Conditions for metastable crystallization from undercooled melts

The limits of validity of Ostwald's rule of stages are investigated theoretically in the case of crystallization of undercooled melts. The treatment is within the limits of capillary theory. Two basic models are compared: (1) According to the first one (model A), the phase with lower energy of formation of critical nucleus is predominantly formed. In an enantiotropic-type phase diagram there is no region of homogeneous preferential formation of the low temperature phase. If the phase diagram is monotropic-type there is a certain temperature below which the metastable crystalline phase is preferentially formed. (2) The second assumption takes into consideration that the nature of extremely small phases is somewhat undefined. One certainly cannot determine whether, say 3-particle complex, is of phase 1 or of phase 2. Moreover, it is known that properties of extremely small clusters could be different from the corresponding volume phase. The main assumption is that there is a certain crucial size (n-particle complex) at which the nature of the two phases can be distinguished. Complex of the phase, which has lower chemical potential at the crucial size, will be formed first. According to the model, in the case of enantiotropic-type transition there is a critical temperature.     

Glass formation of ternary CaCuMg alloys

Ternary CaCuMg alloys have been investigated with respect to their glass forming ability. Intermetallic glasses can be obtained for all constituting binary systems. The ternary system exhibits one extensive range of glass formation, interconnecting the binary ones. Differential scanning calorimetry has been applied to study thermal decomposition of the glasses obtained. Conditions of formation are discussed in the light of thermodynamic and kinetic criteria.     

Grain refinement in the solidification of undercooled Ni–Pd alloys

Two bulk Ni–Pd alloys Ni₇₅Pd₂₅ and Ni_54.8Pd_45.2 have been undercooled beyond their hypercooling limits. As undercooling increases, two grain refinements, occurring at low and high undercooling, respectively, were observed in the solidification of both alloys, even though the equilibrium crystallization temperature range of the latter is only 5 K. Reaching of the hypercooling limit did not change the microstructural morphology abruptly, and dendritic substructure could still be found in the refined grains. Such experimental results cannot be interpreted satisfactorily by the theory that the break-up of dendrites during solidification is dominated by capillary force. It is proposed that the grain refinement at low undercooling results from the remelting associated with chemical superheating, but that at high undercooling is due to the post-solidification recrystallization.     

Stability and crystallization of Fe-Co-Nb-B amorphous alloys

New soft magnetic amorphous alloys with a wide supercooled liquid region were obtained by increasing the boron content (up to 30 at.%) and by addition of cobalt (composition range between 5 and 42 at.%) in FeNbB based materials. Upon thermal treatments, the stability and the crystallization behavior of Fe₅₂Co₁₀Nb₈B₃₀ and Fe₂₂Co₄₀Nb₈B₃₀ have been studied by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). On the other hand, the Curie temperature change, T_c, of the remaining amorphous phases has been determined by thermomagnetic analysis (TMG). Sub-micrometric fcc-(Co,Fe) and fcc-(Fe,Co)₂₁Nb₂B₆ grains were observed to crystallize at a relatively high temperature (950-970 K) providing an excellent stability of the amorphous states. Due to the partitioning behavior, occurring during crystallization, the Curie temperature of the remaining amorphous structure is observed to increase in the case of the Fe₅₂Co₁₀Nb₈B₃₀ composition while a decrease of T_c to the lower temperatures is observed with the Fe₂₂Co₄₀Nb₈B₃₀ composition.     

Eutectic crystallization of aluminium nickel alloys

The directional crystallization of eutectic Al Ni alloys was studied. Variations of the growth rate caused significant change of the microstructure and mechanical behaviour of eutectic alloys. On the base of a Tiller's analysis involving the voluminal ratio of the two phases the interphase boundary energy σ_αβ is obtained.     

Kinetics of the irregular binary alloys crystallization

With the help of Monte-Carlo computer simulation method we have investigated the growth of binary crystals having the structure anomally depended on temperature and the liquid phase composition. The dependences of the growth rate and component concentrations in the crystal volume on temperature for different liquid phase compositions were obtained. Kinetic phase diagrams are found here too. A comparison with the results obtained from analytical calculations was also made in this work.     

