As-quenched Samples Research Articles

Crystallization kinetics of a lithium zinc silicate glass studied by DTA and DSC

The crystallization kinetics of a lithium zinc silicate glass have been determined using isothermal and non-isothermal DSC and non-isothermal DTA. Values for the kinetic parameters for as-quenched and nucleated samples are reported and discussed. Particular reference is given to the similarities and differences noted between isothermally and non-isothermally derived data.

Thermal characterization of Cu 60Zr xTi 40− x metallic glasses ( x=15, 20, 22, 25, 30)

The crystallization behavior, the thermal stability and the microhardness of Cu 60Zr x Ti 40− x amorphous alloys ( x=15, 20, 22, 25, 30) prepared by melt-spinning have been studied by means of differential scanning calorimetry (DSC), differential thermal analysis (DTA), X-ray diffraction (XRD) and microhardness tests. DSC analysis have confirmed the existence of glass transition, followed by a supercooled liquid region in all the compositions. However, depending on the Zr content, different crystallization behavior has been observed. The apparent activation energies of the crystallization processes have been determined and the obtained values indicate high thermal stability for all the alloys. XRD-patterns of the as-quenched samples showed a broad symmetric halo typical of a fully amorphous structure. Independently of the composition, after the first crystallization stage the XRD patterns present a structure similar to that of an amorphous one. But the final developed microstructure depends on the composition of the alloys. The addition of Zr has not shown significant differences in microhardness of the amorphous alloys, although they have exhibited good mechanical properties.

Influence of the heat treatment on the crystallization mechanisms of Al 85Y 5Ni 8Co 2 metallic glass

Glass forming ability, crystallization and stability of a rapidly quenched, Al 85Y 5Ni 8Co 2 amorphous alloy have been investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Calorimetric results obtained in continuous heating regime reveal a glass transition followed by a wide supercooled liquid region (27 °C) and then by a three-stage crystallization process, starting with the primary formation of nanoscale α-Al particles. However, isothermal annealing of the as-quenched samples below the glass-transition temperature activates the first two crystallization stages and an unidentified compound is formed conjointly with α-Al. Kinetic analysis suggests that crystallization occurs through nucleation and three-dimensional growth.

Synthesis of amorphous calcium phosphates for hard tissue repair using conventional melting technique

The technical use of phosphate glasses has been restricted because of their strong hygroscopic nature. Traditionally phosphate glasses have been used as an opaline in silicate glasses since the solubility of phosphate in silicate glasses at lower temperature is very poor. Their poor chemical durability can be improved by the introduction of higher valency elements such as Ba, Al, or Be. The most important property of the phosphate glass is its ability to dissolve some elements and oxides that are insoluble or poorly soluble in glasses of other materials and crystalline compounds. Another important use of phosphate glasses is their application as biomaterials based on the calcium phosphate system. Their similarities in composition to bone and teeth make them ideal candidates. The first proposed application of the calcium phosphate glass to the biomaterials was proposed as dental restorative materials such as the crown because of their excellent castability as well as sufficient mechanical strength [1, 2]. The purpose of this study was to explore calcium phosphate glasses having an amorphous structure as potential biomaterials in hard tissue repair. We synthesized amorphous calcium phosphate glasses by melting and subsequent quenching process. Batches in the system CaO–CaF2–P2O5–MgO–ZnO were prepared with six kinds of Ca/P ratios from 0.2 to 1.2 using CaCO3, CaF2, H3PO4, MgO, and ZnO as raw materials. The molar ratio of CaO/CaF2 was fixed to 9. MgO and ZnO were added at 1 wt%, respectively. Mixed batches were dried at 80 ◦C and melted at temperatures ranging from 800 to 1550 ◦C depending on the Ca/P ratios. They were subsequently quenched onto the graphite plate at room temperature after melting. In order to determine the crystallinity and crystalline phases of the as-quenched samples, X-ray diffraction analysis was performed using Philips APD 3720 X-ray diffraction apparatus with a fine focus copper target X-ray tube. As shown in Fig. 1, there was no crystalline peak up to Ca/P of 0.6. When Ca/P ratio was 0.8, the sample exhibited the Zn3(PO4)2 as well as Ca2P2O7; the former had disappeared but the latter still remained. Ca2P2O7 phase partially transformed to Ca4P2O7 with increasing Ca/P to 1.2. Crystallinity Figure 1 XRD patterns of the as-quenched samples. • and denote Ca2P2O7 and Zn3(PO4)2, respectively.

