Icosahedral Quasicrystalline Phase Research Articles

Microstructure Regulation and Mechanical Properties of Mg–6xZn–xY Alloys Containing Icosahedral Quasicrystalline Phases

Mg–6xZn–xY (x = 0.8, 5, and 7 wt%) alloys containing an icosahedral quasicrystal phase (I‐phase) are synthesized, and the effects of Y doping on the microstructure and mechanical properties of the Mg matrix and I‐phase are investigated. The results show that as the Y content increases, the I‐phase content gradually increases, with the distribution of the I‐phase changing from a discontinuous to a continuous network and lamellar structures. Additionally, the eutectic structure (α‐Mg + I‐phase) in the alloys gradually increases, while the Mg matrix area decreases. The tensile strength of the as‐cast alloy initially increases and then decreases, reaching a maximum value at Y = 5. Heat treatment of the Mg–30Zn–5Y alloy reveals an optimal solid solution temperature of 320 °C, resulting in a tensile strength of 175.09 MPa and quasicleavage fracture characteristics. The maximum tensile strength of the alloy is achieved when the I‐phase distribution is continuous and the porosity of the Mg matrix is ≈66%. These results provide guidance for the microstructural design of I‐phase‐reinforced Mg alloys with rare earth elements to enhance their mechanical properties.

Icosahedral Quasicrystalline Phase on the Surface of Spherical Particles in Al-Cu-Fe Alloy

Icosahedral Quasicrystalline Phase on the Surface of Spherical Particles in Al-Cu-Fe Alloy

Identifying the in-situ origin of supercluster-induced ductile precipitates in Zr-based bulk metallic glasses

A sandwich architecture including double crystal substrates and a single glass-forming droplet has been successfully designed based on the simplified liquid-bridge methods in multicomponent alloys. The multiscale precipitates mainly consisting of β-phase dendrites with a body-centered cubic (BCC) structure, cubic CuZr2 phase, and icosahedral quasicrystalline phase (I-phase) could be simultaneously identified in dissimilar regions of the glass-forming matrix during heat-induced dissolution or diffusion from crystal substrate to glass-forming droplet. The gradient distributions of crystal-like clusters not only accelerate diffusion velocity due to the collective atomic motion but also can simultaneously modulate the atomic-scale structure, mechanical response, and chemical composition via tracing a high-throughput strategy. The present work provides new insights to discover the formation mechanism of the representative microstructures strongly associated with the atomic-scale structure-property-composition relationships and is also helpful to shed light on the in-situ origin or structural mechanism of supercluster-induced multiscale precipitates in developing bulk metallic glasses (BMGs) and in-situ formed BMG composites (BMGCs).

Study on fracture toughness of quasicrystalline phase in Mg-42Zn-7Y alloy based on indentation technique

The Mg-42Zn-7Y alloy, which contains an icosahedral quasicrystalline phase, was prepared using a conventional solidification process. The fracture toughness of this quasicrystalline phase in the Mg-42Zn-7Y alloy was nondestructively detected using in situ tensile and indentation techniques. The alloy exhibited a tensile strength of 99.17 MPa and an elongation of 3.9%. Microscopic observation and energy spectrum analysis confirmed the high-contrast lamellar structure as the quasicrystalline phase. Ultimately, under cyclic loading during indentation, the fracture toughness of the quasicrystalline phase was determined to be 35.08 MPa m0.5. The findings of the present study provided an effective basis for the determination of the fracture toughness of the quasicrystalline phase.

