Volume 100, Issue 8

The present paper summarizes our progress in establishing a novel production technology for -TiAl components to be used in advanced aircraft engines. In the beginning the main emphasis is put on the design of a -TiAl based alloy which exhibits excellent hot-workability. Then, the development of a “near conventional” hot-die forging route for this type of intermetallic material is described. Finally, the effect of two-step heat-treatments on the microstructure and the mechanical properties is discussed. Because of the small “deformation window” hot-working of -TiAl alloys is a complex and difficult task and, therefore, isothermal forming processes are favoured. In order to increase the deformation window a novel Nb and Mo containing -TiAl based alloy (TNM TM alloy) was developed, which solidifies via the β-phase and exhibits an adjustable β/B2-phase volume fraction. Due to high volume fractions of -phase at elevated temperatures the alloy can be hot-die forged under near conventional conditions, which means that conventional forging equipment with minor and inexpensive modifications can be used. Examples for the fabrication of -TiAl components employing a near conventional forging route are given. With subsequent heat-treatments balanced mechanical properties can be achieved. The results of tensile and creep tests conducted on forged and subsequently heat-treated TNM TM material are presented.

Cast irons with different graphite morphologies were thoroughly analysed and their influence on the electrical properties was studied. Graphite morphology was analysed with focused ion beam tomography. The graphite crystal structure, nucleation and growth mechanisms were studied using chemical analysis and thin foils observations with transmission electron microscopy. Quantitative three-dimensional characterisation of the shape and spatial connectivity, as well as the growth mechanisms provided understanding of the structure of the different graphite morphologies and allowed the estimation of their influence on the effective properties using simulations directly on the tomographic data. First correlations between manufacturing parameters, spatial microstructure and effective properties were suggested for advanced material development and non-destructive quality control.

Shape memory wires were trained under constant stress in order to introduce a uniaxial shape memory effect by a thermomechanical treatment. These investigations were carried out on three different alloy systems (NiTi, NiTiW and CuAlNi) with different microstructures (cold-worked and annealed condition, with and without particles, large and small grain size). Several thousand thermal cycles were performed on the trained shape memory elements, continuously observing the changes in the deformation behaviour. The influence of the microstructure on development and stability of the intrinsic two-way shape memory effect has been discussed. Furthermore, this work deals with the production of thin specimens of shape memory alloys by melt-spinning and splat-cooling, the training procedure and their characterization with respect to microstructure and functional properties.

The role of eutectics in recrystallization in a single crystal nickel-base superalloy – CMSX-4 – was studied by electron backscatter diffraction and electron probe micro-analysis techniques. It was shown that eutectics can initiate recrystallization by a particle stimulated nucleation mechanism which operates in interdendritic regions. During grain growth in the course of recrystallization, eutectics were observed to inhibit the motion of the advancing recrystallization grain boundary. Alternatively, a recrystallization grain boundary can migrate through eutectics by forming a large ′ phase particle behind the advancing front, either with the same crystal orientation as the eutectic the boundary advances into or, in some cases, with completely new orientations, i. e. neither the same as the eutectic it grows into nor the same as the recrystallized grains surrounding it.

The influence of varying substrate temperature, N 2 partial pressure, ion energy, and ion-to-Ti flux ratio on the texture development and mechanical properties of TiN is investigated in detail. We show that during low substrate temperature (T s = 300 °C) reactive sputtering of TiN in a mixed Ar + N 2 discharge a change from 111- to 001-oriented growth occurs when increasing the ion-to-Ti ratio J i / J Ti above 2.5 while using a low ion energy E i of 30 eV. This texture change can be reversed to a 111-oriented growth by increasing the ion energy to 60 and 90 eV when using high ion-to-Ti ratios J i / J Ti of 2.5 and 9, without introducing strain. Thereby the hardness can be increased from ∼31 to 37 GPa with only minor changes in compressive stresses. Consequently, by defining the ion-to-Ti ratio and the ion energy during low substrate temperature reactive sputtering of TiN the texture development towards 111- or 001-oriented growth can be controlled. Based on previous studies and the texture development as a function of substrate temperature and N 2 -partial pressure, we propose that varying the ion-to-Ti ratio and the ion energy result in different N-terminated surfaces and hence different Ti-fluxes from 111- to 001-oriented grains and vice versa.