Glass formation and properties in the gallia-calcia system

The critical cooling rate for glass formation was measured for five compositions in the Ga₂O₃?CaO system and varied from a low of (315 ± 85) °C/s for a eutectic melt containing 37.5 mol.% Ga₂O₃ to a high of (840±60)°C/sec for a melt containing 52 mol.% Ga₂O₃. Selected properties of glasses containing from 37.5 to 62 mol.% Ga₂O₃ were measured. The density and refractive index, n_D, both increased with increasing Ga₂O₃ content, but the crysallization temperature and microhardness varied only slightly with Ga₂O₃ content. The IR spectra of these glasses suggest that both GaO₄ tetrahedra and GaO₆ octahedra are present.     

Vacancy formation enthalpy in AgZn alloys

Effective vacancy formation enthalpy (H_v) for AgZn alloy was determined by means of positron annihilation method. It was found that H_v decreases linearly with increasing Zn content for the α-phase. For the ξ and β′-phase the H_v does not change and is equal to 0.53–0.55 eV.     

Investigation of viscosity and crystallization in supercooled-liquid region of Zr-based glassy alloys

Using viscosity measurement method and in-situ heating synchrotron radiation, the viscosity of the (Zr_0.55Al_0.1Ni_0.05Cu_0.3)_100 ? xY_x (x = 0, 0.5, 1, 2) bulk metallic glasses (BMGs) in their supercooled liquid regions (SLRs) and the in-situ heating nucleation were investigated, respectively. In the SLR, the (Zr_0.55Al_0.1Ni_0.05Cu_0.3)₉₉Y₁ metallic glass which shows distinct plastic strain in compression exhibits higher viscosity than the other three BMGs, however their Poisson's ratios are almost the same. The synchrotron diffraction results show that crystallization happened in the SLR of the (Zr_0.55Al_0.1Ni_0.05Cu_0.3)₉₉Y₁ glassy alloy, which could be the reason for the higher viscosity and larger plastic strain in compression compared to the other three alloys. The fracture surfaces of the glassy alloys were observed and analyzed.     

Structure formation in liquid and amorphous metallic alloys

Bulk metallic glasses developed in last 15 years represent a new class of amorphous metallic alloys. These multi-component metallic alloys can be obtained at relatively low cooling rates, which allow the production of large-scale materials by conventional casting processes. Furthermore, bulk metallic glasses show a glass transition well below the crystallization temperature enabling hot deformation, but also to investigate the glass transition phenomenon in a metallic system. The thermal behavior of Zr- and Pd-based bulk metallic glasses was studied by in situ X-ray diffraction at elevated temperatures. The temperature dependence of the X-ray structure factor of the glassy state can be well described by the Debye theory. At the caloric glass transition the temperature dependence of the structure alters, pointing to a continuous development of structural changes in the liquid state. The short-range order of the glass, of the super-cooled liquid, and of the equilibrium melt is found to be very similar. The existence of complex chemically ordered clusters in the melt is supposed to be related to the high glass-forming ability of the alloys. The microstructure of metallic glasses consisting of elements with negative enthalpy of mixing is homogeneous at dimensions above 1 nm. Phase separation in the liquid state appears in metallic systems with large positive enthalpy of mixing of the elements like Nb–Y. Thermodynamic calculations of the Ni–Nb–Y phase diagram show that the miscibility gap of the monotectic binary Nb–Y system extends into the ternary up to large Ni content. Experimental evidence of the phase separation in ternary Ni–Nb–Y melts is obtained by in situ X-ray diffraction at elevated temperatures and differential scanning calorimetry. The phase separated melt can be frozen into a two-phase amorphous metallic alloy by rapid quenching from the liquid. The microstructure depends on the chemical composition and consists of two amorphous regions, one Nb-enriched and the other Y-enriched, with a size distribution from several nanometers up to micrometer dimension. The experimental results confirm the close relationship between the structure of metallic glasses and the corresponding under-cooled liquids.     

