Phase formation in rapidly quenched Cu-based alloys

In the present investigation, we report the formation of γ-brass type phase in the rapidly quenched Cu₅₀Ga₃₀Mg₅Ti₁₅ and Cu₅₀Al₃₀Mg₅Ti₁₅ alloys. Rapid solidification of Cu₅₀Ga₃₀Mg₅Ti₁₅ alloy shows the formation of simple cubic γ-brass type phase (a = 0.863 nm), which on annealing at 1,023 K for 60 h transforms to disordered type γ-brass phase (a = 0.879 nm). It has been observed that intensity modulation of electron diffraction spots corresponding to simple cubic γ-brass phase is similar to the intensity modulation observed in the mirror orientation of icosahedral quasicrystalline phase. Contrary to the crystalline phase formation in Cu–Ga–Mg–Ti alloy, rapid solidification of Cu₅₀Al₃₀Mg₅Ti₁₅ shows the formation of amorphous and nanocrystalline bcc γ-brass type phase (a = 0.870 nm), which on annealing transforms to ordered γ-brass type phase (a = 0.872 nm). The structural and microstructural characterization was done through X-ray diffraction and transmission electron microscopy.     

Hard magnetic phase in rapidly quenched Zr-Co-B alloys

In order to identify a hard magnetic phase in rapidly quenched Zr-Co-B alloys and clarify its magnetic properties, Zr-Co-B ribbons, Zr-Co ribbons, and Zr-Co ingots were studied. The hard magnetic phase is interpreted as a Zr₂Co₁₁ intermetallic compound. This compound has a Curie temperature of 500°C and uniaxial magnetic anisotropy with an anisotropy field of 34 kOe. The magnetization of this compound was estimated to be 67 emu/g at 15 kOe. In addition to the hard magnetic phase, the low and high T_c phases appear in both binary and ternary alloys. The low T_c phase is FCC Zr₆Co₂₃ with T_c=180°C. The magnetization of Zr₆Co₂₃ was estimated to be 44 emu/g at 15 kOe. The high T_c phase is cobalt including a small amount of zirconium. In Zr-Co-B alloys, suitable boron addition is shown to enhance the coercive force. On the other hand, the addition increases the magnetization. While the boron addition produces cobalt, it reduces Zr₆Co₂₃ or quenches its ferromagnetism     

Microstructural stability of rapidly quenched Al---Fe---Mo alloys

We investigated the microstructural stability of rapidly quenched Al-8%Fe-2%Mo alloys by measuring the evolution of thermoelectric power during ageing between 300 °C and 500 °C, and by transmission electron microscopy and small-angle X-ray scattering observation. Four stages of evolution were found corresponding to the evolution of the solid solution and S-phase into a final incoherent generalized precipitation. The addition of Mo does not delay this evolution but gives a more homogeneous microstructure.     

Age hardening of rapidly quenched Al-Zr-B alloys

Ternary alloys from the Al-Zr-B system were prepared as thin foils by a rapid quench technique. These foils were annealed isothermally as well as isochronally at various temperatures (150 to 550° C). The microstructures show that at high temperature, the grain growth is significantly retarded by the grain boundary pinning of boride precipitates. A strong age hardening is also a characteristic phenomenon in these alloys. It is found that microstructure and microhardness largely depend upon the zirconium/boron ratio of the alloy, indicating that the ratio determines the types of compounds occurring in this alloy system. It appears that in these alloys, high concentration of boron and the low ratio of zirconium/boron together yield stable precipitates at high temperatures.     

Devitrification of rapidly quenched Al-Cu-Ti amorphous alloys

X-ray diffraction, transmission electron microscopy and differential scanning calorimetry were carried out to study the transformation from amorphous to icosahedral/crystalline phases in the rapidly quenched Al₅₀Cu₄₅Ti₅ and Al₄₅Cu₄₅Ti₁₀ alloys. In the present investigation, we have studied the formation and stability of amorphous phase in Al₅₀Cu₄₅Ti₅and Al₄₅Cu₄₅Ti₁₀ rapidly quenched alloys. The DSC curve shows a broad complex type of exothermic overlapping peaks (288- 550†C) for Al₅₀Cu₄₅Ti₅ and a well defined peak around 373†C for Al₄₅Cu₄₅Ti₁₀ alloy. In the case of Al₅₀Cu₄₅Ti₅ alloy amorphous to icosahedral phase transformation has been observed after annealing at 280†C for 73 h. Large dendritic growth of icosahedral phase along with α-Al phase has been found. Annealing of Al₅₀Cu₄₅Ti₅ alloy at 400†C for 8 h results in formation of Al₃Ti type phase. Al₄₅Cu₄₅Ti₁₀ amorphous alloy is more stable in comparison to Al₅₀Cu₄₅Ti₅ alloy and after annealing at 400†C for 8 h it also transforms to Al₃Ti type phase. However, this alloy does not show amorphous to icosahedral phase transformation.     

