Cu-Zr Systems Research Articles

Identification of B33 and Cm martensitic products in CuZr-based alloys: A DFT study

B33 and Cm phases as martensite products of CuZr-based alloys are still controversial and even confused. This work identified that the B33 and Cm are martensite products of binary CuZr and ternary CuZrX systems, respectively, and revealed the martensite transformation (MT) barriers and pathways of the two systems by the DFT method. According to our results, in binary CuZr alloy, B19' phase is firstly formed along the B2→R B19' →B19' path with the barrier of 0.28 eV. Then B33 formation follows the no-barrier pathway of B19'→B33. However, due to the higher MT barrier and the fact that the Cm lattice observed in B19' supercell is still B19' phase, the Cm phase was excluded from CuZr martensite products. As for CuZrX systems, the symmetry reduction caused by the third metallic element X makes the Cm phase appear in all Cu 43.75 Zr 50 X 6.25 and Cu 37.5 Zr 50 X 12.5 alloys. Owing to the lattice overlap, the Cm phase in the CuZrX system is always accompanied by B19'. Notably, the transformations of all Cu 43.75 Zr 50 X 6.25 and most Cu 37.5 Zr 50 X 12.5 martensites prefer the two-step pathway of B2→R M →M rather than the B2→M path. Especially for Cu 37.5 Zr 50 Ni 12.5 martensite, there is no transformation barrier along the B2→R M →M path. • B19’ + B33 and B19’ + Cm were identified as martensitic products of CuZr and CuZrX systems, respectively. • B19’ + B33 products of binary CuZr alloy are formed along the B2→R B19’ →B19’ and B19’→B33 pathways with a barrier of 0.28 eV. • Most CuZrX martensite formations prefer the two-step pathway of B2→R M →M. • The Cm lattice observed in the B19’ supercell is still B19’ phase, while B19’ primitive lattice can usually be observed in the Cm lattice.

Molecular dynamics simulation of strengthening of nanocrystalline Cu alloyed with Zr

Nanocrystalline Cu-Zr alloys are an emerging class of immiscible high-strength materials with significant potential for structural and high-temperature applications. The effects of Zr concentration and tensile velocity on the strengthening and mechanics of Cu alloyed with Zr is studied using molecular dynamics simulations based on the many-body embedded-atom potential. The simulation results show that the potential energy of Cu-Zr systems significantly decreases with increasing Zr concentration, resulting in an increase in structural stabilization. Doping Zr atoms at the grain boundaries can strengthen the boundaries and reduce their mobility during the tensile test. The Young’s modulus of Cu-Zr systems decreases with increasing Zr concentration. The plasticity of Cu-Zr systems increases with increasing Zr concentration. The mechanical strength of Cu-Zr systems reaches its maximum value, which is 1.13 times that of the pure Cu counterpart, when the Zr concentration is 5%.

Frank-Kasper Z16 local structures in Cu-Zr metallic glasses

Although previous molecular dynamics studies proposed the existence of Zr-centered Frank-Kasper Z16 structures in Cu-Zr metallic glasses, it cannot be concluded yet, owing to the degeneracy problem of the Voronoi index and the artifact in sets of potentials for Cu-Zr systems. We solve both problems by combining a recently developed Finnis-Sinclair potentials to generate realistic atomic structures and the ${p}_{3}$ code to properly differentiate local structures. In previous studies, the Voronoi index $\ensuremath{\langle}0,0,12,4\ensuremath{\rangle}$ was typically associated with Frank-Kasper Z16 local structures. However, we demonstrate that this index includes two types of polyhedra, only one of which is associated with the Frank-Kasper Z16 structures. We reveal that the Z16 structures are more frequent than the other structures and that the two have different behaviors. The tendency of Zr-centered Z16 local structures to be neighbors of Cu-centered icosahedral local structures is also confirmed. Our findings illustrate the importance of properly differentiating local structures in order to elucidate the behavior of metallic glasses at the atomic scale.