Dispersion of stable quasicrystal phase by precipitation during extrusion in a Mg–Zn–Al alloy: Microstructure and mechanical properties

Precipitation of stable icosahedral quasicrystalline phase (i) has been recently reported in a Mg6Zn3Al (in wt%, ZA63) alloy on ageing after solutionizing treatment of cast alloy to dissolve dendritic i-phase. This offers an opportunity to disperse this phase in a ZA alloy through a solutionizing treatment followed by extrusion, thereby precipitating the i-phaseduring extrusion. By this process, the i-phasecan be dispersed in the alloy uniformly, instead of dispersion from the cast structure by extrusion. In this study, a cast ingot of ZA63 alloy was solutionized at 400 ∘C, followed by extrusions at 255 ∘C and 300 ∘C. Extrusion at 255 ∘C resulted in larger amount of i-phaseprecipitation (about 4.5 mass%), in which spherical precipitates of size about 100 nm or less were distributed in the matrix and boundaries of grains of average size 2.4 ± 1.0μm, exhibiting true yield strengths of 263.7±1.4MPa in tension and 255.4±5.0MPa in compression (with elongation to failure of about 18% in both cases). Extrusion at the higher temperature of 300 ∘C resulted in fewer precipitates (about 1.4 mass%) and larger average grain size of 12.9 ± 6.1μm and i-phaseprecipitates of size 500 nm which were mostly at the grain boundaries, and consequently lower true yield strengths of 214.4±1.3MPa in tension and 159.3±0.3MPa in compression. The strength after extrusion at 300 ∘C increased by 48 MPa in tension and 38 MPa in compression on ageing at 250 ∘C for 20 min. Ageing precipitate phases were identified to be i-phase, Mg4Zn7 and possibly ternary ϕ-phase. Phase equilibrium and change in morphology of phases during ageing has been discussed. The peculiar nature of work hardening during tensile deformation has also been discussed.

Electrical resistivity of coatings of quasicrystalline and crystalline phases of the Ti—Zr—Ni system in the temperature range 4–300 K

The temperature dependence of the electrical resistivity in the temperature range from 4.4 to 300 K for Ti41Zr41Ni18 (at. %) thin coatings of different crystal perfection and phase composition was studied. The coatings were deposited by magnetron sputtering with subsequent vacuum annealing in various modes ensuring the formation of several heterophase combinations, which included 1/1 and 2/1 approximation phases, a quasicrystalline icosahedral phase, a Laves phase (Ti, Zr)2Ni, and an α-Ti (Zr) solid solution. The structure before and after annealing was studied by X-ray diffractometry. It was established that coatings with a predominant content of amorphous, quasicrystalline, or approximant 2/1 phases have a temperature coefficient of resistance less than zero, while samples with a predominance of crystalline phases demonstrate characteristic metallic conductivity. The temperature dependence of the electrical resistivity of the Ti41Zr41Ni18 quasicrystalline coating turned out to be similar to the dependence in bulk samples of the Ti40Zr40Ni20 composition. The electrical resistivity of the approximant 2/1 phase with a low content of the additional Laves phase in the film has an almost constant value in the entire investigated temperature range.

Heterogeneous nucleation and orientation relationships of icosahedral phase with TiB2 inoculants

This study confirms that crystalline TiB2 particles can act as a potent site for the heterogeneous nucleation of the metastable icosahedral quasicrystalline phase (IQC) formed in cast Al-Mn-Si-Cu-Mg alloys. The results show that a primary metastable IQC phase nucleates on facets of TiB2 particles. Electron backscatter diffraction patterns and selected-area electron diffraction patterns confirmed that at least five orientation relationships (ORs) exist between the stable TiB2 and the metastable IQC phase. The IQC adopts different ORs with the crystallineTiB2 particles to form low energy interfaces by matching close-packed planes at the interfaces. In the IQC phase, the close-packed planes are the 5-fold and 2-fold. Microstructural analyses show that inoculation has a noticeable influence on the size and distribution of the primary IQC phase, which in turn has a noticeable influence on the compressive properties of the investigated alloys. Due to their exceptional mechanical properties, these alloys could become a prime candidate for lightweight applications, especially in the automotive and aerospace industries.