Characterization of strengthened rapidly quenched Zr-based alloys

The nominal composition components of alloy Zr_66.4Nb_6.4Ni_8.7Cu_10.5Al₈ (Alloy A) were fabricated and characterized. The strengthening of in-situ alloys depends on the role of both the glassy matrix and the second phases. The glass transition and the crystallization kinetics were studied using DSC and X-ray diffraction as a function of element distribution. The amorphous and semi-crystalline structures were identified with the existence of nano crystals in the alloy nominal compositions. The Elastic compression modulus were found to increase with transition to crystallite phase. Where as, the microhardness decreases dramatically with the change from crystalline to amorphous phase. The compression fracture surface shows classic veins behavior. In mode of continuous heating and adiabatic annealing the glass transition, T _g , and the crystalline peak, T _p , temperatures display a strong dependency on heating rate. The activation energy for glass transition and crystallization were determined as E _g  = 226 KJ/mol based on Kissinger method, but during the isothermal process E _g  = 121 KJ/mol.     

Nonequilibrium phases in rapidly quenched Co-C-Si and Ni-C-Si alloys

It has been found that the rapid quenching of Co-C-Si and Ni-C-Si alloys results in the formation of an amorphous phase in the range above 10 at% C and 12 to 23 at% Si in the Co-C-Si system, and a nonequilibrium ordered b c c phase with a lattice parameter of 0.2744 nm in the range above 4 at% C and 15 to 21 at% Si in the Ni-C-Si system. Since the interaction between cobalt and carbon is repulsive, the glass formation in the high metalloid concentration range in the Co-C-Si system is thought to be attributed to a strongly attractive interaction between metalloid atoms (carbon and silicon). Crystallization temperature and Vickers hardness of the amorphous alloys are in the range of 671 to 708 K and 833 to 942 diamond pyramid number (DPN) respectively. Furthermore, the amorphous alloys exhibit a soft ferromagnetism and the Curie temperature, saturated magnetization under an applied field of 100 Oe, coercive force and permeability at 1 kHz are 395 to 432 K, 4.33 to 5.50 kG, 0.031 to 0.210 Oe and 31 000, respectively, in the as-quenched state. The effective permeability of (Co_0.94Fe_0.06)_67.5C_12.5Si₂₀ amorphous alloy is higher than that of Co₆₇Fe₄Si₁₉B₁₀ amorphous alloy with zero magnetostrain at frequencies above 200 kHz. Accordingly, the Co-C-Si amorphous alloys newly found in the present work are very attractive as a soft ferromagnetic material with good characteristics in the high frequency range.     

Non-equilibrium crystalline superconductors in Zr-Si binary alloys rapidly quenched from melts

The new non-equilibrium superconductor with a b c c structure has been found in rapidly quenched Zr-Si alloys. The silicon content in the b c c alloys was limited to the narrow range between 8 and 11 at%. The b c c alloys showed a superconducting transition whose temperature increased from 3.20 to 3.84 K with decreasing silicon content. The upper critical magnetic field and the critical current density for Zr₉₂Si₈ alloy were of the order of 3.58 × 10⁶ Am^–1 at 2.0 K and 3.3 × 10^–2 Am^–2 at 2.42 K in the absence of an applied field. The upper critical field gradient at the transition temperature and the electrical resistivity at 4.2 K were about — 1.82 × 10^–1 Am^–1 K^–1 and about 150 cm. The Ginzburg-Landau parameter and coherence length _GL (0) were estimated to be about 65 and 6.3 nm, respectively, from these experimental values by using the Ginzburg-Landau-Abrikosov-Gorkov theory and hence it is concluded that the present b c c alloys are extremely soft type-II superconductors with a high degree of dirtiness.     

Phase transformations in Al-rich Al-W alloys rapidly quenched from the melt

Al-W alloys with a W content from 4.3 to 11.7 wt% were rapidly quenched, using the two-piston quenching method. X-ray examination showed that in as-quenched alloys two kinds of samples appeared; multiphase samples and single-phase samples. Multiphase samples contained supersaturated Al-W solid solutions (-Al), Al₄W and two unknown phases, all of which were metastable. In single-phase samples -Al was present as the only phase. In these samples -Al began to decompose at temperatures as low as 100° C. During the decomposition of these solid solutions two other new phases were observed, the unit cell constants of which were determined. Experiments with isochronal and isothermal annealing showed that the presence of metastable phases in as-quenched alloys highly increases the resistance to decomposition of Al-W solid solutions. The possible reason for this behaviour is discussed.     

