Guo He

In situ formed Ti–Cu–Ni–Sn–Ta nanostructure-dendrite composite with large plasticity

Abstract A group of Ti–Cu–Ni–Sn–Ta multicomponent alloys is prepared by copper mold casting and arc melting, respectively, in which nanostructured (or ultrafine-grained) matrix-dendrite composites can be obtained. With increasing Ti and Ta contents, the volume fraction of the dendritic phase increases. The grain size of the matrix phase depends on the preparation method, and is 30–70 nm for as-cast 2–3 mm diameter cylinders and about 100–200 nm for the as-arc melted samples. Compression test results indicate that fully nanostructured samples exhibit very high yield strength of 1800 MPa with a limited plastic strain of 1.4%. The nanostructure-dendrite composites exhibit high yield strengths of 1525–1755 MPa together with large plastic strains of 4.7–6.0%. The as-arc melted samples exhibit relatively lower yield strengths of 1037–1073 MPa with very large plastic strains of 16.5–17.9% because of the coarser grain size of the matrix. The large plasticity of the composites is attributed to the retardation of localized shear banding and the excessive deformation in the nanostructured matrix due to the in situ formed dendrites. The deformation and the fracture mechanisms of the nanostructure-dendrite composites are discussed based on fractography observations.

Stability, phase transformation and deformation behavior of Ti-base metallic glass and composites

Abstract Melt-spun ribbons and copper-mold cast cylinders of (Ti 0.5 Cu 0.23 Ni 0.2 Sn 0.07 ) 100− x Mo x bulk glass-forming alloys are prepared. Both Ti 50 Cu 23 Ni 20 Sn 7 and (Ti 0.5 Cu 0.23 Ni 0.2 Sn 0.07 ) 95 Mo 5 melt-spun glassy ribbons exhibit large supercooled liquid regions, high reduced glass transition temperatures, and good thermal stabilities. During continuous heating of the melt-spun ribbons, both alloys present a two-stage crystallization behavior. Mo slightly lowers the glass-forming ability but significantly decreases the temperature of the second stage crystallization. For both alloys, the stable phases after heating are Ti 2 Ni, TiCu, Ti 3 Sn and β-(Cu,Sn). As-cast Ti 50 Cu 23 Ni 20 Sn 7 cylinders contain dendritic hcp-Ti solid solution precipitates, as well as interdendritic glassy and Sn-rich crystalline phases. The ultimate compression stress reaches 2114 MPa with 5.5% plastic strain for 2-mm diameter cylinders. Yielding occurs at 1300 MPa, and Young’s modulus is 85.3 GPa. Mo improves and stabilizes the precipitation of a β-Ti solid solution but prevents glass formation in as-cast (Ti 0.5 Cu 0.23 Ni 0.2 Sn 0.07 ) 95 Mo 5 bulk alloys. The bulk samples contain dendritic β-Ti solid solution precipitates, Ti 2 Ni particles and Sn-rich phases. The ultimate compression stress is 2246 MPa with about 1% plastic strain for a 3-mm diameter cylinder. σ 0.2 is about 1920 MPa and Young’s modulus is 104 GPa. The high strength is attributed to both Mo solution strengthening and Ti 2 Ni particle strengthening. The limited ductility is induced by the precipitation of brittle Ti 2 Ni particles.

Nanostructured Ti-based multi-component alloys with potential for biomedical applications

A group of Ti(60)Cu(14)Ni(12)Sn(4)M(10) (M=Nb, Ta, Mo) alloys was prepared using arc melting and copper mold casting. The as-prepared alloys have a composite microstructure containing a micrometer-sized dendritic beta-Ti(M) phase dispersed in a nanocrystalline matrix. These new alloys exhibit a low Youngs modulus in the range of 59-103 GPa, and a high yield strength of 1037-1755 MPa, together with large plastic strains. The combination of high strength and low elastic modulus offers potential advantages in biomedical applications.

Effect of Ta on glass formation, thermal stability and mechanical properties of a Zr52.25Cu28.5Ni4.75Al9.5Ta5 bulk metallic glass

The effect of Ta on glass-forming ability, crystallization behavior and mechanical properties of Zr52.25Cu28.5Ni4.75Al9.5Ta5 bulk metallic glass (BMG) is investigated. The solubility of Ta in the Zr-base BMG alloy depends on the arc melting conditions. 3.2 at.% Ta dissolve in the alloy inducing an increase of about 20 K in both glass transition temperature and crystallization temperature of the BMG. However, Ta does not significantly change the extension of the supercooled liquid region. The remaining Ta particles in the master alloy may induce a composition-segregation layer around the particles upon subsequent casting. This further induces the crystallization of Zr2Cu that deteriorates the ductility of the samples. The compressive strength and ductility of the as-cast 3 mm diameter Zr52.25Cu28.5Ni4.75Al9.5Ta5 samples are improved in comparison with the Zr55Cu30Ni5Al10 BMG alloy. The fracture plane of the present alloy has an angle of 31–33° with respect to the stress axis, which remarkably deviates from the maximum shear stress plane. The improvement of the mechanical properties and the peculiar fracture feature for the Zr52.25Cu28.5Ni4.75Al9.5Ta5 BMG alloy can be attributed to the effect of dispersed Ta particles.