Magnetic Properties of Co-Substituted Fe-Nb-Ta-Mo-B Amorphous Alloys

Fe/sub 82-x/Co/sub x/Nb/sub 3/Ta/sub 1/Mo/sub 1/B/sub 13/(0 /spl les/ x /spl les/ 41) amorphous alloys were produced by rapid solidification. The Co substitutions for Fe on the magnetic and structural properties of as-cast and crystalline alloys were studied. Co substitutions were shown to enhance the saturation magnetization of as-quenched ribbon samples from 108 emu/g at x = 0 to a maximum of 171 emu/g at x = 18. Co substitutions resulted in only slight changes in the primary and secondary crystallization onset temperatures, T/sub x1/ and T/sub x2/. The Curie temperature of the amorphous phase, T/sub C,amorphous/ increased with x and exceeded T/sub x1/ at x = 18, where nanocrystalline /spl alpha/-FeCo phase was observed. The T dependence of the magnetization, with decreasing temperature (after secondary crystallization), was used to determine the demarcation between the Curie transition of the /spl alpha/-FeCo phase and the loss of magnetization due to the /spl alpha//spl rarr//spl gamma/ phase transition at temperatures between 940 and 970/spl deg/C. In the range 18 /spl les/ x /spl les/ 41, except for x = 20.5, an abrupt drop in magnetization was observed at T/sub x2/ corresponding to the secondary crystallization of a (FeCo)/sub 23/B/sub 6/ phase. For the x = 20. 5 composition, the magnetization was observed to increase at T/sub x2/, fall gradually, and then drop abruptly 140/spl deg/C above T/sub x2/. Therefore, a large magnetic moment was retained to about 790/spl deg/C in this alloy. This composition was the best candidate for the soft magnetic application due to their high moments up to high temperature and low measured coercivity.

The microstructure and magnetic properties of NdFeB magnets directly solidified at a low cooling rate

Nd12.5Fe82B5.5 ribbons have been prepared by a melt spinning technique at a wheel speed ranging from 5 to 22 m/s. It was found that the wheel speed vR dramatically influences the microstructure and magnetic properties of the ribbons. At an optimized vR, a structure with single-phase Nd2Fe14B and an enhanced coupling between grains give rise to excellent magnetic properties. For example, the coercivity, remanence ratio, and maximum energy product of the best sample quenched at a wheel speed of 13 m/s are 1227.4 kA/m, 0.76, and 182.6 kJ/m3, respectively. In comparison with the samples from under- and over-quenched conditions and all the samples with annealing, the as-quenched sample fabricated at the optimized wheel speed has the best magnetic properties.

Phase stability of CuAlMn shape memory alloys

Abstract Thermoelastic martensitic transformations in single crystals of two CuAlMn shape memory alloys were investigated using tension/compression stress–strain tests, thermal cycling tests, stress recovery tests and calorimetric measurements. Pseudoelastic behaviour was observed in the as-quenched samples stressed above the A f temperature. Near the M s temperature the stress–strain response changed and became pseudoplastic. A pronounced dependence of the thermoplastic behaviour on the transformation history was found. The samples once subjected to tension/compression cycling at a temperature near or below M s remained pseudoplastic during subsequent stress–strain experiments at higher temperatures, and a large increase of the A f temperature was observed. This stabilisation of martensite is similar to that observed on CuAlNi, and can be ascribed to the detwinning of the martensitic phase and formation of a single variant of the γ 1 ′ martensite.

