W. Löser

HIGH-STRENGTH MATERIALS PRODUCED BY PRECIPITATION OF ICOSAHEDRAL QUASICRYSTALS IN BULK ZR-TI-CU-NI-AL AMORPHOUS ALLOYS

Zr62−xTixCu20Ni8Al10 (3⩽x⩽5) amorphous alloys crystallize via precipitation of icosahedral quasicrystals in the primary crystallization step, leading to nano-sized quasicrystals embedded in an amorphous matrix. Ti is the decisive component favoring the precipitation of quasicrystals. The mechanical properties of the crystallized alloys with different amounts of quasicrystalline phase were measured by compression and bending tests. If the volume fraction of quasicrystalline precipitates is below about 50%, the strength increases with an increasing amount of quasicrystalline precipitates, but the ductility does not decrease significantly in comparison with the amorphous counterpart. The fracture stress reaches 1835 MPa for 50 vol % of quasicrystals. Quasicrystalline precipitates of more than 60 vol % lead to reduction of ductility and strength. This shows a way of producing bulk quasicrystalline materials of high strength by crystallization of bulk amorphous alloys.

High-strength Ti-base ultrafine eutectic with enhanced ductility

(Ti0.705Fe0.295)100−xSnx (x=0 and 3.85) ultrafine eutectics were prepared by slow cooling from the melt through cold crucible casting. The addition of 3.85 at. % Sn to the binary Ti–Fe eutectic decreases the strength slightly but considerably improves the plastic deformability under uniaxial compressive loading from ef=2.1% to 9.6% strain to failure. The change in the morphology of the eutectic and the distribution of the FeTi phase are suggested as origin of the improvement of the mechanical properties.

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.

High-strength Zr-Nb-(Cu,Ni,Al) composites with enhanced plasticity

Zr73.5Nb9Cu7Ni1Al9.5 and Zr66.4Nb6.4Cu10.5Ni8.7Al8.0 composites of bcc β-Zr(Nb) dendrites embedded in a nanocrystalline matrix were prepared by slow cooling from melt. The increase of Nb content from 6.4 to 9 at. % slightly reduces the strength, but considerably improves the plastic elongation under uniaxial compressive loading from ep=0.6% to 14.8%. The interaction of strain with dendrites and the nanocrystalline matrix is suggested as origin of the improvement of the mechanical properties.

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.

Effect of cooling rate on the precipitation of quasicrystals from the Zr–Cu–Al–Ni–Ti amorphous alloy

The Zr57Cu20Al10Ni8Ti5 alloy solidifies into an amorphous phase upon rapid quenching or casting at low cooling rates. However, the amorphous alloys formed at different cooling rates exhibit different crystallization behavior. The slowly cooled bulk amorphous alloys prepared by copper mold casting reveal a first crystallization peak at 715 K upon heating at 0.33 K/s and crystallize via precipitation of an icosahedral quasicrystalline phase in the first crystallization step. The rapidly quenched ribbons exhibit a first crystallization peak at 720 K and crystallize by simultaneous precipitation of the quasicrystalline phase together with Zr2Cu and Zr2Ni intermetallic phases in the first stage of crystallization. It is supposed that the undercooled Zr57Cu20Al10Ni8Ti5 melt has a tendency to develop an icosahedral short-range order, which is favored by low cooling rate. As a result, the bulk amorphous alloy has a short-range order close to a quasicrystalline phase. In contrast, the structure of the ribbon is mo...

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.

Solidification kinetics and phase formation of undercooled eutectic Ni-Nb melts

The non-equilibrium solidification behavior of undercooled eutectic Ni{sub 84}Nb{sub 16} and Ni{sub 59.5}Nb{sub 40.5} melts has been analyzed by in situ observation of recalescence events during electromagnetic levitation of undercooled melts. Levitated drops of controlled undercooling were quenched onto chill substrates and subjected to phase and microstructure analysis. For Ni{sub 84}Nb{sub 16} a maximum melt undercooling of 276 K has been achieved. A transition from coupled eutectic to primary supersaturated {alpha}-Ni dendrite growth has revealed beyond a critical undercooling of 30 K. Beyond 110 K the primary Ni{sub 3}Nb phase occurred for substrate quenching. The undercooling of Ni{sub 59.5}Nb{sub 40.5} melt was limited to 135 K. Bulk amorphous samples up to 2 mm thick have been prepared by quenching of undercooled Ni{sub 59.5}Nb{sub 40.5} melts. On slow cooling the metastable phases decompose into an anomalous eutectic microstructure.