Enhanced plasticity in a Ti-based bulk metallic glass-forming alloy by in situ formation of a composite microstructure

A bulk metallic glass-forming Ti–Cu–Ni–Sn alloy with in situ formed composite microstructure prepared by both centrifugal and injection casting presents more than 6% plastic strain under compressive stress at room temperature. The in situ formed composite contains dendritic hexagonal-close-packed-Ti solid solution precipitates and a few Ti 3 Sn, β –(Cu, Sn) grains dispersed in a glassy matrix. The composite microstructure can avoid the development of the highly localized shear bands typical for the room-temperature deformation of monolithic glasses. Instead, highly developed shear bands with evident protuberance are observed, resulting in significant yielding and homogeneous plastic deformation over the entire sample.

Porous titanium materials with entangled wire structure for load-bearing biomedical applications.

A kind of porous metal-entangled titanium wire material has been investigated in terms of the pore structure (size and distribution), the strength, the elastic modulus, and the mechanical behavior under uniaxial tensile loading. Its functions and potentials for surgical application have been explained. In particular, its advantages over competitors (e.g., conventional porous titanium) have been reviewed. In the study, a group of entangled titanium wire materials with non-woven structure were fabricated by using 12-180 MPa forming pressure, which have porosity in a range of 48%-82%. The pores in the materials are irregular in shape, which have a nearly half-normal distribution in size range. The yield strength, ultimate tensile strength, and elastic modulus are 75 MPa, 108 MPa, and 1.05 GPa, respectively, when its porosity is 44.7%. The mechanical properties decrease significantly as the porosity increases. When the porosity is 57.9%, these values become 24 MPa, 47.5 MPa, and 0.33 GPa, respectively. The low elastic modulus is due to the structural flexibility of the entangled titanium wire materials. For practical reference, a group of detailed data of the porous structure and the mechanical properties are reported. This kind of material is very promising for implant applications because of their very good toughness, perfect flexibility, high strength, adequate elastic modulus, and low cost.

High-performance bulk Ti-Cu-Ni-Sn-Ta nanocomposites based on a dendrite-eutectic microstructure

Using a Ti-Cu-Ni-Sn-Ta alloy as an example, we demonstrate a strategy for the in situ formation of nanocomposite microstructures that can lead to simultaneous high strength and ductility. Our approach employs copper mold casting for the production of bulk alloys from the melt, and the solidification microstructure is designed to be composed of micrometer-sized ductile dendrites uniformly distributed inside a matrix of nanoscale eutectic reaction products. The nanostructured matrix is achieved at a relatively deep eutectic, which facilitates the formation of an ultrafine eutectic microstructure over a range of cooling rates. The multi-component recipe stabilizes a ductile solid solution as the toughening phase and helps to reduce the eutectic spacing down to nanoscale. The multi-phase microstructure (including phase distributions, morphologies, and interfaces) has been examined in detail using transmission electron microscopy (TEM) and high-resolution TEM. The metastable eutectic reaction and the nanoscale spacing achieved are explained using thermodynamic and solidification modeling. The benefits expected from the microstructure design are illustrated using the high strength and large plasticity observed in mechanical property tests. Our nanocomposite design strategy is expected to be applicable to many alloy systems and constitutes another example of tailoring the microstructure on nanoscale for extraordinary properties.

Preparation of near micrometer-sized TiO2 nanotube arrays by high voltage anodization.

Highly ordered TiO2 nanotube arrays with large diameter of 680-750 nm have been prepared by high voltage anodization in an electrolyte containing ethylene glycol at room temperature. To effectively suppress dielectric breakdown due to high voltage, pre-anodized TiO2 film was formed prior to the main anodizing process. Vertically aligned, large sized TiO2 nanotubes with double-wall structure have been demonstrated by SEM in detail under various anodizing voltages up to 225 V. The interface between the inner and outer walls in the double-wall configuration is porous. Surface topography of the large diameter TiO2 nanotube array is substantially improved and effective control of the growth of large diameter TiO2 nanotube array is achieved. Interestingly, the hemispherical barrier layer located at the bottom of TiO2 nanotubes formed in this work has crinkles analogous to the morphology of the brain cortex. These structures are potentially useful for orthopedic implants, storage of biological agents for controlled release, and solar cell applications.

Inverse deformation-fracture responses between dendrite and matrix in Ti-based nanostructure–dendrite composite

The room-temperature deformation and fracture mechanisms of Ti-based nanostructured alloys are investigated. The monolithic nanostructured alloy goes through a shear banding ↔ kinetic softening vicious cycle and exhibits very limited plasticity. The nanostructure–dendrite composite exhibits large plasticity while retaining a very high strength. Three fracture modes, namely shear fracture of the nanostructured matrix phase, normal ductile fracture of the dendritic phase and the peeling off of the dendrites from the matrix, are clearly observed. With increasing deformation, the nanostructured matrix is kinetically softened while the dendrite phase is work hardened. The inverse deformation responses and the interaction between the nanostructured matrix and the dendrites can effectively retard the inhomogeneous shear deformation of the nanostructured phase and lead to a large plasticity.

A new approach to the fabrication of porous magnesium with well-controlled 3D pore structure for orthopedic applications

A new approach to the fabrication of porous magnesium by using the titanium wire space holder (TWSH) method has been developed. Since the entangled titanium wire structure is used as the space holder, the porous structure is pipe-like and interconnected, and the pore size is identical and equals to the diameter of the titanium wire. In particular, the pore size, porosity, and porous morphology of the porous magnesium can be exactly and individually controlled. When the porosity is in the range of 54.2-43.2%, the compressive yielding strength is in the range of 4.3-6.2 MPa, and the Youngs modulus is in the range of 0.5-1.0 GPa.