Structural heritage of metallic glasses and relevant crystals understood via the principal cluster

ABSTRACT Structural heritage between bulk metallic glasses (BMGs) and their relevant crystals has already been acknowledged. But how this kind of heritage is represented is still unclear. In the present paper, firstly, two important characters are considered to determine the principal cluster of crystal, i.e. the atomic-packing efficiency which is evaluated by atomic density radial distribution method and cluster isolation degree which is obtained by the cluster-size reduction rate because of overlapping of clusters in crystal. Then according to these two criteria principal clusters of BMGs Cu-Zr, (Pt,Pd)-Zr and Pd-Si systems are analysed. And the results show that the principal cluster dominates the short-range-order structure of its parent phase as well as the relevant BMG. It is exactly the principal cluster that enters the cluster-plus-glue-atoms units to formulate BMG as [principal cluster](glue atom)1,3. It proves that the structural heritage between glassy state and relevant crystal is actually reflected by cluster formula which contains the principal cluster. Furthermore, via study structural similarity of metallic glass and relevant crystal, elaborated steps towards quantitatively composition design for BMGs are established.

Estimation of the glass-forming ability of metallic glasses with monolayer two-dimensional model

In present work, the glass-forming ability of Cu-Zr and Cu-Al systems has been estimated by utilizing a monolayer two-dimensional model. The structure of 2D Cu-Zr MGs can be simply characterized by three kinds of Voronoi polygons, i.e. pentagon, hexagon and heptagon, and the structural translational symmetry is broken down by the strong coupling effect between pentagon and heptagon, promoting the formation of MGs. Moreover, the larger bond lengths of Cu-Zr and Zr-Zr pairs are found responsible for the outstanding GFA of Cu-Zr system. The 2D model provides a promising avenue for accurately predicting the GFA of alloys of interest, which is important both in theoretical research and engineering.

Comparing short–range and medium–range ordering in Cu[sbnd]Zr and Ni[sbnd]Zr metallic glasses – Correlation between structure and glass form ability

According to recent studies, CuZr and NiZr binary alloying systems are very similar based on known glass-forming ability criteria, however, they exhibit significant difference in glass-forming ability in practice. In this work, local atomic structures of CuZr and NiZr metallic glasses are studied by molecular dynamics simulation to explain the source of mentioned difference. The total and partial distribution functions, coordination number and voronoi analysis are utilized to characterize the local atomic structures around Zr, Ni and Cu atoms. It was found that the local environment around Zr atoms is almost similar in both systems. The difference in the atomic structure in these systems mainly arises from topologies of polyhedra around Cu and Ni atoms. In particular, full icosahedron is one of the most popular local structure in CuZr system, while its population is significantly smaller in NiZr system with the same composition. Additionally, networks formed by full icosahedra connections, medium range order, are more robust in CuZr system. Higher percentage of topologically ordered and compact structures and their connectivity could explain why bulk metallic glass formation is possible in CuZr system but not in NiZr system.

Chemical interaction and thermodynamic properties of (Cu,Ni)-Zr glass-forming alloys

The model of ideal associative solutions was applied to analyze the influence of strong chemical interaction in (Cu,Ni)-Zr melts depending on their glass-forming ability. Within the model framework thermodynamic properties of both Ni-Zr and Cu-Zr systems in the liquid state were calculated. The formation enthalpies of intermetallic compounds of these systems were redefined by the matching procedure, taking into account the additive manifestation of chemical interaction. It was suggested that directed (covalent) interaction causes formation of associative complexes which impedes diffusion and slows down crystallization. The intensity of interparticle interaction in these alloys is found to have no decisive influence on their glass-forming ability.

Investigating the atomic level influencing factors of glass forming ability in NiAl and CuZr metallic glasses.

Bulk metallic glasses are a relatively new class of amorphous metal alloy which possess unique mechanical and magnetic properties. The specific concentrations and combinations of alloy elements needed to prevent crystallization during melt quenching remains poorly understood. A correlation between atomic properties that can explain some of the previously identified glass forming ability (GFA) anomalies of the NiAl and CuZr systems has been identified, with these findings likely extensible to other transition metal-transition metal and transition metal-metalloid (TM-M) alloy classes as a whole. In this work, molecular dynamics simulation methods are utilized to study thermodynamic, kinetic, and structural properties of equiatomic CuZr and NiAl metallic glasses in an attempt to further understand the underlying connections between glass forming ability, nature of atomic level bonding, short and medium range ordering, and the evolution of structure and relaxation properties in the disordered phase. The anomalous breakdown of the fragility parameter as a useful GFA indicator in TM-M alloy systems is addressed through an in-depth investigation of bulk stiffness properties and the evolution of (pseudo)Gruneisen parameters over the quench domain, with the efficacy of other common glass forming ability indicators similarly being analyzed through direct computation in respective CuZr and NiAl systems. Comparison of fractional liquid-crystal density differences in the two systems revealed 2-3 times higher values for the NiAl system, providing further support for its efficacy as a general purpose GFA indicator.

