Thomas Ebel

Ternary aluminides LnT2Al10 (Ln=Y, La–Nd, Sm, Gd–Lu andT=Fe, Ru, Os) with YbFe2Al10 type structure and magneticproperties of the iron-containing series

Of the title compounds 25 have been prepared for the first time. They crystallize with the orthorhombic YbFe2Al10 type structure (Cmcm, Z=4), which was refined from single-crystal X-ray data of SmFe2Al10 [a=898.9(2), b=1018.6(2), c=904.3(2) pm, R=0.043 for 823 structure factors and 19 variable parameters] and LaOs2Al10 [a=920.5(6), b=1027.4(5), c=916.9(6) pm, R=0.047 for 1475 F values and 21 variables]. Two aluminium sites of LaOs2Al10 show significant deviations from the ideal occupancies, resulting in the exact composition LaOs2Al9.74(2) . The magnetic properties of the iron-containing compounds have been determined with a SQUID magnetometer in the temperature range 2–300 K with magnetic flux densities up to 5.5 T. YFe2Al10 and LaFe2Al10 are Pauli-paramagnetic, indicating that the iron atoms in these isotypic compounds do not carry magnetic moments. The behaviour of the other iron-containing compounds is dominated by the magnetic properties of the rare-earth components. CeFe2Al10 and YbFe2Al10 show mixed-valent behaviour. This can also be concluded from the cell volumes for CeRu2Al10 and YbRu2Al10 . SmFe2Al10 is Van Vleck paramagnetic, and the others follow the Curie–Weiss law with magnetic ordering temperatures of <20 K. The magnetic properties of these compounds were further investigated by magnetization measurements at 2 K.

Rare earth and uranium transition metal pnictides with LaFe4P12 structure

Abstract The new compounds NdFe 4 Sb 12 , SmFe 4 Sb 12 , EuFe 4 Sb 12 and SmRu 4 Sb 12 were prepared by reaction of the binary lanthanoid antimonides LnSb or EuSb 2 respectively, with the transition metals and antimony in sealed silica tubes. Single-crystal structure refinements of NdFe 4 Sb 12 as well as of the previously reported compounds EuFe 4 P 12 , UFe 4 P 12 , EuRu 4 Sb 12 and NdOs 4 Sb 12 led to residuals of R = 0.014, 0.023, 0.013, 0.025 and 0.042 respectively. Systematic differences in the interatomic distances of CeFe 4 P 12 , EuFe 4 P 12 and ThFe 4 P 12 suggest that the Fermi level cuts through a phosphorus-phosphorus antibonding band. The thermal parameters of the rare earth components increase with the size of their transition metal-pnictogen cages. The cell volumes of the LaFe 4 P 12 type pnictides suggest that cerium and europium have intermediate valencies. In agreement with the higher electronegativity of phosphorus, cerium is the most tetravalent in the phosphides and europium is the most divalent in the antimonides.

EuTa2Al20, Ca6W4Al43 and other compounds with CeCr2Al20 and Ho6Mo4Al43 type structures and some magnetic properties of these compounds

Abstract The 25 new compounds CaT2Al20 (T=Ti, Nb, Ta, Mo), LnT2Al20 (Ln=La, Ce, Pr, Nd, Sm, Eu; T=Nb, Ta), SmCr2Al20, TbCr2Al20, Ca6W4Al43, Y6Nb4Al43, Y6Ta4Al43 and Ln6Mn4Al43 (Ln=Ho–Yb) have been prepared by reaction of the elemental components in alumina crucibles under argon. They crystallize with CeCr2Al20 and Ho6Mo4Al43 type structures, respectively, which were refined from single-crystal X-ray data of EuTa2Al20 ( Fd 3 m , a=1478.3(1) pm, R=0.025 for 312 structure factors and 15 variable parameters) and Ca6W4Al43 (P63/mcm, a=1108.4(4) pm, c=1778.8(6) pm, R=0.010 for 849 F values and 53 variables). All atomic positions are fully occupied, with the exception of one aluminium site of EuTa2Al20, which has an occupancy of only 81(2)%. The compounds LnCr2Al20 (Ln=La–Nd, Eu) and LnV2Al20 (Ln=La–Pr, Eu–Tb) are confirmed. The cell volumes of the cerium, europium and ytterbium compounds LnT2Al20 (Ln=Ce, Eu; T=Ti, V, Nb, Ta, Cr, Mo, W), YbTi2Al20 and Yb6Mn4Al43 deviate from the smooth functions observed for the cell volumes of the compounds with the typically trivalent rare earth elements. Magnetic susceptibility data, measured with a SQUID magnetometer, show Pauli paramagnetism for LaTi2Al20, LaMo2Al20, CeTi2Al20, CeV2Al20 and CeMo2Al20, thus indicating cerium to be tetravalent in agreement with the cell volumes of these compounds. The magnetic susceptibilities of EuTi2Al20 and EuMo2Al20 follow the Curie–Weiss law; their magnetic moments correspond to divalent europium.