Microstructural characterization of rapidly solidified Al-13.5 at.% Cr and Al-13.5 at.% V alloys for catalytic applications

Intermetallic compounds, due to their well-defined stoichiometry, arrangement of atoms and controlled crystal structure, are a promising alternative to expensive noble metal catalysts. In this paper, the catalytic properties of Al-13.5 at.% Cr and Al-13.5 at.% V alloys, corresponding to the quasicrystalline approximants Al45Cr7 and Al45V7, were investigated for the first time.The alloys in the form of fragmented brittle ribbons were produced by the melt-spinning. The microstructure of the ribbons, both in the as-spun state and after heat treatment (100 h at 600 °C), was characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. In the as-spun state, the ribbons showed a multiphase microstructure. In addition to the Al45Cr7 phase in the Al-Cr alloy and the Al3V phase in the Al-V alloy, they also contained α(Al) solid solution and icosahedral quasicrystalline phases. After heat treatment, the alloys became almost single phase, consisting mainly of stable monoclinic phases: Al45Cr7 or Al45V7. The catalytic performance of the phenylacetylene hydrogenation reaction was tested on as-spun and heat-treated alloys that had previously been pulverized and sieved to select a powder fraction of less than 32 µm. All the tested materials show high substrate conversion, above 80% after 1 h reaction, along with high activity rate. The homogenized powders demonstrated a slightly better properties in relation to as-spun materials. These results confirm the potential of intermetallic catalysts, including the tested alloys, in hydrogenation reactions and verify the possibility of using the metallurgical method to obtain catalytically active materials.Graphical abstract

Structure and electrical conductivity of Ti-Zr-Ni films of quasicrystalline and related crystalline phases

The temperature dependence of electrical resistivity in the temperature range from 4.4 to 300 K for Ti41Zr41Ni18 (at.%) thin coatings of various crystal perfection and phase compositions has been investigated. The films were deposited by the magnetron sputtering method. The following vacuum annealing in various modes provided several combinations of the film phases such as 1/1 and 2/1 crystal-approximant phases, the quasicrystalline icosahedral phase, Laves (Ti,Zr)2Ni phase and α-Ti(Zr) solid solution phase. The structure and composition of the films before and after annealing were studied in detail by X-ray diffractometry and cross-sectional high-resolution transmission electron microscopy. Samples with predominant amorphous, quasi-crystalline, and approximant 2/1 phases have a temperature coefficient of resistance less than zero, while samples dominated by crystalline phases exhibit the characteristic metallic conductivity. For the quasicrystalline Ti41Zr41Ni18 film, the temperature dependence of the electrical resistance turned out to be similar to that of bulk samples with the composition Ti40Zr40Ni20. A significant effect of the presence of phases with metallic conduction on the overall conductivity of the coating is described. The electrical resistance of the 2/1 approximant phase with a low content of the additional Laves phase in the film has an almost constant value over the entire temperature range under study.

A group of ductile metallic glasses prepared by modifying local structure of icosahedral quasicrystals

Inspired by research into the association between icosahedral local orders and the plasticity of metallic glasses (MGs), beryllium (Be) is added to the icosahedral quasi-crystal forming alloy Zr40Ti40Ni20. In this way, bulk metallic glasses (BMGs) with favorable compressive plasticity are fabricated. Therein, the icosahedral quasi-crystalline phase is the main competing phase of amorphous phases and icosahedral local orders are the main local atomic motifs in amorphous phases. The alloys of (Zr40Ti40Ni20)76Be24 and (Zr40Ti40Ni20)72Be28 with their greater plastic strain capacity show similar characteristics to highly plastic amorphous systems: The serrated flow of compression curves always follows a near-exponential distribution. The primary and secondary shear bands intersect each other, bifurcate, and bend. Typical vein patterns are densely distributed on the fracture surfaces. The relaxation enthalpy of four MGs is linearly correlated with the plastic strain, that is, the greater the relaxation enthalpy, the larger the plastic strain.