Mechanisms of metastable states in CuZr systems with glass-like structures.

The local structural inhomogeneity of glasses, as evidenced from broad bond-length distributions (BLDs), has been widely observed. However, the relationship between this particular structural feature and metastable states of glassy solids is poorly understood. It is important to understand the main problems of glassy solids, such as the plastic deformation mechanisms and glass-forming ability. The former is related to β-relaxation, the relaxation of a system from a subbasin to another in the potential energy landscape (PEL). The latter represents the stability of a metastable state in the PEL. Here, we explain the main reason why CuZr systems with glass-like structures exist in metastable states: a large strain energy. The calculation results obtained in this study indicate that a system with broad BLD has a large strain energy because of the nonlinear and asymmetric strain energy of bonds. Unstable polyhedra have larger volumes and more short and long bonds than stable polyhedra, which are most prone to form deformation units. The driving force for pure metal crystallization was also elucidated to be the decrease in strain energy. The results obtained in this study, which are verified by a series of calculations as well as molecular dynamics simulations, indicate the presence of metastable states in amorphous materials and elucidate the mechanisms of plastic deformation and the driving force for crystallization without chemical bonding.

Atomic-Scale Mechanisms of the Glass-Forming Ability in Metallic Glasses

The issue, composition dependence of glass-forming ability (GFA) in metallic glasses (MG), has been investigated by systematic experimental measurements coupled with theoretical calculations in Cu-Zr and Ni-Nb alloy systems. It is found that the atomic-level packing efficiency strongly relates to their GFA. The best GFA is located at the largest difference in the packing efficiency of the solute-centered clusters between the glassy and crystal alloys in both MG systems. This work provides an understanding of GFA from atomic level and will shed light on the development of new MGs with larger critical sizes.

Investigation of the phase diagram and the determination of synthesis conditions of volume amorphous alloys in the Cu-Ni-Zr system at 1123 K

The phase equilibria in the Cu-Ni-Zr system in a field enriched by zirconium at 1123 K were investigated by the methods of X-ray and electron-probe microanalysis, microdurometry, and scanning calorimetry. A fragment of the isothermal section of the given system was constructed and the borders of areas of homogeneity of double phases of the Ni-Zr and Cu-Zr systems were defined. It was established that the double compounds NiZr and NiZr2 dissolve up to 33 and 12 at % Cu, respectively, and CuZr and CuZr2, dissolve up to 7 at % Ni.

Ab initio molecular dynamics to designing structural and dynamic properties in metallic glass-forming alloys

An overview of a recent series of ab initio molecular dynamics (AIMD) simulations for the liquid, undercooled states of materials forming metallic glasses is presented. Here we use a combination of state-of-the-art computational techniques to resolve the atomic-level structure of such disordered systems. The dynamic properties, such as the self-diffusion coefficients and viscosity, are also studied as a function of temperature in the undercooled regime. By analyzing two model systems that involve different chemistry, i.e. Cu-Zr and Au-Si systems, we identify the local icosahedral ordering as a fundamental process underlying structural relaxation. Our findings have also strong implications for understanding the nature of the glass forming ability and properties of metallic glasses.

Thermodynamic assessment of the Cu-Ti-Zr system. II. Cu-Zr and Ti-Zr systems

The thermodynamic assessment of the Cu-Zr and Ti-Zr systems is carried out using the CALPHAD method. The Gibbs energy of Cu-Zr liquid alloys is described by the ideal associated solution model. The excess Gibbs energy of Ti-Zr liquid alloys and Cu-Zr and Ti-Zr solid solutions is descried by models with Redlich-Kister polynomials. The Gibbs energy of Cu-Zr intermetallic compounds is described by models taking into account their formation enthalpy and entropy. A set of self-consistent parameters of the models is obtained using data on phase equilibria and thermodynamic properties of the phases. The calculated phase diagrams of the systems and values of thermodynamic properties of the phases are in good agreement with experimental information. The relative thermodynamic stability of supercooled liquid alloys and competitive crystal phases of the Cu-Zr system are analyzed.