Kinetics of crystallization of titanium based binary and ternary amorphous alloys

Metallic glasses are kinetically metastable materials. These amorphous materials can be transformed into a crystalline state by both isothermal and isochronal methods. The study of this transformation, and hence the thermal stability of metallic glasses, are important from an application view-point. In the present work, the non-isothermal crystallization kinetics of two titanium-based amorphous alloys namely, Cu₅₀Ti₅₀ and Ti₅₀Ni₃₀Cu₂₀, are reported. The activation energies for crystallization, E_c for both the systems have been evaluated using different non-isothermal methods viz. derived through Kissinger, Augis and Bennet and Ozawa. The values of E_c obtained using these methods are consistent for both the metallic glasses and it is found that E_c for the ternary metallic glass is considerably higher than the binary metallic glass. The increase in the activation energy on the substitution of Ni in the Cu–Ti metallic glass suggests the increase in the thermal stability.     

Glass formation and crystallization behavior in Mg65Cu25Y10−xGdx (x=0, 5 and 10) alloys

The glass forming ability and crystallization behavior of Mg₆₅Cu₂₅Y_10−xGd_x (x=0, 5 and 10) alloys have been investigated. The glass forming ability (GFA) is significantly improved when Y in Mg₆₅Cu₂₅Y₁₀ is substituted with Gd. Ternary Mg₆₅Cu₂₅Gd₁₀ bulk metallic glass (BMG) with diameter of at least 8 mm was successfully fabricated by conventional Cu-mold casting method in air atmosphere. Mg₂(Y, Gd) is the first competing crystalline phase against the glass formation in the Mg₆₅Cu₂₅Y_10−xGd_x (x=0, 5) alloys, while Mg₂Cu and Cu₂Gd are the competing crystalline phases in the Mg₆₅Cu₂₅Gd₁₀ alloy. Therefore, the suppression of the formation of Mg₂(Y, Gd) during cooling from the liquid improves the GFA significantly.     

Magnetothermal investigation of crystallization of Ni–P amorphous alloys

A new magnetic phase analysis method is presented and its application to a certain class of amorphous alloys is thoroughly discussed. Special attention is paid to the size of clusters with strong magnetic properties that form the separating phase. This magnetothermal (MT) method reveals the formation or disappearance of a phase with strong magnetic properties, and in certain cases it can give valuable information, even in the temperature range where the sample is paramagnetic. The accuracy and applicability of the MT method are discussed. Combining this method with differential scanning calorimetry (DSC), the transformations in melt-quenched (MQ) and electrolessly deposited (ED) amorphous Ni–P alloys with hypoeutectic and close to eutectic composition are investigated under the condition of a constant heating rate. Up to four stages of Ni precipitation in the alloys are revealed, depending on the phosphorous content and the method of sample preparation.     

Kinetics of crystallization of FeB-based amorphous alloys studied by neutron thermo-diffractometry

Kinetics of crystallization of two amorphous alloys, Fe₇₀Cr₁₀B₂₀ and Fe₈₀Zr₁₀B₁₀, have been followed up by neutron thermo-diffractometry experiments performed in the two axis diffractometer D20 (Institut Laue-Langevin, Grenoble). The structural changes are directly correlated with the temperature dependence of the magnetization. Fe₇₀Cr₁₀B₂₀ crystallizes following a two-step process: an eutectic crystallization of α-Fe (BCC) and the metastable tetragonal phase (Fe_0.8Cr_0.2)₃B followed by another eutectic transformation to the stable phase (Fe_0.75Cr_0.25)₂B and more segregation of α-Fe. These tetragonal phases are magnetically anisotropic, giving rise to a large increase of the coercivity. This behavior is similar to that of Fe₈₀B₂₀ alloys, with Cr atoms replacing the Fe positions in both crystalline phases. Fe₈₀Zr₁₀B₁₀ shows also a two-step process in which two polymorphic transformations take place.     