High-ductility fine-grained Mg-1.92Zn-0.34Y alloy fabricated by semisolid and then hot extrusion

The combination of semisolid and hot extrusion processing was applied to refine the icosahedral quasicrystalline phase (I-phase) in an extruded Mg-1.92Zn-0.34Y(wt.%) alloy for the first time. The semisolid isothermal heat treatment transformed the micron-sized I-phase particles into nano lamellar eutectic (α-Mg + I-phase) with a lamellar spacing of ~86 nm. After subsequent hot extrusion at 250 °C, the nano lamellar eutectic phases were broken into uniformly dispersed nanoscale I-phase particles. What's more, the matrix microstructure was significantly refined with an equiaxed average grain size of 2.59 ± 0.81 µm, and an unusual texture component (most of the grains’ c-axis is parallel to the extrusion direction) was observed. The processed alloy exhibited a high tensile elongation to failure (EL) of 44 ± 2.6% with an ultimate tensile strength (UTS) of 258 ± 2.0 MPa and a tensile yield strength (TYS) of 176 ± 1.6 MPa at room temperature. The high ductility from the combined effects of the grain refinement, dispersion of nanoscale I-phase particles, and the unusual texture. The uniform dispersion of nanoscale I-phase particles could promote grain refinement by particle stimulated nucleation mechanism, and thus bring the unusual texture (where the c-axis is aligned parallel to the extrusion direction during dynamic recrystallization, which contributed to ductility).

STABILITY OF THIN QUASI-CRYSTALLINE Ti-Zr-Ni FILMS AND RELATED CRYSTALLINE PHASES UNDER LOW-ENERGY TRANSIENT PLASMA IRRADIATION

The properties of Ti41Zr38.3Ni20.7 thin films under radiation-thermal action of hydrogen plasma with a surface heat load of 0.2 MJ/m2 was studied at the QSPA Kh-50 quasi-stationary plasma accelerator (NSC KIPT).The phase composition, structural state, and surface morphology were studied using X-ray diffraction and scanning electron microscopy. It was found that the quasicrystalline phase and related crystalline phases, the Laves phase, the α-solid solution, and the 2/1 phase of the Ti-Zr-Ni approximant crystal were stable under irradiation with up to 20 hydrogen plasma pulses. The phase composition did not change. It is shown that the changes in the coatings mainly manifest themselves as changes in the substructure of the observed phases. With an increase in the plasma exposure dose, the structure of the quasicrystalline icosahedral phase improves, and the size of the coherence regions increases. In the films consisting of crystalline phases, a partial phase transformation is observed with a redistribution of components between the 2/1 phase of the approximant crystal and the α-solid solution phase. It was found that thin films of the TiZr-Ni system containing a quasicrystalline icosahedral phase, irradiated with radiation-thermal plasma pulses, are less prone to cracking than coatings with crystalline phases of the same system.

Nanoscale precipitates with quasicrystal domains enhanced strength-ductility synergy in a Mg–6Zn–4Al–1Sn–0.5Mn alloy

Precipitation strengthening is one of the significant strengthening mechanisms for metallic alloys owing to its excellent strengthening effect. Here, we effectively introduce homogenous nanosized precipitates into a Mg–6Zn–4Al–1Sn-0.5Mn (wt.%) alloy via extrusion and double aging to enhance strength-ductility synergy. The yield strength, ultimate tensile strength, and total elongation of the Mg alloy are 303.1 ± 15.8 MPa, 392.5 ± 3.9 MPa, and 21.1 ± 2.4%, respectively. Using transmission electron microscopy and atomic level high angle annual dark field-scanning TEM, in addition to the icosahedral quasicrystalline (IQC) particles at the grain boundary, we find that there are two kinds of nanoprecipitates in the matrix. The majority of nanoprecipitates are cuboid IQC phases with nanodomains parallel to the basal plane of the Mg matrix, while the minority of nanoprecipitates are rod-like β1′ phases containing IQC clusters vertical to the basal plane. These homogenous nanoscale precipitates and grain refinement contribute 50.5% and 30.2% of the yield strength. This work sheds new light on understanding the nanoscale IQC precipitate strengthening mechanisms of magnesium alloys and provides a novel method to fabricate alloys with superior mechanical properties.

