M. Stoica

Atomic-Level Processes of Shear Band Nucleation in Metallic Glasses

The ability to control the plastic deformation of amorphous metals is based on the capacity to influence the percolation of the shear transformation zones (STZs). Despite the recent research progress, the mechanism of STZ self-assembly has so far remained elusive. Here, we identify the structural perturbation generated by an STZ in the surrounding material and show how such a perturbation triggers the activation of the neighboring STZ. The mechanism is based on the autocatalytic generation of successive strong strain and rotation fields, leading to STZ percolation and, ultimately, to the formation of a shear band.

Casting and Phase Transformations of Fe65.5Cr4Mo4P12C5B5.5 Bulk Metallic Glass

Bulk glassy Fe 65.5Cr4Mo4Ga4P12C5B5.5 rods with diameters of 1.5 3 mm were prepared by copper mold casting. Thermal stability measurements reveal a distinct glass transition, followed by a supercooled liquid region of 60 K. The casting temperature plays an important role for obtaining fully glassy material. The crystallization of the g lass as observed by in-situ X-ray diffraction measurements in transmission configuration occurs via the formation of a metastable intermediate phase. The crystalline phases observed after heatin g do not correspond to those occurring after slow cooling from the melt. Introduction The Fe-based amorphous alloys recently found by Inoue et al. [1-3] exhibit a large supercooled liquid region between the glass transition temperature Tg and the crystallization temperature Tx visible upon constant-rate heating to elevated temperatures. Beca use of the lack of crystal anisotropy, they have good soft magnetic properties charac terized by low coercive force and high permeability [4-7]. The high glass-forming ability of this ki nd of alloys allows the formation of bulk glassy samples [8-10]. Such alloys can be directly cast in f orm of bulk specimens, which could be used for magnetic cores using different techniques, such as copper mold casting or water quenching. However, the critical cooling rate of about 10 2 K/s required for glass formation is higher than the value of about 1-10 K/s characteristic for alloys with ver y good glass-forming ability [11,12]. Thus, the maximum achievable diameter of these Fe-based allo ys is limited to only a few millimeters [13]. The other hindrance that can influence bulk glass formation is the presence of impurities in the melt [14,15] that can be removed using fluxing techniqu es [16,17], or of crystalline inclusions that can form upon solidification of the melt. In the case of FeCrMoGaPCB alloys, Shen and Schwarz [15] used the flux-mel ting technique to remove the oxide inclusions from the melt and subsequent water quenching allows to produce rods with 4 mm diameter. From this class of alloys, the nominal c omposition Fe65.5Cr4Mo4Ga4P12C5B5.5 was chosen because the samples show the best thermal stability and soft magnetic properties. The present work focuses on the possibility to cast this alloy directly in bulk form using the copper mold casting technique, the correlation between t h casting conditions and the structure, as well as the crystallization behavior measured in-situ using a high intensity high-energy monochromatic X-Ray synchrotron beam. Journal of Metastable and Nanocrystalline Materials Online: 2002-08-01 ISSN: 2297-6620, Vol. 12, pp 77-84 doi:10.4028/www.scientific.net/JMNM.12.77

Microstructure and Microhardness in Current Annealed Fe65.5Cr4Mo4Ga4P12C5B5.5 Bulk Metallic Glass

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.

Synthesis, Structure and Properties of Iron-Based Bulk Glass-Forming Metallic Alloys Prepared by Different Processing

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.