Uta Kühn

Transformation-mediated ductility in CuZr-based bulk metallic glasses

Bulk metallic glasses (BMGs) generally fail in a brittle manner under uniaxial, quasistatic loading at room temperature. The lack of plastic strain is a consequence of shear softening, a phenomenon that originates from shear-induced dilation that causes plastic strain to be highly localized in shear bands. So far, significant tensile ductility has been reported only for microscopic samples of around 100 nm (ref. 4) as well as for high strain rates, and so far no mechanisms are known, which could lead to work hardening and ductility in quasistatic tension in macroscopic BMG samples. In the present work we developed CuZr-based BMGs, which polymorphically precipitate nanocrystals during tensile deformation and subsequently these nanocrystals undergo twinning. The formation of such structural heterogeneities hampers shear band generation and results in macroscopically detectable plastic strain and work hardening. The precipitation of nanocrystals and their subsequent twinning can be understood in terms of a deformation-induced softening of the instantaneous shear modulus. This unique deformation mechanism is believed to be not just limited to CuZr-based BMGs but also to promote ductility in other BMGs.

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.

Short-range order of Zr62−xTixAl10Cu20Ni8 bulk metallic glasses

Abstract The short-range order and crystallization behavior of slowly cooled Zr 62− x Ti x Al 10 Cu 20 Ni 8 bulk metallic glasses have been investigated in terms of the atomic pair correlation function as a function of Ti content x (2≤ x ≤7.5). The structural parameters point to the presence of chemical short-range order in these bulk glasses. An enhanced local excess free volume around the Ti atoms is concluded from density measurements. The first stage of crystallization in Zr 62− x Ti x Al 10 Cu 20 Ni 8 bulk glasses is related to changes in the medium-range order while the first neighborhood is retained. The atomic pair correlation functions of the first crystallization products are similar for all titanium contents. There is no indication of any special atomic arrangement for the particular alloy forming quasicrystals upon heating ( x =3). In case of Zr 54.5 Ti 7.5 Al 10 Cu 20 Ni 8 an ultrafine microstructure consisting of clusters of 2 nm in size is formed as the first step of crystallization.

Correlation between internal states and plasticity in bulk metallic glass

We report a close correlation between the internal states and plasticity in a bulk metallic glass (BMG) and discover that the optimization of copper-mold casting current can induce large plasticity stably in an otherwise brittle BMG. It is possible to confirm that larger plasticity corresponds to the internal states with more average free volume (FV) as revealed by lower density, higher enthalpy change, and higher Poisson’s ratio. The enhanced plastic deformation mechanism is interpreted based on the FV model of BMGs, and our results may have some implications for understanding the role of the FV during plastic deformation of BMGs.We report a close correlation between the internal states and plasticity in a bulk metallic glass (BMG) and discover that the optimization of copper-mold casting current can induce large plasticity stably in an otherwise brittle BMG. It is possible to confirm that larger plasticity corresponds to the internal states with more average free volume (FV) as revealed by lower density, higher enthalpy change, and higher Poisson’s ratio. The enhanced plastic deformation mechanism is interpreted based on the FV model of BMGs, and our results may have some implications for understanding the role of the FV during plastic deformation of BMGs.

Bulk nanostructured Zr-based multiphase alloys with high strength and good ductility

IFW Dresden, P.O. Box 27 00 16, D-01171 Dresden, Germany(Received August 21, 2000)(Accepted in revised form December 25, 2000)Keywords: Casting; Metallic glasses; Crystallization; Bulk nanostructured alloys; MechanicalpropertiesIntroductionAn increase in strength along with good ductility has recently been observed for a number of alloys withmultiphase nanoscale microstructures (1–4). In particular, partial crystallization of bulk metallicglass-forming alloys is a promising way for producing new bulk nanophase materials with outstandingmechanical properties at room temperature (5,6). Already homogeneous glassy alloys exhibit abeneficial combination of high yield strength, microplasticity on the order of about 1–2%, rather lowYoung’s moduli and high fracture toughness values as well as high wear resistance (7–9). Partialnanocrystallization can further improve these properties (1–4) and allows to obtain bulk nanostructuredalloys with promising potential as new engineering materials. As an example, this work reports on thesynthesis of bulk nanostructured (Zr/Ti)-Cu-Al-Ni alloys through devitrification of cast bulk glassyspecimens, and presents results for their mechanical properties at room and elevated temperatures.ExperimentalRods with 3 mm diameter and 50 mm length were prepared by arc-melting pure elements under argonatmosphere and casting into a copper mold. In order to restrict the oxygen content of the alloys to lessthan 0.2 at.% oxygen high purity starting materials (99.99% purity) were used and the casting wasperfomed under an argon atmosphere of 99.9999% purity. This gave fully amorphous specimens aftercasting. The samples were characterized by x-ray diffraction (XRD, Co Ka radiation), transmissionelectron microscopy (TEM) and differential scanning calorimetry (DSC) using a heating rate of 20Kmin

