h of operation. It was also verified that the addition of Cr to Ni–Al enhances the ductility of the alloy and thus reduces micro-crack formation in the Ni–Al–Cr anode during cell operation. Therefore, it can be concluded that the Ni–Al–Cr alloy anode is a good candidate anode material for MCFCs.

Phase separation kinetics in a Fe–Cr–Al alloy

The α–α′ phase separation kinetics in a commercial Fe–20wt.% Cr–6wt.% Al oxide dispersion-strengthened PM 2000™ steel have been characterized with the complementary techniques atom probe tomography and thermoelectric power measurements during isothermal aging at 673, 708, and 748K for times up to 3600h. A progressive decrease in the Al content of the Cr-rich α′ phase was observed at 708 and 748K with increasing time, but no partitioning was observed at 673K. The variation in the volume fraction of the α′ phase well inside the coarsening regime, along with the Avrami exponent 1.2 and activation energy 264kJmol−1, obtained after fitting the experimental results to an Austin–Rickett type equation, indicates that phase separation in PM 2000™ is a transient coarsening process with overlapping nucleation, growth, and coarsening stages.

Order controlled dislocations and grain boundary mobility in Fe–Al–Cr alloys

The dislocations and grain boundary mobility in elastic range of loading has been analysed by means of mechanical spectroscopy and neutron diffraction studies in Fe–25at.%Al–8at.%Cr and Fe–25at.%Al–25at.%Cr alloys, in relation to the order degree of the structure. If the alloys are in the ordered state (D03 or B2), the mobility of dislocations and grain boundaries is markedly reduced promoting small values of damping. In contrast, when the ordering is suppressed by annealing at temperatures above 973K, the mobility of dislocations and grain boundaries increases. This leads to an increase in overall damping. In particular, thermally activated damping peaks related to the solute grain boundary peaks due to presence of the aluminium and chromium atoms appear within the temperature interval from 900 to 1100K, depending on the substitutional atomic content.