Icosahedral Phase Particles Research Articles

The effect of volume fraction and dispersion of icosahedral phase particles on the strength and work hardening of Mg-Zn-Y alloys

The cast microstructure of a Mg-13Zn-1.55Y alloy (ZW132) with a high volume fraction of I-phase (7.4%) was refined considerably by severe plastic deformation via high-ratio differential speed rolling (HRDSR). Ultrafine grains (0.7–1.3µm) with high angle boundary fractions of 0.48–0.50 were obtained after HRDSR with speed ratios of 2 or 3. The alloy processed at a speed ratio of 3 exhibited high strength and high ductility, with a yield stress of 332MPa and a tensile elongation of 16.3%. The ductility of the rolled ZW alloy was controlled by the work hardening rate, which increased as the amount of I-phase, the degree of refinement of the eutectic I-phase pockets, the degree of dispersion of the broken I-phase particles over the matrix, and the size of the resultant grains increased. A model that considers the contribution of these factors to the work hardening rate was proposed. Grain-size reduction was found to be a major strengthening mechanism. Dispersion of the I-phase particles broken from the eutectic I-phase pockets did not contribute significantly to the material's strength.

Formation of ω -Al7Cu2Fe phase during laser processing of quasicrystal-forming Al–Cu–Fe alloy

The formation of an ω-Al7Cu2Fe phase during laser cladding of quasicrystal-forming Al65Cu23.3Fe11.7 alloy on a pure aluminium substrate is reported. This phase is found to nucleate at the periphery of primary icosahedral-phase particles. A large number of ω-phase particles form an envelope around the icosahedral phase. On the outer side, they form an interface with an α-Al solid solution. Detailed transmission electron microscopic observations show that the ω phase exhibits an orientation relationship with the icosahedral phase. Analysis of experimental results suggests that the ω phase forms by precipitation on an icosahedral phase by heterogeneous nucleation and grows into the aluminium-rich melt until supersaturation is exhausted. The microstructural observations are explained in terms of available models of phase transformations. § Currently as Assistant Professor, Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur, India

Effect of icosahedral phase particles on the texture evolution in Mg–Zn–Y alloys

In order to understand the role of the icosahedral phase (I-phase) particles on the improved mechanical properties of Mg–3Zn–0.6Y and Mg–6Zn–1.2Y alloys, the texture evolution has been investigated by the X-ray diffraction and electron back-scattered diffraction (EBSD) analyses. Mg–6Zn–1.2Y alloy exhibits higher strength than Mg–3Zn–0.6Y alloy due to the presence of larger amount of I-phase particles. However, Mg–6Zn–1.2Y alloy exhibits a larger elongation than Mg–3Zn–0.6Y alloy. With increasing the volume fraction of I-phase particles and rolling speeds, the relatively random texture is developed, which is attributed to local lattice rotations around I-phase particles during shear deformation. The relatively random texture plays an important role in enhancing the ductility when larger amount of I-phase particles are present in Mg–6Zn–1.2Y alloy.

Effect of Crystallization on High-Temperature Plastic Flow and Ductility in Pre-Annealed Zr-Al-Ni-Cu Bulk Metallic Glass

High-temperature plastic flow and tensile ductility in pre-annealed Zr65Al10Ni10Cu15 bulk metallic glass are examined in a supercooled liquid region. The specimen annealed at 673K for 1.8ks before the tensile test exhibits uniform deformation, and large elongation of 1100% is obtained in the tensile test at 673K and in a constant true strain rate of 1×10-2s-1. In the specimen, initial stress decreases in comparison with that of the specimen without annealing, but apparent strain hardening arise during deformation. SEM analysis after the deformation reveals that the cavitation is very limited even at the fracture tip. XRD and TEM analyses reveal that icosahedral phase is observed only in gage section but not in grip section after the deformation. The limited cavitation due to the accommodation of stress concentration through the grain boundary amorphous phase possibly makes a positive contribution to the large elongation in the annealed specimen, as well as the neck stability due to the strain hardening by the interaction between the icosahedral phase particles increasingly crystallized by the local strain.

