E. Fleury

Effects of air plasma spraying parameters on the Al–Cu–Fe quasicrystalline coating layer

Abstract The focus of this study has been the effect of air plasma spray processing parameters on the characteristics of the Al–Cu–Fe quasicrystalline coatings. Key variables controlling the particle temperature and velocity and substrate temperature, such as the arc current, spraying distance, powder flow rate, and preheating of the substrate surface, were of primary interest. Significant effects of the process parameters on the microstructure and subsequent mechanical properties of the coatings were observed. Process conditions favorable for the formation of coating layers with larger amount of quasicrystalline phase led to the higher values of the microhardness, although it resulted in a relatively large porosity level. The bonding strength was found to be slightly dependent on the spraying distance, powder feeding rate, and preheating of the substrate. However, the predominant variable appeared to be the coating thickness, where a significant reduction of the bonding strength was reported as the coating thickness was increased up to 100 μm .

Comparison of Al-Cu-Fe quasicrystalline particle reinforced Al composites fabricated by conventional casting and extrusion

Aluminum matrix composites reinforced by Al62Cu26Fe12 gas atomized powders were produced by conventional metallurgical processes, such as gravity casting with stirring and hot extrusion. This investigation was mainly focused on the dependency of the yield stress at room temperature as a function of the volume fraction of reinforcement, but other variables such as the matrix, coating layer around the particles, and processing were also investigated. For as-extruded composites, the addition of the Al-Cu-Fe particles improved the yield stress, although not dramatically owing to the large particle size. In contrast, it was found that the yield stress was considerably enhanced for the as-cast composites up to 10%(AlCuFe)p, while an asymptotic value was observed afterward. The dominant parameter appeared to be the strength of the matrix, which was found to be proportional to the volume fraction of the reinforcement. These results are discussed in relation with the possible strengthening mechanisms in order to estimate the role of the icosahedral and related crystalline phases on the increase of yield stress.

Formation and stability of quasicrystalline and hexagonal approximant phases in an Al-Mn-Be alloy

Phase formation and thermal stability for an Al-Mn-Be alloy have been investigated by melt-spinning and conventional casting. Significant differences in the phase formation and the thermal stability of the microstructure were found as a result of the different cooling rates. In the melt-spun ribbons, a large volume fraction of a metastable icosahedral phase was found to coexist with an Al solid solution. In the bulk cast ingots, the primary phase formed in the two-phase microstructure was a hexagonal approximant phase of quasicrystals. This phase that solidified in the form of faceted particles embedded in the Al solid matrix proved to be thermodynamically stable during annealing at 540 °C for 100 h. The effect of Be addition on the formation of the stable approximant phase is discussed in terms of the Hume-Rothery mechanism.

Comparative study of the tribological behavior of thermal sprayed quasicrystalline coating layers

To investigate the role of tribological reactions on the friction and wear of quasicrystalline materials, coatings with two alloy compositions have been prepared by plasma and HVOF spraying techniques. The tribolayers were characterized by the formation of a transfer film on the counterface and densification of the coating subsurfaces. It was observed that the thickness of the transfer film and pore-free region were dependent on the composition and process used for the deposition of the coatings as well as the sliding velocity. As the sliding velocity increased, the growth rate of the transfer film decreased, resulting in a decrease of the coefficient of friction. On the other hand, the wear rate appeared to be controlled by the thickness of the pore-free region formed within the coating surface zone.

Enhancement of the quasicrystal-forming ability in Al-based alloys by Be-addition

Abstract The influence of beryllium (Be) addition on the quasicrystal-forming ability (QFA) in Al–Cu–Fe and Al–Mn systems has been investigated using conventional solidification technique. For a series of as-cast (Al 62− x Be x )Cu 25.5 Fe 12.5 ( x =0,1,3,5,7) alloys, the Be addition modified the icosahedral phase formation mechanism from peritectic reaction to primary solidification and resulted in the increase in the volume fraction of the i-phase from 40% for x =0 to 90% for x =7. Such an enhancement of the QFA with the increase of Be content can be represented by using the reduced quasicrystal transition temperature proposed in the present paper. In Al–Mn alloy system, microstructural changes due to the addition of Be were examined in detail by comparing as-cast Al 90 Mn 2.5 Be 7.5 alloy with Al 97.5 Mn 2.5 bulk ingot. For this alloy system, the substitution of Al by Be was found to reduce substantially the critical Mn-content and cooling rate necessary for the formation of the i-phase.