Phase selection in undercooled binary peritectic alloy melts

The change of the primary solidification mode in undercooled peritectic Fe–Mo, Co–Si and Al–Co melts has been revealed by in situ observation of recalescence events during electromagnetic levitation. Levitated drops of controlled undercooling up to 380 K were quenched onto chill substrates and subjected to phase and microstructure analysis. Beyond a critical undercooling, the equilibrium solidification can be replaced by the direct growth of the peritectic phase. This was ascertained from sudden changes in growth velocity at a critical undercooling and the ultimate recalescence temperature. Metastable phases were detected in Co75Si25 and Al74.4Co25.6 as-quenched samples. The direct growth of the peritectic phase from the undercooled melt considerably improves the sample homogeneity.

Thermoelectric properties of Bi–Sb semiconducting alloys prepared by quenching and annealing

Bi 100− x Sb x ( x=8–17) alloys were prepared by direct melting of constituent elements, which was followed by quenching and annealing. The synthesis of high-homogeneity alloys was confirmed by X-ray diffraction, differential thermal analyses and electron microprobe analysis. The semiconducting and thermoelectric properties of the samples were investigated by measuring Hall coefficient, electrical resistivity and Seebeck coefficient in the temperature range from 20 to 300 K for both the as-quenched and annealing samples. The properties change gradually with the Sb concentration x, which is attributed to the variation of the energy gap. The Hall mobility was enhanced by annealing, which leads to a small electrical resistivity and a large Seebeck coefficient. Consequently, large values of about 8.5 mW/mK 2 for the power factor were obtained in the annealed alloys of x=8,12, and 14.

In situ formed two-phase metallic glass with surface fractal microstructure

The alloy La 27.5Zr 27.5Al 25Cu 10Ni 10 was found to phase separate during quenching from the melt into (La–Cu)-rich and (Zr–Ni)-rich glassy phases with compositions close to those of Zr–Ni–Al and La–Cu–Al bulk metallic glasses. As-quenched samples consist of two amorphous phases in spherical shape with a wide range of length scales from tens of micrometers to a few nanometers. Several self-similar generations of spheres in spheres were observed using scanning electron microscopy and transmission electron microscopy. Small angle X-ray scattering results show an exponential dependence to the power of −3.4 of the intensity on the scattering vector, indicating a surface fractal microstructure of dimension 2.6. The selected area diffraction patterns obtained from two-phase regions show two clearly distinguishable halo rings and the high resolution electron microscope image shows isotropic maze patterns with two different length scales corresponding to the difference in the average atomic distance of the two amorphous phases.

Structural relaxation in Pd–Cu–Ni–P metallic glasses

Abstract The structural relaxation process in the Pd 40 Cu 30 Ni 10 P 20 and the Pd 43 Cu 27 Ni 10 P 20 metallic glasses was examined by measuring the electrical resistivity at room temperature, ρ (3 0 0), and the endothermic peak behavior peculiar to glass transition after relaxation on the differential scanning calorimetry (DSC) curve. It was concluded that the annihilation and the formation of the free volume increased and decreased the ρ (3 0 0), respectively. The maximum change in ρ (3 0 0) due to the annihilation of the free volume was about 2% of that in the as-quenched sample. The relaxation process at 540 K just below glass transition was investigated. The quenched-in free volume for a Pd 43 Cu 27 Ni 10 P 20 glass did not reach the equilibrium value within annealing period of about 3.0×10 6 s. The local structure of the Pd 40 Cu 30 Ni 9 57 Fe 1 P 20 glass was examined by Mossbauer spectroscopy for the relaxation process at 540 K. The average quadrupole splitting, Q ¯ , and the isomer shift, δ ¯ , decreased with time up to 1.44×10 6 s and thereafter these increased. The origin of this increase was not explained if we took account for the contribution of the chemical short range ordering process.