The special kind of internal oxidation phenomenon that can take place in pure metals containing a high concentration of defects, particularly thermodynamically stabilized vacancy clusters, is presented. The process starts with dissolution of oxygen into the metal at the free surfaces and continues with diffusion of oxygen atoms into the volume of the metal matrix where they are trapped at numerous defects in the crystal lattice. The increasing oxygen activity at these places causes local oxidation of the matrix and consequently precipitation of fine oxide particles. The results of experimental studies performed with pure copper metal confirmed the formation of microstructures consisting of continuous three-dimensional copper matrix and homogeneous finely dispersed Cu 2 O particles of submicron size. The volume fraction of precipitated oxides depends on the starting microstructure of the Cu matrix, and on the temperature and time of the internal oxidation process.

Hot-work tool steels are used for die casting, extrusion molding and drop forging tools and thus are exposed to high temperatures and elevated stresses during application. To get insight into the materials' behavior under those conditions, short-term creep tests are performed at temperatures in the range of 550 – 590 °C and at stress levels ranging from 400 – 750 MPa. A steady-state creep range is not observed. Instead only a minimum strain rate appears. This minimum is followed by an extended tertiary creep range. Based on the observed stress exponent at 590 °C recovery-controlled dislocation creep has been identified as the dominant creep mechanism. During creep carbide coarsening, recovery and recrystallization occur, as observed by scanning electron microscopy and transmission electron microscopy.

Probing mechanical properties in the micrometer regime is of current interest in materials science. A focused ion beam microscope was employed to fabricate miniaturized specimens, while an indenter installed in a scanning electron microscope was utilized to actuate the samples and record the load and displacement data during the deformation. Examples for miniaturized compression, tension, bending, as well as newly developed bending fatigue and bending fracture experiments are presented, demonstrating the unique flexibility of in-situ mechanical testing in the scanning electron microscope at small length scales.

The nickel-based superalloy Allvac 718Plus was developed in order to combine formability and cost advantages of Alloy 718 with high temperature strength of Alloy 720. Typical applications for this new material are turbine disks that are produced by closed die forging, e. g. by screw pressing. The final mechanical properties are strongly related to the microstructure, which forms during hot forming. It is therefore of great interest to model both the dynamic and static grain structure development during processing. In this work, dynamic and static recrystallization as well as normal grain growth models are described for this material. The calculated grain structure evolution in Allvac 718Plus is compared to that of Alloy 718 in the typical process range of disk forging. Additionally, both the experimental program and structure analysis methodology are specified.

Several means of modifying surfaces of nickel – titanium have been investigated to gather information on how topography and the nickel release rates can be influenced. Electrolytically etched nickel – titanium surfaces show a very fine structure but show the highest nickel release of all modified surfaces. Electropolishing reduces the initial roughness of the material and lowers the Ni release due to very thick oxide films emerging in the course of the process of electropolishing. Further enhancement of the oxide layer can be achieved by oxidation in dry air or in a selectively oxidising atmosphere. Selectively oxidised samples exhibit the smallest Ni release of all modified surfaces.

In practice the thermal stability of high-speed steels is assessed by simple hardness tests. As high-speed steels can be seen as a composite material of a particle-strengthened metallic matrix and coarse primary carbides, the measured hardness is a mixture of both microstructural constituents. Whereas the primary carbides do not change during tempering, the particle-strengthened matrix is prone to microstructural changes, thus affecting the mechanical properties. Consequently, characterization methods which are sensitive only to the matrix have to be applied. In this study single-peak profile analysis were carried out to investigate the thermal stability of high-speed steel matrices. By applying this method the lattice distortion was obtained and compared to classical hardness tests. In a first step, the applicability of this method was investigated. Afterwards, four different high-speed steels were analyzed. The investigations showed that the distortion of the lattice of the matrix depends on their microstructure and composition, which both depend on the heat treatment history. The observed differences in hardness can be correlated to differences in lattice distortion of the matrix and the present amount of primary carbides.