Fabrication of a bulk icosahedral material through mechanical alloying of the powder mixture Ti41.5Zr41.5Ni17

Abstract A bulk material that consists of the stable icosahedral phases (I-phases) in the Ti–Zr–Ni system has been fabricated in a hot press using mechanically alloyed (MA) powders. Considerable amounts of amorphous phase and nanocrystals are formed after MA for more than 30 h in the mixture of elemental powders (Ti, Zr, Ni) with the overall composition of Ti 41.5 Zr 41.5 Ni 17 . The amorphous phase transforms into the I-phase in the temperature range of 573–673 K during continuous heating in differential scanning calorimetry (DSC) with the heating rate of 40 K/min. The powders MA for 40 h are hot-pressed for 2 h at 873 K under the applied pressure of 900 MPa to form a bulk I-phase material. The evolution of the Vickers microhardness has been examined in the load range of 0.5–10 N. The indentation size effect is accompanied by a modification of the fracture pattern. At a low value of the load, no crack is observed. As the load increases, the first cracks observed around the indenter are lateral cracks, and then radial cracks are found for load larger than 1 N.

Poisson’s ratio and fragility of bulk metallic glasses

The correlation among apparent global plasticity, Poisson’s ratio, and fragility in monolithic bulk metallic glass (BMG) alloys has been investigated in the present study. The shear and bulk moduli in monolithic Cu-based BMG alloys have been measured by resonant ultrasound spectroscopy (RUS) and ultrasonic technique. The Cu 43 Zr 43 Al 7 Ag 7 BMG alloy showing a large apparent global plasticity (∼8%) exhibits a high Poisson’s ratio when compared with that of Cu 43 Zr 43 Al 7 Be 7 BMG alloy. In addition, the fragility of Cu-based BMG alloys can be obtained by differential scanning calorimetry (DSC). The fragility index m of Cu 43 Zr 43 Al 7 Ag 7 BMG alloy is slightly larger than that of Cu 43 Zr 43 Al 7 Be 7 BMG alloy. The correlation between Poisson’s ratio and fragility in BMG alloys can be presented by a simple relation of m − 17 = 14 ( B ∞ / G ∞ − 1). Poisson’s ratio and fragility might be regarded as an important parameter that controls global plasticity of glass-forming alloys.

Tribological properties of Al–Ni–Co–Si quasicrystalline coatings against Cr-coated cast iron disc

Abstract The tribological behavior of Al–Ni–Co–Si quasicrystalline (QC) coatings prepared by air plasma spray (APS) and high velocity oxy-fuel (HVOF) techniques was investigated in dry sliding condition against a Cr-coated cast iron disc. Tests were performed for different parameters of the load, sliding velocity, and temperature under a reciprocal motion. Our results demonstrated the influence of the coating microstructure on the frictional and wear behavior. Values of the coefficient of friction measured for HVOF coatings against Cr-coated cast iron disc were found to be about 14% lower than those of APS coatings. Under the standard condition, HVOF coatings exhibited a better wear resistance than APS coatings, however, this difference was reduced for tests performed under the highest values of the load and temperature investigated in this study. The sliding conditions imposed in this study led to significant modifications of the contacting surfaces characterized by the formation of a transfer film on the counterpart material, densification of the coating subsurface, and generation of wear particles. The variations of the coefficient of friction with the testing parameters were explained by the third body concept. It was also shown that the wear performance was dependent on the local surface properties of the QC coatings.

Sliding friction and wear behavior of Al-Ni-Co-Si quasicrystalline coatings deposited by the high-velocity oxy-fuel spraying technique

The sliding friction and wear performance of Al–Ni–Co–Si quasicrystalline coatingsdeposited by the high-velocity oxy-fuel technique were investigated under dry slidingconditions. This study indicated that changes in the imposed sliding test conditionsmodified the friction and wear behavior of quasicrystalline coatings. Qualitativeanalysis of the contact interface and wear debris were performed with the aim ofunderstanding the role of the third body on the friction and wear processes.The dependence of the coefficient of friction on the sliding velocity and counterpartmaterial was explained by the stick-slip behavior. It was also shown that testconditions favorable for the formation of thick intermediate layers and the densificationof the coating subsurface led to low wear rates. Large cylindrical particles, formed byagglomeration of small wear debris, were suggested as a beneficial factor for thereduction of the coefficient of friction.I. INTRODUCTIONFollowing the discovery of ordered crystals withquasi-periodicity by Shechtmanet al.

Evaluation of the mechanical properties of conventionally-cast Al matrix composites reinforced by quasicrystalline Al-Cu-Fe particles using continuous ball indentation technique

Room temperature mechanical properties of the Al/(AlCuFe)p and Al96Cu4/(AlCuFe)p cast composites were estimated from uniaxial compressive test and continuous ball indentation technique. Values of the Young’s modulus and yield stress determined from continuous ball indentation tests were slightly overestimated, suggesting a surface effect on the mechanical properties. However, it was shown that the Al-Cu-Fe particles provided a significant increase of the elastic modulus, yield stress, and strain hardening, especially in the range up to 10% volume fraction of reinforcements. Also, determination of the hardness by continuous-ball-indentation tests revealed a strong influence of the matrix strength on the mechanical properties of the conventionally cast composites.