P. Krooß

On the functional degradation of binary titanium–tantalum high-temperature shape memory alloys — A new concept for fatigue life extension

High-temperature shape memory alloys are promising candidates for actuator applications at elevated temperatures. Ternary nickel–titanium-based alloys either contain noble metals which are very expensive, or suffer from poor workability. Titanium–tantalum shape memory alloys represent a promising alternative if one can avoid the cyclic degradation due to the formation of the omega phase. The current study investigates the functional fatigue behavior of Ti–Ta and introduces a new concept providing for pronounced fatigue life extension.

Time-of-Flight Three Dimensional Neutron Diffraction in Transmission Mode for Mapping Crystal Grain Structures

The physical properties of polycrystalline materials depend on their microstructure, which is the nano- to centimeter scale arrangement of phases and defects in their interior. Such microstructure depends on the shape, crystallographic phase and orientation, and interfacing of the grains constituting the material. This article presents a new non-destructive 3D technique to study centimeter-sized bulk samples with a spatial resolution of hundred micrometers: time-of-flight three-dimensional neutron diffraction (ToF 3DND). Compared to existing analogous X-ray diffraction techniques, ToF 3DND enables studies of samples that can be both larger in size and made of heavier elements. Moreover, ToF 3DND facilitates the use of complicated sample environments. The basic ToF 3DND setup, utilizing an imaging detector with high spatial and temporal resolution, can easily be implemented at a time-of-flight neutron beamline. The technique was developed and tested with data collected at the Materials and Life Science Experimental Facility of the Japan Proton Accelerator Complex (J-PARC) for an iron sample. We successfully reconstructed the shape of 108 grains and developed an indexing procedure. The reconstruction algorithms have been validated by reconstructing two stacked Co-Ni-Ga single crystals, and by comparison with a grain map obtained by post-mortem electron backscatter diffraction (EBSD).

Pulsed magnetic field-induced changes in the meso- and nanostructure of Co49Ni21Ga30 martensite

A near single-variant martensitic state of Co49Ni21Ga30 alloy was induced by a static axial stress of 66MPa. Upon application of a magnetic field pulse, the single-variant state of stressed specime...

Cyclic degradation of titanium–tantalum high-temperature shape memory alloys — the role of dislocation activity and chemical decomposition

Titanium–tantalum shape memory alloys (SMAs) are promising candidates for actuator applications at elevated temperatures. They may even succeed in substituting ternary nickel–titanium high temperature SMAs, which are either extremely expensive or difficult to form. However, titanium–tantalum alloys show rapid functional and structural degradation under cyclic thermo-mechanical loading. The current work reveals that degradation is not only governed by the evolution of the ω-phase. Dislocation processes and chemical decomposition of the matrix at grain boundaries also play a major role.

Electron beam welding of Fe–Mn–Al–Ni shape memory alloy: Microstructure evolution and shape memory response

The present study reports on the impact of abnormal grain growth (AGG) on the microstructural evolution following electron beam (EB) welding of Fe–Mn–Al–Ni shape memory alloy (SMA). Polycrystalline sheet-like material was EB-welded and a cyclic heat treatment, studied in previous work, was conducted for inducing AGG and a bamboo-like microstructure, respectively. Optical and electron microscopy were carried out to characterize the prevailing microstructure upon cyclic heat treatment. For characterization of the functional properties following AGG, a load increase test was conducted. The current results clearly show that good shape memory response can be obtained in Fe–Mn–Al–Ni SMA upon EB welding and subsequent post-heat treatment. These results further substantiate the potential use of conventional processing routes for Fe–Mn–Al–Ni SMA.

Processing Effects on the Microstructure and Properties of Fe-Based Superelastic Alloys

This study focuses on the evolution of microstructure and mechanical properties following thermomechanical processing of FeMnAlNi superelastic alloys. For Fe43.5Mn34Al15Ni7.5, which is an eligible candidate for superelastic applications, the effects of substituting ± 1.5 at. % Al with Ni were analyzed. The ingots were subjected to five thermomechanical processing steps: (i) hot rolling; (ii) annealing; (iii) cold rolling; (iv) cyclic heat treatment under protective Ar atmosphere and (v) solution treatment and ageing. The evolution of microstructure following the five processing steps was monitored by optical and scanning electron microscopy and energy dispersive spectroscopy. Micro-hardness evolution was monitored during consecutive processing steps. Tensile specimens were cut by spark erosion and mechanical properties were evaluated following consecutive processing steps taking into account the microstructural evolution.

Magnetic pulse controlled microstructure development in Co49Ni21Ga30 single crystals

ABSTRACT A pulsed magnetic field (MF) up to 0.6 T leads to the generation of fine microstructure through the change in the dynamics of the interaction process between the MF and the magnetic- and microstructure in Co49Ni21Ga30 magnetic shape memory alloy single crystals. This paper addresses the impact of pulsed and permanent MF on the transformation behaviour in Co49Ni21Ga30. A pulsed MF can result in different microstructural accommodation processes in stress-induced martensite, as compared to an application of a mechanical stress or permanent MF. In a pulsed MF, the development of new nano-twin systems in the martensitic structure of the CoNiGa alloy and the formation of fine magnetic domains at the microscale are observed.

MARTENSITE AGING IN

Co–Ni–Ga high-temperature shape memory alloys (HT-SMAs) are well-known candidate materials for damping applications at elevated temperatures. Recent studies showed that upon heat treatment in stres...