Microstructure and Properties after Friction Stir Processing of Twin-Roll Cast Al–Mn–Cu–Be Alloy

We studied the effect of friction stir processing (FSP) on the microstructure and properties of high-speed twin-roll cast strips made of an experimental Al–Mn–Cu–Be alloy. The samples were examined using light, scanning, and transmission electron microscopy, microchemical analysis, X-ray diffraction, and indentation testing. During FSP, the rotational speed varied, while other parameters remained constant. The uniformity of the microstructure increased with the growing rotational speed. In the stir zone, several processes took place, and the most important were: recrystallisation of the matrix grains, fragmentation of the primary intermetallic particles Al15Mn3Be2 and their more uniform distribution in the stir zone, fracture, and dispersion of the eutectic icosahedral quasicrystalline phase (IQC), transformation of tiny Al15Mn3Be2 and IQC particles into the τ1-Al26Mn6Cu4 phase and precipitation of Al–Mn–Cu precipitates. In the thermomechanically affected zone, new dislocations formed as well as dispersion of the IQC eutectic phase and recrystallisation of the matrix grains. In the heat-affected zone, dissolution of θ’-Al2Cu precipitates occurred. The hardness variation was not severe between the stir and heat-affected zones.

Study of icosahedral quasi-crystalline phase T2-Al6CuLi3 and transformation in 2A97 Al-Li alloy fabricated by laser additive manufacturing

Precipitated phases of 2A97 Al-Li alloy fabricated by laser additive manufacturing with thermal input are mainly composed of quasicrystal T2 phase gathered around grain boundary and TB phase in grain interior. Diffraction patterns of quasi-crystalline T2 phase show two-fold, three-fold and five-fold symmetry, respectively. Under condition of thermal cycling, T2 phase transforms into a face-centered cubic Y phase with a lattice constant of 2 nm. Y phase can grow into twins, and the twin plane is (111). After solution treatment at 515 °C for 1.5 h, δ' phase, T1 phase and TB phase are completely dissolved in matrix, while quasi-crystalline T2 phase transforms into orthorhombic O phase. The size of O phase is similar to that of T2, with length of about 400 nm and rougher surface.

Microstructure and Indentation Properties of Single-Roll and Twin-Roll Casting of a Quasicrystal-Forming Al-Mn-Cu-Be Alloy

In this investigation, strips of an experimental Al-Mn-Cu-Be alloy were manufactured by high-speed single-roll and twin-roll casting to stimulate the formation of a quasicrystalline phase during solidification. The strips were characterised by light microscopy, scanning and transmission electron microscopy, microchemical analysis, and X-ray diffraction. Indentation testing was used to determine the mechanical responses of the strips in different areas. A smooth surface was achieved on both sides of the twin-roll-cast strip, while the free surface of the single-roll-cast strip was rough. The microstructures in both strips consisted of an Al-rich solid solution matrix embedding several intermetallic phases Θ-Al2Cu, Be4Al (Mn, Cu), Al15Mn3Be2 and icosahedral quasicrystalline phase (IQC). The microstructure of the single-roll-cast strip was more uniform than that of the twin-roll-cast strip. Coarse Al15Mn3Be2 particles appeared in both alloys, especially at the centre of the twin-roll strip. These coarse particles adversely affected the strength and ductility. Nevertheless, both casting methods provided high-cooling rates, enabling the formation of metastable phases, such as quasicrystals. However, improvements in alloy composition and casting procedure are required to obtain enhanced microstructures and properties.