Formation, Thermal Stability and Mechanical Properties of Cu-Zr and Cu-Hf Binary Glassy Alloy Rods

Glassy alloy rods with diameters up to 1.5 mm exhibiting a large supercooled liquid region before crystallization and high mechanical strength were formed in Cu-Zr and Cu-Hf binary alloy systems by the copper mold castingmethod. The large supercooled liquid region exceeding 40 K was obtained in the composition range of 30 to 70 at%Zr and 35 to 60 at%Hf. The largest value of the supercooled liquid region defined by the difference between glass transition temperature (Tg) and crystallization temperature (T x ), ΔT x (= T x - Tg), was 58 K for Cu 6 0 Zr 4 0 and 59 K for Cu 5 5 Hf 4 5 . The reduced glass transition temperature (T g /T 1 ) of the two alloys was 0.61 and 0.59, respectively. The alloys with large ΔT x above 50K were formed into a bulk glassy alloy form with diameters up to 1.5 mm by copper mold casting. The Cu 6 0 Zr 4 0 , Cu 4 5 Zr 5 5 , Cu 6 0 Hf 4 0 and Cu 5 5 Hf 4 5 glassy alloy rods exhibited high fracture strength of 1920, 1880, 2245 and 2260 MPa, respectively, Young's modulus of 107, 102, 120 and 121 GPa, respectively, a nearly constant elastic elongation of about 1.9% and plastic elongation up to 2.2%. The formation of these binary glassy alloy rods can he interpreted in the framework of the concept of the formation of the unique glassy structure consisting mainly of icosahedral atomic configuration as similar to that for special multi-component alloys with the three component rules.

Synthesis and Fundamental Properties of Cu-Based Bulk Glassy Alloys in Binary and Multi-component Systems

A glassy phase containing cubic phase particles with a size of 3-5 nm was formed in cast Cu 6 0 Zr 3 0 Ti 1 0 and Cu 6 0 Hf 3 0 Ti 1 0 bulk alloys. The cubic phase is in a metastable state and its lattice parameter (a 0 ) is 0.45 nm for the former alloy and 0.51 nm for the latter. These bulk alloys exhibit good mechanical properties of 2000-2130 MPa for tensile strength (σ 1 , f ), 2060-2160 MPa for compressive strength (σ c , f ) and 0.008-0.017 for compressive plastic strain (e c , p ). The temperature interval of the supercooled liquid (SL) region prior to crystallization is 37 K for Cu 6 0 Zr 3 0 Ti 1 0 and 67 K for Cu 6 0 Hf 3 0 Ti 1 0 . The primary crystallization occurred by precipitation of cubic CuZr (a 0 = 0.35 nm) in a diffusion-controlled growth mode of nuclei and an orthorhombic Cu 8 Hf 3 phase in an interface diffusion-controlled growth of nuclei with decreasing nucleation rate. The difference in the precipitation modes is interpreted to be the origin of the difference in the SL region. Furthermore, the addition of Al to Cu-Zr and Cu-Hf alloys caused the formation of a glassy single phase in the rod form with diameters up to at least 3 mm, though bulk glassy alloy rods with critical diameters up to 1.5 mm and σ c , f , of 1920-2260 MPa were formed in Cu-Zr and Cu-Hf binary systems. The ternary bulk glassy alloys exhibited high σ c , f of 2100-2370 MPa with e c , p of 0.002-0.006. The addition of Pd, Pt, Ag or Au increased ΔT x and a large ΔT x of 102-110 K was obtained for the Cu-Hf-Al-M (M=Pd or Ag) glassy alloys. The synthesis of Cu-based bulk glassy alloys with good mechanical properties and large ΔT x in glassy single, and mixed glassy and cubic phase states, is important for future applications of bulk glassy alloys.

Formation of amorphous Cu60Zr40 and Ni50Zr50 powders by mechanical alloying

Amorphization of Cu-Zr and Ni-Zr systems by mechanical alloying (MA) was studied by x-ray diffraction. The results show that the amorphization is not merely a simple interdiffusion by thermal activation; in addition, the strain induced by MA also accelerates this process.