Papers by Author: Stefan Roth

Abstract: Textured polycrystalline Ni-Mn-Ga alloys were prepared by directional solidification. Alloys were chosen to have the 5M modulated martensitic structure after proper heat treatment. A two-side mechanical training decreased the twin boundary pinning stress. The stress-strain behaviour for the training process and the magnetically induced stress depends on the training direction. Magnetic field induced strain was demonstrated in samples with a plate-like geometry. The influence of the microstructure on the magneto-mechanical behaviour is discussed.

Abstract: Textured polycrystalline NiMnGa alloys were prepared by directional solidification. Alloys were chosen to have either the 7M or the 5M modulated martensitic structure after proper heat treatment. Mechanical training allowed to reduce the twin boundary pinning stress to below the magnetically induced stress. Thus, magnetic field induced changes in the mechanical behaviour could be demonstrated. The conditions of preparation and mechanical training will be discussed together with their influence on structure, microstructure, and the magneto-mechanical behaviour.

Abstract: The rods of Fe-based bulk metallic glasses with the nominal composition Fe65.5Cr4Mo4Ga4P12C5B5.5 were cast by melt injection into 1.5 and 1.8 mm diameter copper molds. The thermal stability, microstructure and crystallization behavior were investigated by differential scanning calorimetry, optical micrography and X-ray diffraction, respectively. The wide supercooled liquid region between crystallization temperature (Tx) and glass transition temperature (Tg) in the as-cast state Tx=Tx-Tg=60 K, as well as the high value of reduced glass transition temperature Trg=Tg/Tl=0.567 (Tl is liquidus temperature) approves enhanced thermal stability of the alloy against crystallization. In the as-cast “XRD-amorphous” state, microhardness HV1=742 was observed. Multistep current annealing thermal treatments were performed for structural relaxation. After applying high enough heating power per square area (PS ≥ 6 W/cm2), intensive crystallization of the samples characterized by appearance of several iron-metalloid compounds (Fe5C2, Fe3Ga4, Fe63Mo37 and Mo12Fe22C10) was observed. The microstructure changes after crystallization bring about differences in the microhardness values. The areas of still present amorphous matrix are with increased value HV1=876, but a remarkable decrease to HV1=323 was observed in precipitated crystallized zone that propagate along inner part of cylinders.

Abstract: Multistep current annealing (CA) treatments were performed on amorphous FINEMETtype Fe-Cu-V-Si-B, as well as on a novel Fe-Al-Ga-P-C-B alloy with a large supercooled liquid region, in order to optimize their magnetotransport properties. On-line and post-annealing electrical resistivity measurements, DSC, XRD and Mössbauer spectroscopy were used for characterization of structural changes evolved during CA treatments. Results on magnetoresistance (MR) and magnetoimpedance (MI) effects after CA in ribbon samples with relaxed amorphous as well as precipitated nanocrystalline structure are presented. Significant improvement in MI-response after CA up to
Z/Z ≈ 50% was recorded at frequencies 2-3 MHz. The highest MI-element sensitivity was found for low magnetic field intensity where values of about 6 %/kA/m for samples of Fe-Al- Ga-P-C-B alloy were attained.

Abstract: This article deals with the materials science and engineering of glass-forming alloys in Fe-(Nb)-(Al, Ga)-(P, C, B, Si), Fe-(Cr, Mo, Ga)-(P, C, B) and Fe-(Co, Ni)-(Cu)-(Zr, Nb)-B bulk metallic glasses (BMG) systems with high thermal stability of the undercooled melt against crystallization. Different liquid quenching techniques (melt-spinning or copper-mold casting) as well as hot pressing of the powder obtained by milling of the melt-spun ribbons were used to prepare samples in various shapes. Synthesis of the investigated BMG alloys is discussed according to Inoue’s empirical components rules for the achievement of the large glass forming ability (GFA). Thermal and microstructure characterization (performed by DSC, TMA, XRD and Mössbauer spectroscopy) was used to correlate GFA, microstructure and thermo/thermo-magnetic treatments with optimum soft magnetic properties.