Heat-Resistant Al-Alloys with Quasicrystalline and L1<sub>2</sub>- Precipitates

We have been developing Al-Mn-Cu based alloys alloyed with minor additions of different elements. Small additions of beryllium enhance the formation of the icosahedral quasicrystalline phase (IQC) during solidification, especially during ageing. Upon solidification, primary IQC-particles may form, with sizes, ranging from 5 to 50 μm. IQC is also present as a part of binary eutectic in the interdendritic regions. More importantly, nanosized quasicrystalline precipitates can form during T5-treatment at temperatures ranging from about 250−450 °C. They are, in fact, metastable precipitates transforming to ternary T-precipitates (Al20Mn3Cu2) phase above 450 °C. The heat resistance can be increased considerably by the addition of Sc and Zr by forming L12-precipitates in spaces between quasicrystalline precipitates. In this paper, we studied three alloys, two Al-Mn-Cu-Be alloys and an Al-Mn-Cu-Be-Sc-Zr alloy. The alloys were produced by vacuum induction melting and casting into a copper mould. We investigated the response of the alloys to different heat treatments and their heat resistance at higher temperatures. It was shown that the alloys could be precipitation strengthened by ageing at 300 °C and 400 °C. The hardness of the alloy stayed at relatively high levels even at 500 °C, while more substantial softening occurred at 600 °C.

STRUCTURAL-PHASE TRANSFORMATIONS IN MAGNETRON DEPOSITED FILMS OF Ti-Zr-Ni SYSTEMS DURING ANNEALING IN VACUUM

Structural and phase changes in thin films obtained by magnetron sputtering of a target with the composition Ti41Zr38.3Ni20.7 (at.%) were studied under isothermal vacuum annealing in the temperature range from 400 to 800 C. The boundaries of the thermal stability of the phases have been determined. Coatings consisting of a single icosahedral quasicrystalline phase, as well as films consisting of the 2/1 crystal-approximant phase were obtained. The features of the phase transformation of a quasicrystal into an approximant crystal were investigated, and the activation energy of this transformation was determined at a level of 94 kJ/mol. The dependence of the degree of phase transformation on the temperature and duration of annealing has been established.

Microstructural and mechanical characterization of quasicrystalline Al-Cu-Fe foams

Quasi-crystalline metal foams are considered a new class of materials, as they differ fundamentally from conventional materials in their atomic structure and mechanical properties. Their manufacturing methods are under constant development. In this article, the microstructural characterization and the mechanical behavior of quasicrystalline foams obtained via slow solidification and heat treatment with the presence of molten metal is presented. The studied Al63Cu28Fe9 alloy was thermally treated at 800 °C for short times from 5 to 15 min and for longer times of 180 and 360 min in order to promote the formation of a metal structure constituted by icosahedral phase with a high percentage of porosity. The microstructural characterization was performed by using the X-ray diffraction (XRD) technique, scanning electron microscopy (SEM) and optical microscopy (OM). The thermal properties of the as-cast alloy and the changes in the density of the thermally treated samples were characterized by a differential thermal analysis (DTA) and Archimedes method, respectively. The highest percentage of macroporosity (40%) was obtained in the heat-treated sample at 800 °C for 360 min. Under these heat treatment conditions, a completely porous structure (foam) mainly constituted by quasicrystalline icosahedral phase was formed. The maximum compressive stress value and average plateau stress reached for the alloy heat-treated during 360 min were 80 MPa and 30 MPa, respectively.

Joint effect of quasicrystalline icosahedral and L12-strucutred phases precipitation on the grain structure and mechanical properties of aluminum-based alloys

Dispersoid hardening is a key factor in increasing the recrystallization resistance and mechanical strength of non-heat treatable aluminum-based alloys. Mn and Zr are the main elements that form dispersoids in commercial Al-based alloys. In this work, the annealing-induced precipitation behavior, the grain structure, and the mechanical properties of Al-3.0Mg-1.1 Mn and Al-3.0Mg-1.1 Mn-0.25 Zr alloys were studied. The microstructure and the mechanical properties were significantly affected by annealing regimes after casting for both alloys. The research demonstrated a possibility to form high-density distributed quasicrystalline-structured I-phase precipitates with a mean size of 29 nm during low-temperature annealing of as-cast alloys. Fine manganese-bearing precipitates of I-phase increased recrystallization resistance and significantly enhanced the mechanical strength of the alloys studied. The estimated strengthening effect owing to I-phase precipitation was 150 MPa. Due to the formation of L12-structured Al3Zr dispersoids with a mean size of 5.7 nm, additional alloying with Zr increased yield strength by about 90 MPa. The L12-phase strengthening effect was estimated through the dislocation bypass looping and shearing mechanisms.