Effects of Sn addition on the glass forming ability and crystallization behavior in Ni-Zr-Ti-Si alloys

The effect of Sn substitution for Si on the glass forming ability (GFA) and crystallization behavior has been studied in Ni₅₉Zr₂₀Ti₁₆Si_5 − xSn_x (x=0, 3, 5) alloys. A bulk amorphous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy with diameter up to 3 mm can be fabricated by injection casting. Partial substitution of Si by Sn in Ni₅₉Zr₂₀Ti₆Si_5 − xSn_x alloys improves the glass forming ability. The improved GFA of the Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy is can be explained based on the lowering of liquidus temperature. The crystallization sequence becomes completely different with addition of Sn. The amorphous Ni₅₉Zr₂₀Ti₁₆Si₅ alloy crystallizes via precipitation of only a cubic NiTi phase in the first crystallization step, whereas the amorphous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy crystallizes via simultaneous precipitation of orthorhombic Ni₁₀(Zr,Ti)₇ and cubic NiTi phases. Addition of Sn in the Ni₅₉Zr₂₀Ti₁₆Si₅ alloy suppresses the formation of the primary cubic NiTi phase. The bulk amorphous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy exhibits high compressive fracture strength of about 2.7 GPa with a plastic strain of about 2%.     

Investigation of the microhardness and the electrical resistivity of undercooled Ni-10 at.% Si alloys

Ni-10 at.% Si alloys (Ni and Si of purity of 99.99%) were prepared by arc melting method under argon atmosphere in a water-cooled copper hearth with a non-consumable tungsten electrode and titanium getter. Spherical-shaped Ni-10 at.% Si alloy melts are undercooled in a containerless electromagnetic levitation apparatus. The effects of undercooling (ΔT) on the microstructure, electrical resistivity (ρ) and microhardness (HV) were investigated. The experimental results reveal that with the increase of the undercooling of the melts from 16 to 110 K, the microstructures undergo transition from coarse dendritic morphology to ultra fine dendritic morphology. The measurements of microhardness of the samples were performed by using a microhardness test device. The dependence of microhardness on undercooling was analyzed and it has been found that with increasing the undercooling the HV increases. Variations of electrical resistivity of the Ni-10 at.% Si alloy with the temperature in the range of 300-800 K were also determined by using a standard d.c. four-point probe technique. The resistivity of samples increases with increasing temperature and undercooling.     

AFM investigations during the nanostructure formation in FeZrB alloys

We present a study by intermittent contact atomic force microscopy technique, using the corresponding line profile analysis, performed in as-cast amorphous and Joule heated nanocrystalline Fe₉₁Zr₇B₂ ribbons. The average grain size of the nanocrystalline phases estimated from AFM images, (≈10–15 nm), correlates well with respect to that obtained from neutron powder diffraction. The differences in the relative amount of amorphous to nanocrystalline phases estimated from both techniques, suggest that the onset of the crystallisation process begins at the ribbon surface.     

Thermodynamics of formation of aluminum-iron-germanium amorphous alloys

Thermodynamic properties of ternary liquid Al-Fe-Ge alloys were studied by electromotive force method at 1050-1250 K and by high-temperature isobasic calorimetry at 1740 ± 5 K. The heat capacity change at ternary alloy formation (Δ_mixC_p) was estimated using the temperature dependence of integral enthalpy of mixing. Thermodynamics of the formation of Al-Fe-Ge amorphous alloys was evaluated by extrapolation of thermodynamic functions of mixing to the temperature of amorphization. The process of glass forming is preferable by both enthalpy and entropy for compositions of (0.1<x_Fe<0.6, x_Ge<0.5). The area with most negative integral Gibbs free energy and enthalpy of amorphous alloy formation corresponds well to the area of amorphization estimated by a glass-forming tendency (GFT) criterion.