Papers by Keyword: Amorphous Alloy

Abstract: Effects of Fe, Co and Al addition to Zr55Al10Ni5Cu30 and Zr70Cu30 metallic glass ribbons were studied. 20 at.% of Fe addition prevented Zr55Al10Ni5Cu30 molten alloys from being supercooled and resulted in nanocrystallization, while Zr55Al10Ni5Cu30 alloys containing 20 at.% Co could be quenched into a supercooled liquid region. Fe addition also degraded Zr70Cu30 metallic glass, while Al addition improved both glass phase stability and mechanical properties. Degradation of Zr-based metallic glass by Fe addition originates in the large negative enthalpy of mixing Fe with Cu.

Abstract: The thermomechanical processing consisting in severe cold rolling (true strain 0.7–1.9) followed by a post-deformation annealing (200-700oC) is applied to Ti-50.0 and 50.7at%Ni alloys. The thermal stability of the amorphous phase as well as the influence of post-deformation annealing on the structure, substructure and temperature range of martensitic transformations are studied using TEM and DSC techniques. For a given level of cold work, the equiatomic alloy has a higher volume fraction of amorphous phase than the nickel-rich one. For both alloys, the higher the volume fraction of the amorphous phase, the higher the thermal stability. For a given post-deformation annealing temperature, the DSC martensitic transformation peaks from the material subjected to amorphization cold work are sharper and the hysteresis between the direct and reverse transformations is narrower than those for a material subjected to strain hardening cold work. This observation confirms the absence of the well-developed dislocation substructure in the severely deformed alloy subjected to nanocrystallization heat treatment, which is consistent with TEM results.

Abstract: The thermal and mechanical characteristics of various Mg-Cu(Ni)-Y(Gd) metallic glassy alloys prepared by melt spinning are examined using differential scanning calorimetry (DSC), thermomechanical analyzer (TMA), and instrumental nanoindenter. The replacement of Y by Gd appears to benefit both the thermal and mechanical properties, while the replacement of Cu by Ni improves only the hardness and modulus, with the sacrifice of thermal characteristics. The amorphous Mg-Cu-Gd based alloys can be fabricated into rods with a diameter greater than 6 mm, with minimum porosity and reasonable toughness.

Abstract: The flux treatment was applied to increase the glass-forming ability of a glassy [(Fe0.5Co0.5)0.75B0.20Si0.05]96Nb4 alloy. The large bulk glassy specimen with diameter of 7.7 mm was prepared by water quenching the melt immersed in the molten flux of B2O3. The diameter of the specimens is approximately 1.5 times as large as the previous result, even though the cooling rate of copper mold casting should be much higher than that of water quenching. The critical cooling rate for a glassy phase is 150 - 170 K/s, which is much slower than 500 K/s without the flux treatment. The flux treatment improves the glass-forming ability by cleaning the molten metal where heterogeneous nucleation is difficult to take place.

Abstract: CuxZr1-x (x=0.30~0.85) binary amorphous alloy models were constructed using molecular dynamics simulations. In order to characterize the short-ranged local structures and the free volume of the whole cell, Voronoi polyhedral analyses were performed for their initial states and also for the deformed states. Both geometrical and physical factors are intimately related to each other and exhibit a large influence on mechanical properties. The elastic properties of shear modulus and bulk modulus tend to increase as the free volume decreases. The Poisson’s ratio, which is related to the glass fragility, increases as the free volume. However, the relation among three elastic properties obtained from the continuous mechanics of the isotropic body does not hold. Also, the maximum stress in the finite deformation drastically decreases with increasing free volume content.

Abstract: The effect of the concentration of Ni, Y and La-rich mischmetal on the thermal stability, microstructure and mechanical properties of ribbons tested in the temperature range 25-350°C have been evaluated. The low-alloyed materials were crystalline or partially crystalline while high-alloyed materials were amorphous. The amorphous alloys experienced numerous transformations during heating above 170°C. A transition in the amorphous state was observed in all alloys prior to the crystallization stage. The alloy composition determines the sequence of phase transformations during crystallization. In general, the structure of crystallised amorphous, even at high temperatures, is much finer than that of crystalline materials. A MgxREy intermetallic matrix with other intermetallic phases homogeneously distributed was observed in the crystallised ribbons with high RE contents. However, a magnesium matrix embedding other intermetallic phases was the microstructure observed for low-alloyed materials. The amorphous Mg-10Ni-2.5Y2.5-2.5La(MM) showed the higher tensile strength values up to 200 °C The crystalline Mg-2Ni-1Y-1La(MM) ribbon and the partially crystalline Mg-3Ni-1.5Y-1.5La(MM) alloy also exhibited high mechanical resistance levels which were maintained up to 250 °C. MgNi10Y2.5La(MM)2.5 amorphous and MgNi3Y1.5La(MM)1.5 crystalline broken above 500 and 400 MPa, respectively, at 100°C