Structural evolution, residual stresses, and stress relaxation at elevated temperatures were investigated for sputtered TiN/Cu multilayer films with a bilayer thickness between 100 and 800 nm, as well as nanocomposite films with a Cu content up to 30 at.%, to illuminate the effect of adding a soft metal to a hard nitride. In the nanocomposites the Cu domain size increases with rising Cu content from 7 to 22 nm at the expense of the TiN grain size, while it is strongly related to the bilayer thickness in the multilayers at constant TiN domain size. Films were thermally cycled in vacuum from room temperature up to 650 °C. Stresses during heating are governed by thermo-elastic behaviour and at temperatures above 300 °C by plastic deformation of the Cu phase and stress relaxation of both Cu and TiN, whereas the cooling phase is determined by thermo-elasticity, plastic deformation of the Cu phase and eventually tensile crack formation.

The recent interest in fibrillar biological attachment systems, as found in the gecko, has led to the development of micropatterned elastomer adhesion surfaces. All reported studies have been performed at ambient humidity neglecting its possible influence on adhesion. The present paper investigates, for the first time, the effect of systematic changes in ambient humidity from 2 to 90 %. Adhesion measurements were performed on PDMS (Sylgard 184) surfaces possessing micropillars with flat-ended and hemispherical contact shape. The pillar radius was varied between 2.5 and 25 μm; the pillar aspect ratio was kept at 1. While the adhesion of a flat sample was not affected by humidity, we found that pillar size and shape influenced the sensitivity to humidity changes: Thinner pillars, with higher pull-off forces in the dry state, exhibited decreasing adhesion values, by up to 35 %, with increasing humidity. The effect was stronger for the hemispherical tip shape, where the positive effect of finer pillars was even reversed. Possible explanations for these effects, which may lower the reliability of biomimetic adhesion devices in the presence of humidity, are given.

The aim of this article is to describe the design strategy deployed in developing new biodegradable Mg–Y–Zn alloys. The development approach is based on a microalloying concept which aims to restrict grain growth considerably during alloy casting and forming. We discuss the efficiency of the design approach, and evaluate the characteristics of the new alloys using metal-physical experiments, thermodynamic calculations and transmission electron microscopy analysis. Our results show that after extrusion the alloys have very fine grains (< 10 m), exhibit high ductility (uniform elongation: 17 – 20 %) at considerable strength (ultimate tensile strength: 250 – 270 MPa) and reveal the presence of finely distributed intermetallic particles which are stable upon annealing. Due to an attractive combination of mechanical, electrochemical and biological properties, the new alloys are very promising not only for applications in medicine but also in other fields.

Laminates with strong interfaces and having compressive residual stresses in the outer layer are designed to have high strength and high reliability. Bending strength tests on such laminate specimens were performed and compared with tests made on monolithic specimens made from the outer layer material. In both types of specimens fracture started from similar flaws, but the laminate specimens showed a higher strength and Weibull modulus. In the case of the monolithic material the toughness is constant but in the case of the laminates it increases – due to the action of the residual stresses – with crack extension (i. e. it is an apparent toughness). The influence of the apparent toughness curve on the fracture statistics is discussed. It is also shown that the statistics of the laminate can be determined using the statistics of the monolithic material, if the influence of the residual stresses on the toughness and strength is properly taken into account.

During single crystal solidification in turbine blades, grain defects are often formed in the platform region with abrupt variation in cross-section. In order to reduce grain defects arising from component geometries an HC (Heat Conductor) technique was developed. In the production process for ceramic shell moulds, a graphite heat conductor of excellent heat conductivity is inserted close to the inner corner of the platform. This effectively extracts local heat from this critical region during directional solidification. Numerical simulations show that by using HC the thermal condition in the platform region can be significantly improved. The single crystal growth in the blade body can spread into the platform more quickly before the melt at the platform extremity becomes deeply undercooled. Structural investigations reveal a remarkable reduction in grain defect formation, providing confirmation of the effectiveness of the HC technique in improving the casting quality of single crystal components.