Christian Leinenbach

Thermodynamic re-assessment of Fe-Ti binary system

The Fe–Ti binary system was re-assessed using the CALPHAD method in order to improve the capability of being extrapolated to a ternary or higher-order system. Compared with previous assessments, the main focus was put on the thermodynamic description of the two intermetallic compounds Fe2Ti and FeTi. The C14_Laves phase Fe2Ti was described by the two-sublattice model, which is widely used at present. By checking the homogeneity range on the boundary of the ternary systems involving the binary, the phase boundary of this compound was further confirmed. The FeTi phase with a BCC_B2 crystal structure was treated as the ordered phase of the BCC_A2 phase and a unified Gibbs energy function was used to describe both the ordered and disordered phases. Reproduction of the specific heat capacities of these compounds was another aspect paid particular attention to. Comprehensive comparisons of the calculated and experimental results regarding the phase diagram and thermodynamic properties show a good agreement between them and prove the validity of the present thermodynamic description.

Stress recovery behaviour of an Fe-Mn-Si-Cr-Ni-VC shape memory alloy used for prestressing

This paper describes the stress recovery behaviour of an Fe‐17Mn‐5Si‐10Cr‐4Ni‐1(V, C) (mass%) shape memory alloy used for prestressing of civil structures. The prestressing due to the shape memory effect was simulated by a series of tests with pre-straining of the material followed by heating and cooling back at constant strain. Different pre-strain and heating conditions were examined. Moreover, the response due to additional mechanical and thermal cyclic loading has been investigated. These results were used to predict the partial prestress loss in a structure due to variable loading during operation. Finally, a heating test at constant strain was performed after the cyclic loading to check the possibility of reactivating the prestress lost during an exceptionally high load. (Some figures may appear in colour only in the online journal)

Processing of metal-diamond-composites using selective laser melting

Purpose – The purpose of this paper is a feasibility study that was performed to investigate the basic processability of a diamond-containing metal matrix. Powder-bed-based additive manufacturing processes such as selective laser melting (SLM) offer a huge degree of freedom, both in terms of part design and material options. In that respect, mixtures of different powders can offer new ways for the manufacture of materials with tailored properties for special applications such as metal-based cutting or grinding tools with incorporated hard phases. Design/methodology/approach – A two-step approach was used to first investigate the basic SLM-processability of a Cu-Sn-Ti-Zr alloy, which is usually used for the active brazing of ceramics and superhard materials. After the identification of a suitable processing window, the processing parameters were then applied to a mixture of this matrix material with 10-20 volume per cent artificial, Ni-coated mono-crystalline diamonds. Findings – Even though the processing...

Fatigue Assessment of Defect-Free and Defect-Containing Brazed Steel Joints

This work aims at the development of lifetime estimation procedures for defect-free and defect-containing brazed joints. Preliminary investigations were performed to measure the influence of specimen geometry on the joint strength. To estimate the influence of defects on the fatigue lifetime, defect-free specimens were compared with specimens containing defects. The experiments show that defect-containing specimens provide considerably lower joint strengths than defect-free specimens. The decrease of the fatigue strength with increasing defect size can be shown, but the direct comparison of different defects is not possible with S-N-curves. Based on the experimental results and on theoretical investigations, a procedure was developed to estimate the lifetime of defect-free and defect-containing brazed joints based on the stress intensity caused by a defect.

In situ investigation of phase transformations in Ti-6Al-4V under additive manufacturing conditions combining laser melting and high-speed micro-X-ray diffraction

We present combined in situ X-ray diffraction and high-speed imaging to monitor the phase evolution upon cyclic rapid laser heating and cooling mimicking the direct energy deposition of Ti-6Al-4V in real time. Additive manufacturing of the industrially relevant alloy Ti-6Al-4V is known to create a multitude of phases and microstructures depending on processing technology and parameters. Current setups are limited by an averaged measurement through the solid and liquid parts. In this work the combination of a micro-focused intense X-ray beam, a fast detector and unidirectional cooling provide the spatial and temporal resolution to separate contributions from solid and liquid phases in limited volumes. Upon rapid heating and cooling, the β ↔ α′ phase transformation is observed repeatedly. At room temperature, single phase α′ is observed. Secondary β-formation upon formation of α′ is attributed to V partitioning to the β-phase leading to temporary stabilization. Lattice strains in the α′-phase are found to be sensitive to the α′ → β phase transformation. Based on lattice strain of the β-phase, the martensite start temperature is estimated at 923 K in these experiments. Off-axis high speed imaging confirms a technically relevant solidification front velocity and cooling rate of 10.3 mm/s and 4500 K/s, respectively.