Abstract: The devitrification of the Fe-Ni-B-Si amorphous ribbon was investigated by the differential scanning calorimetry (DSC) with scanning and isothermal methods. The devitrification of rapidly quenched ribbons is a multilevel process. On the basis of DSC investigations it was determined that crystallization occurs in three processes up to 700°C in the Fe40Ni40B16Si4 alloy. In the present work the first and second steps have been discussed. The first crystallization step involves the segregation of the Fe-Ni crystalline solid solution from the amorphous matrix. During the second crystallization phase, in addition to austenite, nickel silicide and two types of iron borides crystallize as well. The ribbons were relaxed at 380°C for 2 hours, following the pre-annealing at different temperatures. Pre-annealing was performed in the DSC within the temperature range elapsing from 395°C to 420°C. The preannealing at temperatures below the first exothermal DSC peak has an effect on the crystallization processes. After the pre-annealing the samples were investigated by DSC. The DSC peak of the first crystallization step shifts to higher temperatures and decrease its enthalpy. The scanning DSC measurements, applied after the isothermal pre-annealing, were performed in order to determine the fraction of the ribbon transformed in the primary crystallization step. The second DSC peak shifts to lower temperatures with a maximum of 4°C. The X-ray diffraction (XRD) analyses reveal that the lattice constant changes with the pre-annealing temperatures. Such observation was also supported by the circumstance that the composition of the Fe-Ni solid solution undergoes certain modifications.

Abstract: By means of the vibrating reed technique, measurements of internal friction have been performed in the temperature range of 120 K < T < Tg (= glass temperature) on two amorphous alloys, each produced as ribbon and bulk material. The different contents of free volume result in an only slight shift of the onset of irreversible structural relaxation to lower temperatures (i.e., lower activation energies) for the ribbons, while considerably different amounts of structural relaxation occur. After correcting for the thermoelastic effect, the reversible structural relaxation, i.e., an approximately exponential increase of damping with rising temperature, is well described by KWW kinetics (β ≈ 0.3). For the Zr-based alloy only, a clear relaxation peak occurs in the range from 270 K to 320 K (for the first flexural vibration mode between 100 Hz and 400 Hz) induced by hydrogenation. In addition, the effect of plastic deformation on the damping behavior by cold rolling of the bulk materials has been examined.

Abstract: A series of Mg-based alloys with composition of Mg65Y10Cu25-XAgX, x = 0, 5, 10, were selected for investigating the microstructure evolution of the Mg-based nano/amorphous-composite alloy powder synthesized by the combination method of melt-spinning and mechanical alloying (MA). The microstructure characterization of the alloy powders was conducted by means of DSC, XRD, FEG-SEM, and TEM techniques. The result of XRD reveals that the entire as-quenched alloy ribbons exhibit a broaden diffraction pattern of amorphous phase. After 50 hours milling the mixture of amorphous alloy ribbons with 5 vol.% of nano-sized ZrO2 by planetary mill, the ZrO2 dispersed magnesium composite alloy powder can reach to a homogeneous size distribution. In parallel, the MA composite Mg-based alloy powders still remain an amorphous state by the characterization of X-ray diffraction and the DSC analysis. A clear Tg (glass transition temperature) and most wide supercooled region (about 44 K) were revealed for both the Mg65Y10Cu20Ag5 alloy ribbon and the MA magnesium composite powder. In addition, the result of TEM observation also revealed that the ZrO2 with average particle size of 80 nm distributed homogeneously in the amorphous matrix of the Mg65Y10Cu20Ag5 /5 vol.% ZrO2 composite alloy powder. The interface between the ZrO2 dispersoid and the amorphous matrix of the composite alloy powder exhibits a very good bonding condition.

Abstract: In this paper, a review of recent 57Fe Mössbauer spectroscopy (MS) studies of external influence on the properties of amorphous and nanocrystalline Fe- and Co-based alloys is submitted. Different types of alloys (FeCuNbZr, FeCuNbSiB, FeCoCuNbB, CoFeZrB and CoFeSiB) in the form of original amorphous and nanocrystalline ribbons were subjected to different external factors: different annealing atmospheres, mechanical stress (for example influence of ball-milling) and tensile stress. It will be shown that the Mössbauer spectrometry is a suitable tool for such studies because the measured spectral parameters are very sensitive to the changes in the vicinity of the probe 57Fe-nuclei and thus, this technique provides a wide variety of information about structural and magnetic behavior of Fe-containing materials.