Microstructure and mechanical behavior of metal injection molded Ti–Nb binary alloys as biomedical material

The application of titanium (Ti) based biomedical materials which are widely used at present, such as commercially pure titanium (CP-Ti) and Ti-6Al-4V, are limited by the mismatch of Youngs modulus between the implant and the bones, the high costs of products, and the difficulty of producing complex shapes of materials by conventional methods. Niobium (Nb) is a non-toxic element with strong β stabilizing effect in Ti alloys, which makes Ti-Nb based alloys attractive for implant application. Metal injection molding (MIM) is a cost-efficient near-net shape process. Thus, it attracts growing interest for the processing of Ti and Ti alloys as biomaterial. In this investigation, metal injection molding was applied to the fabrication of a series of Ti-Nb binary alloys with niobium content ranging from 10wt% to 22wt%, and CP-Ti for comparison. Specimens were characterized by melt extraction, optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). Titanium carbide formation was observed in all the as-sintered Ti-Nb binary alloys but not in the as-sintered CP-Ti. Selected area electron diffraction (SAED) patterns revealed that the carbides are Ti2C. It was found that with increasing niobium content from 0% to 22%, the porosity increased from about 1.6% to 5.8%, and the carbide area fraction increased from 0% to about 1.8% in the as-sintered samples. The effects of niobium content, porosity and titanium carbides on mechanical properties have been discussed. The as-sintered Ti-Nb specimens exhibited an excellent combination of high tensile strength and low Youngs modulus, but relatively low ductility.

Metal Injection Moulding of Titanium and Titanium-Aluminides

Metal injection moulding (MIM) attracts growing interest as an economic net-shape manufacturing technique for the processing of titanium and titanium alloys. Even for titanium-aluminides, intended for high-temperature applications, MIM is seen as a reasonable technique to overcome processing problems with conventional methods. In this paper, basic requirements in terms of raw materials, facilities and processing in order to produce high performance components are presented. Main focus is laid on the well-known Ti-6Al-4V alloy. It is shown that the tensile properties of specimens after MIM processing can exceed the requirements given by ASTM standards even without performing an additional HIP process. For an oxygen content ranging from 0.15 to 0.33 wt% plastic elongation yields excellent 14%. Fatigue measurements performed by means of 4-point-bending tests show that grain size is more important than residual porosity in order to achieve a high endurance limit. This is shown by addition of boron powder which refines the microstructure dramatically. The modified alloy Ti-6Al-4V-0.5B yields an endurance limit of 640 MPa compared to 450 MPa of MIM parts made from standard alloy powder. Sintered components from Ti-45Al-5Nb-0.2B-0.2C (at%) powder made by inert gas atomising (EIGA technique) and processed by MIM exhibit a residual porosity of only 0.2% and tensile properties comparable to cast material.

Preparation, crystal structure and magnetic properties of the uranium nickel phosphides UNi3P2, UNi4P2, U6Ni20P13 and U2Ni12P7

Abstract The compounds UNi 3 P 2 and UNi 4 P 2 were prepared for the first time and their structures were determined from X-ray powder data. UNi 3 P 2 has a HoCo 3 P 2 type structure with the orthorhombic lattice constants a =1047.5(2) pm, b =379.40(4) pm and c =1238.2(2) pm. UNi 4 P 2 is isotypic with ZrFe 4 Si 2 . Its tetragonal structure was refined by the Rietveld method: P 4 2 / mnm , a =707.67(1) pm, c =365.58(1) pm, R F =0.013 for 78 structure factors and six positional and thermal parameters. The magnetic properties of these and the previously reported compounds U 2 Ni 12 P 7 and U 6 Ni 20 P 13 were investigated with a SQUID magnetometer. They show temperature-dependent paramagnetic behaviour with magnetic ordering temperatures all below 50 K. The nickel atoms do not seem to carry magnetic moments, while the moments of the uranium atoms vary between 1.6±0.1 and 2.1±0.1 μ B per uranium atom. The crystal structures of the title compounds are closely related. The compounds UNi 2 P 2 , U 3 Ni 3 . 3 4 P 6 and UP 2 belong to another structural family. The magnetic ordering temperatures of these compounds correlate with the uranium content.