Effect of microstructure on the mechanical properties of as-cast Ti–Nb–Al–Cu–Ni alloys for biomedical application

The correlation between the microstructure and mechanical behavior during tensile loading of Ti68.8Nb13.6Al6.5Cu6Ni5.1 and Ti71.8Nb14.1Al6.7Cu4Ni3.4 alloys was investigated. The present alloys were prepared by the non-equilibrium processing applying relatively high cooling rates. The microstructure consists of a dendritic bcc β-Ti solid solution and fine intermetallic precipitates in the interdendritic region. The volume fraction of the intermetallic phases decreases significantly with slightly decreasing the Cu and Ni content. Consequently, the fracture mechanism in tension changes from cleavage to shear. This in turn strongly enhances the ductility of the alloy and as a result Ti71.8Nb14.1Al6.7Cu4Ni3.4 demonstrates a significant tensile ductility of about 14% combined with the high yield strength of above 820 MPa already in the as-cast state. The results demonstrate that the control of precipitates can significantly enhance the ductility and yet maintaining the high strength and the low Youngs modulus of these alloys. The achieved high bio performance (ratio of strength to Youngs modulus) is comparable (or even superior) with that of the recently developed Ti-based biomedical alloys.

Comparison of different post processing technologies for SLM generated 316l steel parts

Purpose – The purpose of this paper is to compare different post processing techniques for improving the high surface roughness (SR) characteristic of parts generated by selected laser melting (SLM).Design/methodology/approach – Test parts were built by SLM and their surface was characterized via SEM and optical measurements. The surface of the as‐generated parts was then modified by grinding, sand blasting and electrolytic and plasma polishing to reduce the SR.Findings – The change of the SR after the different surface treatments was quantified and compared. The effectiveness and usability of the post processing techniques and their combinations were determined. The results indicate that some of the post processes are only usable for simple structures.Research limitations/implications – The amount of abrasion induced by the different surface treatments was not quantified. A major focus of future work should deal with this issue.Practical implications – The surface quality of parts with simple geometry ca...

Relaxation and crystallization of amorphous Zr65Al7.5Cu17.5Ni10

Abstract The structural development of amorphous Zr65Al7.5Cu17.5Ni10 obtained by rapid quenching was investigated after annealing and at elevated temperatures by means of X-ray diffraction (XRD) and differential scanning calorimetry measurement (DSC). The interference functions of the metallic alloy in the undercooled liquid as well as in the amorphous state were determined. The results confirm experimentally that the amorphous structure represents the frozen in undercooled liquid state. Structural relaxation of the as-quenched state was observed at annealing temperatures TA below and above the glass transition temperature Tg = 630 K. The calculated pair correlation functions, representing mainly the Zr-Zr distribution, show the behaviour typically for metallic glasses at a first maximum of r1 = 3.11 A (nearest neighbour distance ~ atomic diameter) and a split second peak in G(r). The crystallization appears after a temperature dependent incubation time τ. The crystallization is characterized by the simultaneous formation of several phases, mainly tetragonal CuZr2 and further unknown phases. The phase composition was found to be dependent on the annealing conditions. Continuous heating with heating rates greater than 5 K min −1 and annealing at temperatures TA > 720 K lead to the formation of an additional metastable cubic NiZr2-type phase.

Structural behavior of Zr52Ti5Cu18Ni15Al10 bulk metallic glass at high temperatures

The structural behavior of the Zr52Ti5Cu18Ni15Al10 bulk glass-forming alloy has been investigated in situ by means of high-temperature x-ray synchrotron diffraction. The dependence of the structure factor of the glass can be well described with a Debye–Waller factor and a Debye temperature θ=412 K. At the glass transition, the structure factor significantly decreases due to additional thermal excitations. The extrapolation of the structure factor of the supercooled liquid to temperatures above the liquidus curve is in agreement with experimentally determined values of the melt. The short-range order of the glass, of the supercooled liquid state, and of the equilibrium melt at T=1193 K, is found to be quite similar. The formation of complex chemically ordered clusters in the melt is proposed to be essential for the high-glass-forming ability of this alloy.

Deformation and fracture behavior of composite structured Ti-Nb-Al-Co(-Ni) alloys

Tensile ductility of the Ti-based composites, which consist of a β-Ti phase surrounded by ultrafine structured intermetallics, is tunable through the control of intermetallics. The two Ti-based alloys studied exhibit similar compressive yield strength (about 1000 MPa) and strain (about 35%–40%) but show a distinct difference in their tensile plasticity. The alloy Ti71.8Nb14.1Ni7.4Al6.7 fractures at the yield stress while the alloy Ti71.8Nb14.1Co7.4Al6.7 exhibits about 4.5% of tensile plastic deformation. To clarify the effect of microstructure on the deformation behavior of these alloys, tensile tests were carried out in the scanning electron microscope. It is shown that the distribution as well as the type of intermetallics affects the tensile ductility of the alloys.