Strengthening effects of icosahedral phase in magnesium alloys

Microstructure and strength of two alloys Mg95Zn4.2Y0.8 and Mg92.5Zn6.5Y containing icosahedral phase have been studied after extrusion at 250°C and 400°C. Due to its high symmetry and quasiperiodicity, the icosahedral phase can form strong interfaces with the matrix in various orientations. The icosahedral phase particles have a strong pinning effect on the grain boundaries that stabilizes grain size. They are resistant to coarsening, and remain hard at higher temperatures, imparting good strength with ductility at 200°C. Very few deformation structures such as high dislocation density and twins are observed after extrusion or tensile tests. c-type dislocations are commonly observed, which has important implications for greater ductility. Due to the stability of microstructure, various post-extrusion treatments are possible.

Enhancement of Plasticity in Ti-Based Bulk Metallic Glass

The effects of structural relaxation and partial crystallization on the mechanical property of the Ti40Zr29Cu9Ni8Be14 bulk metallic glass (BMG) have been investigated. The atomic structure of the as-cast Ti40Zr29Cu9Ni8Be14 metallic glass transforms into a more relaxed state at the temperature region of 452 –585 K, below the crystallization onset temperature of 631 K. Stable icosahedral phase forms in the amorphous matrix by growth of the pre-existing nuclei in the amorphous matrix during first crystallization step. The compressive plastic strain of the as-cast Ti40Zr29Cu9Ni8Be14 BMG is 6.7 %, and decreases when the structural relaxation occurs. However, the plastic strain increases when a few nanometer size icosahedral phase particles form in the amorphous matrix by the partial crystallization treatment.

Glass-Forming Ability and Mechanical Properties of Fe-Ni-Zr-Si-B Bulk Glassy Alloys

The effects of structural relaxation and partial crystallization on the mechanical property of the Ti40Zr29Cu9Ni8Be14 bulk metallic glass (BMG) have been investigated. The atomic structure of the as-cast Ti40Zr29Cu9Ni8Be14 metallic glass transforms into a more relaxed state at the temperature region of 452 –585 K, below the crystallization onset temperature of 631 K. Stable icosahedral phase forms in the amorphous matrix by growth of the pre-existing nuclei in the amorphous matrix during first crystallization step. The compressive plastic strain of the as-cast Ti40Zr29Cu9Ni8Be14 BMG is 6.7 %, and decreases when the structural relaxation occurs. However, the plastic strain increases when a few nanometer size icosahedral phase particles form in the amorphous matrix by the partial crystallization treatment.

Effect of Microstructural Change on High-Temperature Deformation in Pre-Annealed Zr<SUB>65</SUB>Al<SUB>10</SUB>Ni<SUB>10</SUB>Cu<SUB>15</SUB> Bulk Metallic Glass

The effect of the microstructural change on the high-temperature tensile deformation behavior in the supercooled liquid region is investigated in the pre-annealed Zr 65 Al 10 Ni 10 Cu 15 bulk metallic glass. The microstructure before the tensile test on the specimen annealed at 673 K for 1.8ks shows that a small amount of icosahedral phase precipitates and a large amount of amorphous phase still remains. On the contrary, the specimen annealed at 673 K for 2.7 ks shows that a large amount of icosahedral phase precipitates and the presence of amorphous phase is hardly noticeable. From the fact that flow stress of the specimen annealed at 673 K for 2.7 ks increases in comparison with that of asreceived specimen as well as the specimen annealed at 673 K for 1.8 ks in the tensile test, it seems that the deformation behavior in the tension tests is greatly influenced by the difference in the degree of the precipitation of icosahedral phase. Since the m value is about 0.5 and many grain boundaries of icosahedral phase are formed in the specimen annealed at 673 K for 2.7 ks, the interaction between the icosahedral phase particles must be the cause of increase in initial stress. Accordingly, the microstructure of the gage section after tensile test in the specimen annealed at 673 K for 1.8 ks shows that the large amount of icosahedaral phase precipitates in the amorphous matrix. The strain hardening must be caused by the interaction between the icosahedral phase particles whose precipitation and growth are enhanced by deformation.