Microstructure and Mechanical Performance of Cu-Sn-Ti-Based Active Braze Alloy Containing In Situ Formed Nano-Sized TiC Particles

A Cu-Sn-Ti-based active brazing filler alloy was in situ reinforced with nanosized TiC particles by adding different amounts of a cellulose nitride-based binder. The TiC particles emanate from a reaction of the Ti within the filler alloy with the carbon from the binder that does not decompose completely during heating. The correlation between the microstructure and mechanical performance was studied. In addition, the effect of different binder amounts on the shear strength and cutting performance of brazed diamond grains was studied in shear tests and single grain cutting tests. The results clearly show that the mechanical performance of the brazed diamond grains can be improved by the formation of TiC particles. This is attributed to particle strengthening of the filler alloy matrix as well as to the decreasing grain size and more homogeneous distribution of the (Cu,Sn)3Ti5 phase with increasing amount of binder.

High-temperature processable carbon–silicate nanocomposite cold electron cathodes for miniature X-ray sources

We report on the fabrication, Raman characterization and electron emission behavior of cold electron cathodes that are specifically designed to be used in miniature X-ray sources. The cathodes are fabricated by screen-printing a nanocomposite paste comprising multiwall carbon nanotubes as fillers in a matrix of graphite, glass and bentonite clay. The cathodes can resist high temperatures up to 880 °C and, as such, can survive most high-temperature brazing steps required for vacuum-tight sealing of the sources. We demonstrate peak emission current densities around 300 mA cm−2 at an applied electric field of 175 kV cm−1 and stable emission around 50 mA cm−2 at 125 kV cm−1 for at least 5 hours without significant degradation. X-rays were successfully generated at 3.2 watts of peak power (80 μA, 40 kV) with a commercial X-ray tube modified to accommodate a paste cathode.

Numerical and Experimental Investigations on the Defect Tolerance of Brazed Steel Joints

Abstract Based on the combination of FE-calculations and experimental results a defect assessment procedure for brazed steel joints was developed in the present work, which reliably takes into account the effect of different steel heat treatments and defined artificial defects in the braze layer. Tensile tests with defect free joints consisting of X3CrNiMo13-4 as substrate material and of Au-18wt.-%Ni as filler metal serve as a basis to simulate the elastic-plastic material behaviour. By correlating experimental and numerical methods, failure criteria could be derived which indicate and predict failure of brazed components. These failure criteria were also presumed to calculate the maximum tolerable loads for specimens containing different defects. The results show that the simulations correlate very well with the experimental results. Furthermore, they provide a possibility to analyze the failure behaviour for different defect types and the influence of different heat treatments on the joint strength.

Fracture Behaviour of Brazed Soft Martensitic Stainless Steel Joints under Cyclic Loading

The fracture behaviour of brazed joints of the soft martensitic stainless steel X3CrNiMo13-4 under cyclic loading is investigated. The fatigue crack propagation curves (da/dN-ΔK) were derived for different load ratios R. The fatigue crack threshold values ΔKth were estimated to be 9 MPa m0.5, 7 MPa m0.5, 6 MPa m0.5 and 4 MPa m0.5 for the R values of 0.1, 0.3, 0.5 and 0.7, respectively. In addition, crack growth curves were derived for different constant loads ΔF. The Paris exponent, n, was estimated for the different R values and found to be very high compared to homogeneous materials. The work was completed with microstructural and fractographic investigation by scanning electron microscope (SEM).

Determination of Liquidus Temperature in Sn–Ti–Zr Alloys by Viscosity, Electrical Conductivity and XRD Measurements

Abstract The Sn – Ti – Zr system is an important subsystem for Cu based active brazing filler metals. Experimental results on this system, however, are rather scarce. The diagram is rather uncertain regarding most of the liquidus, especially on the Sn rich side. In this work, the atomic structure and temperature dependence of structure-sensitive physical properties (dynamic viscosity and electrical conductivity) of liquid Sn – Ti – Zr alloys in the Sn-rich corner were investigated in a wide temperature range with special attention to the melting – solidification region. The results allowed the liquidus line position to be specified.