In-plane magnetic anisotropy along the width in amorphous magnetic ribbons

Abstract A study about the variation of the in-plane magnetic anisotropy along the width of the ribbon is carried out in Co- and Fe-based amorphous magnetic ribbons. From the measurements of the anisotropy in as-quenched, mechanical polished and annealed samples the origin of the lack of homogeneity of the in-plane magnetic anisotropy in wide amorphous ribbons is determined. In addition a shape magnetic anisotropy that we think is originated by the edge effect was found.

Thermal analysis of low alloy Cr-Mo steel

The paper deals with results of thermal analysis of low-alloyed chromium-molybdenum steel. The methods of analysis were dilatometry, differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The Ac1 and Ac3 temperatures of the steel samples measured by dilatometry and DTA during the heating period were in good agreement. Generated by cooling a martensitic structure first became apparent at 503 K. Tempering of the as-quenched samples showed the presence of the second tempering stage in the region between 473 and 573 K. At that stage heat capacity decreased from 0.48 to 0.32 J g-1 K-1, as a result of conversion of transition carbide due to heat consumption. After normalization of the as-quenched samples the heat capacity values were restored to between 0.42 and 0.47 J g-1 K-1 in the temperature range from 373 to 673 K.

Giant Magnetoimpedance Effect in Co70Fe5Si15B10 and Co70Fe5Si15Nb2.2Cu0.8B7 Ribbons

Giant magnetoimpedance (GMI) effect has been observed in Co70Fe5Si15B10 and Co70Fe5Si15Nb2.2Cu0.8B7 melt-spun amorphous ribbons. The magnetoimpedance (MI) of these samples has been studied up to a frequency of 10 MHz and varying a dc magnetic field (Hdc) within 150 Oe. A maximum change of 89% in MI has been observed for Co70Fe5Si15Nb2.2Cu0.8B7 composition around a frequency of 3.1 MHz. Substitution of Cu and Nb for B in an initial Co70Fe5Si15B10 composition forming the Co70Fe5Si15Nb2.2Cu0.8B7 composition not only favors the GMI effect but also gives rise to the sensitivity of the magnetic response (∼18%/Oe), which is very beneficial for magnetic sensors applications. The GMI effect for both samples annealed at 550 K is further enhanced due to the presence of the ultrasoft magnetic materials, compared to their as-quenched samples.

Exchange coupling in FePt permanent magnets

FePt (for 40–60 at. % Fe) exhibits an order–disorder transformation. The disordered phase is face centered cubic and magnetically soft while the ordered phase is tetragonal and shows high magnetic anisotropy. Since the changes in volume between the two phases are small, it is easy for the soft and hard phases to coexist in a uniform manner. Thus, we have an ideal system with which to investigate the basic features of exchange coupled magnets. Bulk Fe0.6Pt0.4 exhibits reasonably large permanent magnetic properties with a maximum energy product of ∼15 MG Oe (120 kJ/m3) without the need for special processes to promote grain alignment. The high energy product is partially a result of the high ratio of remanence to the saturation induction which amounts to 0.68 as opposed to the ratio of 0.5 for an assembly of randomly oriented uniaxial magnets. This enhanced remanence is predicted by the exchange-spring magnet model for a mixture of cubic and uniaxial phases. In order to verify that the high remanence of the FePt magnets is indeed caused by this exchange mechanism and not by a fortuitous formation of magnetic texture during sample preparation, we have prepared cubic samples of Fe0.6Pt0.4 and measured the B–H loops along the three principal directions of the cube using a vibrating sample magnetometer. The hysteresis loops are identical in all three directions, indicating the absence of strong texture in this material. After correction of the demagnetizing factor, the ratio of remanence to saturation induction is 0.66–0.68. This is consistent with that in the exchange coupling model. The highest energy product in our study was found for the as-quenched samples. Further annealing, with the intention of promoting the formation of hard magnetic phases, reduced the coercivity and remanence. This suggests that improved results may be achieved by initial suppression of the formation of the hard phases by modification of the quenching process and alloying. An ideal exchange coupled system could be designed based on such a starting material.