Strengthening in magnesium alloys by icosahedral phase

Strengthening effects of quasicrystalline icosahedral phase has been studied in two alloys Mg95Zn4.2Y0.8 and Mg92.5Zn6.5Y extruded at 250 and 400 8C. The quasicrystal particles are facetted and show definite orientation relationships with the matrix. Due to its high symmetry and quasiperiodicity, the icosahedral phase can form strong interfaces with the matrix in various orientations. The icosahedral phase particles have a strong pinning effect on the grain boundaries, which stabilizes grain size. The icosahedral particles are resistant to coarsening, and remain hard at higher temperatures, imparting good strength with ductility at 200 °C. Very few deformation structures such as high dislocation density and twins are observed after extrusion or tensile tests. Dislocations commonly observed are c-type. Due to the stability of microstructure, various post-extrusion treatments are possible. In the Mg92.5Zn6.5Y alloy upon annealing at 400 °C the icosahedral phase transforms to a hexagonal Mg25Zn58Y17 phase. The icosahedral phase then reprecipitates on its interface, forming a nano-composite. Effects of microstructural features on the deformation behavior are described.

The ω Phase Formation during Laser Cladding and Remelting of Quasicrystal Forming AlCuFe on Pure Aluminum

ABSTRACTWe report the formation ω phase in the remelted layers during laser cladding and remelting of quasicrystal forming Al65Cu23.3Fe11.7 alloy on pure aluminum. The ω phase is absent in the clad layers. In the remelted layer, the phase nucleates at the periphery of the primary icosahedral phase particles. A large number of ω phase particles forms enveloping the icosahedral phase growing into aluminum rich melt, which solidify as α-Al solid solution. On the other side it develops an interface with aluminum. A detailed transmission electron microscopic analysis shows that ω phase exhibits orientation relationship with icosahedral phase. The composition analysis performed using energy dispersive x-ray analyzer suggests that this phase has composition higher aluminum than the icosahedral phase. The analysis of the available phase diagram information indicates that the present results represent large departure from equilibrium conditions. A possible scenario of the evolution of the ω phase has been suggested.

Nano-quasicrystalline phase formation in Mg–Cd–Yb alloys

The microstructure of Mg 60+ x Cd 25− x Yb 15, x=0, 5, 10, Mg 50Cd 30Yb 20 and Mg 25Cd 65Yb 10 as-cast alloys has been examined by transmission electron microscopy. While the Mg 60Cd 25Yb 15 alloy showed only an icosahedral quasicrystalline phase, the magnesium rich alloys also showed a Zn 2Mg type (Mg,Cd) 2Yb Laves phase. The alloys showed nanosized particles of icosahedral phase. Eutectic microstructures involving the icosahedral phase were observed. A lamellar (50 nm) structure of icosahedral phase and Mg was observed, exhibiting a definite orientation relationship with the hexagonal axis along one of the twofold axes of the icosahedral phase. In the Cd-rich alloys the cubic Cd 6Yb approximant phase occurred, along with the icosahedral phase and intermediate structures in nanocrystalline form.

Quasicrystalline grain boundary precipitates in aluminium alloys through solid‐solid transformations

SUMMARYWe have observed the formation of icosahedral quasicrystals of Al‐Mg‐Zn, Al‐Mg‐Cu, Al‐Li‐Zn and Al‐Li‐Cu by low temperature annealing of solid solution alloys. The icosahedral phase particles form at the grain boundaries of the matrix alloys. The orientational relationships between the icosahedral crystallites, their corresponding Frank‐Kaspar phase particles and the aluminium matrix are reported. The stability of the icosahedral phases is also discussed. Finally, the importance of grain boundary icosahedral phase particles to the physical properties of commercial aluminium alloys is pointed out.