Titanium carbide precipitation in Ti–22Nb alloy fabricated by metal injection moulding

Abstract Metal injection moulding was applied to fabricate Ti–22Nb alloy as a low modulus material for biomedical applications. Tensile test specimens were injection moulded, followed by debinding and sintering. Sintering was at 1500°C for 4 h under vacuum (10–3 Pa). Selected as-sintered Ti–22Nb samples were hot isostatically pressed at 915°C/100 MPa for 2 h. The nature of the titanium carbide precipitates in the as-sintered Ti–22Nb alloy was investigated. Selected area electron diffraction patterns revealed that the carbides are Ti2C with a fcc structure. The calculation of the phase diagram showed a significant decrease of carbon solubility in Ti–22Nb compared with that in Ti from 500 to 1500 °C, contributing to the carbide precipitation in Ti–22Nb. Due to the carbide precipitation, the as-hipped Ti–22Nb alloy exhibited higher tensile strength but lower elongation than conventionally processed Ti–22Nb.

A Porous TiAl6V4 Implant Material for Medical Application

Increased durability of permanent TiAl6V4 implants still remains a requirement for the patients well-being. One way to achieve a better bone-material connection is to enable bone “ingrowth” into the implant. Therefore, a new porous TiAl6V4 material was produced via metal injection moulding (MIM). Specimens with four different porosities were produced using gas-atomised spherical TiAl6V4 with different powder particle diameters, namely, “Small” (<45 μm), “Medium” (45–63 μm), “Mix” (90% 125–180 μm + 10% <45 μm), and “Large” (125–180 μm). Tensile tests, compression tests, and resonant ultrasound spectroscopy (RUS) were used to analyse mechanical properties. These tests revealed an increasing Youngs modulus with decreasing porosity; that is, “Large” and “Mix” exhibit mechanical properties closer to bone than to bulk material. By applying X-ray tomography (3D volume) and optical metallographic methods (2D volume and dimensions) the pores were dissected. The pore analysis of the “Mix” and “Large” samples showed pore volumes between 29% and 34%, respectively, with pore diameters ranging up to 175 μm and even above 200 μm for “Large.” Material cytotoxicity on bone cell lines (SaOs-2 and MG-63) and primary cells (human bone-derived cells, HBDC) was studied by MTT assays and highlighted an increasing viability with higher porosity.

On the Determination of Magnesium Degradation Rates under Physiological Conditions

The current physiological in vitro tests of Mg degradation follow the procedure stated according to the ASTM standard. This standard, although useful in predicting the initial degradation behavior of an alloy, has its limitations in interpreting the same for longer periods of immersion in cell culture media. This is an important consequence as the alloy’s degradation is time dependent. Even if two different alloys show similar corrosion rates in a short term experiment, their degradation characteristics might differ with increased immersion times. Furthermore, studies concerning Mg corrosion extrapolate the corrosion rate from a single time point measurement to the order of a year (mm/y), which might not be appropriate because of time dependent degradation behavior. In this work, the above issues are addressed and a new methodology of performing long-term immersion tests in determining the degradation rates of Mg alloys was put forth. For this purpose, cast and extruded Mg-2Ag and powder pressed and sintered Mg-0.3Ca alloy systems were chosen. DMEM Glutamax +10% FBS (Fetal Bovine Serum) +1% Penicillin streptomycin was used as cell culture medium. The advantages of such a method in predicting the degradation rates in vivo deduced from in vitro experiments are discussed.

Magnesium powder injection moulding for biomedical application

Abstract Currently, commercial biodegradable implants are mainly made from degradable polymers, such as polyglycolic acid or polylactide acid (PLA). These polymer implants, produced by injection moulding technique, suffer from long degradation times between 18 and 36 months, poor mechanical properties and acidic degradation behaviour. On the other hand, magnesium alloys are drawing increasing interest as biodegradable medical implant material for orthopaedic applications in bone tissue; thus, a replacement of polymers by Mg would be attractive. The production of biomedical and biodegradable Mg alloy parts and implants by powder metallurgy and metal injection moulding (MIM) respectively offers the opportunity for economic manufacturing of parts with mechanical properties matching those of cortical bone tissue, as well as the provision of porous surface structures beneficial for cell ingrowth and vascularisation. Furthermore, the technique guarantees a homogenous microstructure being crucial for a predictable degradation process. This study shows how magnesium powder can be processed successfully by MIM. Recent magnesium alloy implant prototypes and tensile test specimen, produced by MIM technique, provide strength and stiffness twice as high compared to modern polymer based implants. Ultimate tensile strength (UTS) of 131 MPa, yield strength of 64 MPa, residual porosity of 2–6% and elastic modulus of 46 GPa, measured by dynamic method, were achieved under application of special sintering technique and sintering atmosphere control. The paper is focussing on sintering methods and porosity control and measurement.