Evolution and characterization of fluorite-like nano-SrBi 2Nb 2O 9 phase in the SrO–Bi 2O 3–Nb 2O 5–Li 2B 4O 7 glass system

Transparent glasses of various compositions in the system (100− x)Li 2B 4O 7− x(SrO–Bi 2O 3–Nb 2O 5) (where x=10, 20, 30, 40, 50 and 60, in molar ratio) were fabricated via splat quenching technique. The glassy nature of the as-quenched samples was established by differential thermal analyses. X-ray powder diffraction (XRD) and transmission electron microscopic studies confirmed the amorphous nature of the as-quenched and crystallinity in the heat-treated samples. Fluorite phase formation prior to the perovskite SrBi 2Nb 2O 9 phase was analyzed by both the XRD and high-resolution transmission electron microscopy. Dielectric and the optical properties (transmission, optical band gap and Urbach energy) of these samples have been found to be compositional dependent. Refractive index was measured and compared with the values predicted by Wemple–Didomemenico and Gladstone–Dale relations. The glass nanocomposites comprising nanometer-sized crystallites of fluorite phase were found to be nonlinear optic active.

Effects of plastic deformation on the elastic modulus and density of bulk amorphous Pd 40Ni 10Cu 30P 20

The effect of annealing and plastic deformation on the density and modulus of bulk amorphous Pd 40Cu 30Ni 10P 20 was measured. The shear and bulk modulus were measured by resonant-ultrasound spectroscopy. Annealing causes the density and shear modulus to increase, but the bulk modulus remains approximately constant. Following annealing, plastic deformation reduces both the density and shear modulus to their values in the as-quenched samples. A linear relationship was found between the shear modulus and the density. The relative change in the shear modulus is approximately 25 times larger than the relative change in the density.

Effect of field-induced magnetic ordering on the electrical resistance of the frustrated magnet Au80Co20

Au80Co20 platelets were produced by solid-state quenching followed by furnace annealing and characterized by X-ray diffraction. Magnetization and magnetoresistance (MR) were measured from 2 to 265K. Minima of the zero-field electrical resistance R0(T) were observed at low temperatures (Tm≈20K) in both as-quenched and annealed samples. These minima disappear upon applying a magnetic field. At low T, a negative MR is associated to strong frustration and a very short magnetic coherence length; at higher temperatures, a positive MR is also observed.

Annealing and neutron-irradiation effects on the permeability of Fe 86Zr 7B 6Cu 1

The effects of annealing and neutron irradiation on the magnetic properties of amorphous Fe 86Zr 7B 6Cu 1 alloys have been investigated by means of the changes of the longitudinal permeability ratio (LPR). In general, there are no differences in magnitude and shape of the LPR curves between as-quenched amorphous and neutron-irradiated samples. This indicates that the effects of neutron irradiation on the magnetic properties of the as-quenched sample are negligible. But a drastic decrease, caused by neutron irradiation, is observed in the magnitude of the LPR and the broadening of the LPR curves of the sample annealed at 650°C, which has a maximum value of the LPR due to the presence of ultra-soft magnetic materials. This can be attributed to a decrease in domain wall displacement and to a magnetization rotation due to the neutron irradiation effect. The local anisotropy plays a significant role in the case of this sample.

A new hypothesis about the mechanism of magnetic-domain formation

Abstract This paper proposes a mechanism of formation of the magnetic domains on cooling a ferromagnetic material from a higher temperature than the Curie temperature. It is derived from the well-known theory of crystallization in an amorphous material through a two-step mechanism of nucleation and crystal growth. Following this approach it is shown that the domain structure should depend on the free-energy change ΔG per mole of magnetic moment aligning in the same direction. A method to avoid calculation of ΔG is shown. This approach was successfully applied to explain the differences between the domain structures of the annealed and the as-quenched samples of the amorphous alloy Fe62.5Co6Ni7.5Zr6Cu1Nb2B15. Moreover, it gives an explanation for recent findings on the magnetic microstructure of high-permeability materials.