Structural inhomogeneities of AlSi alloys rapidly quenched from the melt

Hypo- and hyper-eutectic AlSi alloys were rapidly quenched from the melt using the melt-spinning technique with two spinning velocities. Structural differences between the wheel (chill) and upper sides of the melt-spun ribbons were investigated by optical and scanning electron microscopy and X-ray diffraction methods (texture- and size-strain analyses). The Al-rich phase of the hypo-eutectic alloys was textured. The textures observed from both sides of the ribbons were different; in neither case was it of fibre type. For the larger spinning velocity applied, the structural imperfection of the wheel side was larger than that of the upper side for both the Al-rich and the Si-rich phases.     

Decomposition behaviour of rapidly quenched Al–Nb alloys– TEM study

Abstract

Two binary Al-rich Al–Nb alloys containing 4·5 and 7·7 wt-%Nb have been rapidly quenched from the liquid state using the ‘gun’ technique. Transmission electron microscopy and diffraction techniques have been extensively employed to characterize the as quenched as well as externally heat treated alloy foils. Each alloy in the as-quenched state shows only a supersaturated solid solution, indicating a substantial increase in the solid solubility limit of Nb in Al by rapid quenching. Elongated solid solution grains and defect structures have also been observed in the as-quenched foils. While no precipitation was found to occur on annealing the quenched dilute alloy (Al–4·5Nb) foils for 1 h at 673 K, precipitation along grain boundaries was observed in the concentrated alloy (Al–7·7Nb). Higher temperature annealing (≥773 K) resulted in the formation of a new metastable phase having an ordered fcc GeCa₇ type structure with a lattice parameter a ~ 0·8 nm. This phase forms predominantly with a rodlike morphology and is arranged in a Widmanstätten pattern inside the grains, although fine precipitate particles have also been observed. On prolonged annealing at or above 773 K, the metastable phase transforms with the formation of the equilibrium tetragonal Al₃Nb phase.

MST/391     

Superconductivity in Cu-V-Si alloys

The superconducting properties of copper ternary alloys containing small amounts of vanadium and silicon have been investigated. The wire samples of three alloys—Cu₉₀V_7.5Si_2.5, Cu₈₈V₉Si₃, and Cu₈₄V₁₂Si₄—show a resistive superconducting transition around 17 K, the characteristic transition temperature of the A15 compound V₃Si, followed by a plateau and a second transition between 5 and 10 K. The resistivity, however, does not drop to zero down to 2.5 K. The maximum drop in resistivity is observed for the alloy containing minimum amounts of vanadium and silicon. Complete super-conductivity is absent in these alloys for two reasons. First the effective superconducting volume fraction in these alloys is well below the threshold value obtained by either the effective medium theory or the site percolation theory for solid conduction. Second, the superconducting phase does not precipitate in a fine structure in the matrix and a strong proximity effect does not operate. Experimental evidence confirms the formation of brittle phase V₃Si in the cast alloys which does not get elongated to the same extent as the matrix when rolled or drawn. This reduces the effective volume fraction of V₃Si in the drawn wires. Annealing of these wires causes the superconducting particles to coalesce and grow in size, increasing the interparticle distances to the detriment of superconductivity in these alloys.     

Effect of rare-earth elements on the microstructural characterization in rapidly quenched thermally strengthened aluminium alloys

The influence of rare-earth elements on the microstructural features of rapidly solidified Al93.3-xFe4.3V0.7Si1.7Mmx(x=0, 0.5, 1.0, 3.0, 6.0) alloy was systematically studied by differential scanning calorimetry, X-ray diffraction, transmission electron microscopy and energy dispersive X-ray analysis. Experimental results show that there are different type of phase transformation depending on mischmetal (Mm) concentration. For Al87.3Fe4.3V0.7Si1.7Mm6.0 metallic glass, a shoulder was observed on the high-angle side of the main peak in the X-ray diffraction patterns due to quenched-in aluminium nuclei and a prepeak resulting from Mm–Mm pairs. By means of particle extraction analysis, it has been proved that the -Al13(Fe, V)3Si phase existing in as-cast Al–Fe–V–Si alloy is wholly or partly inhibited for Al93.3-xFe4.3V0.7Si1.7Mmx (x=0.5, 1.0, 3.0) crystalline alloys. In addition, a new phenomenon has been reported that the lattice parameter of as-quenched Al–Fe–V–Si–Mm alloys decrease with increasing Mm content; the "cell lessening effect". This effect is presumably due to the results of composite interactions between rare-earth elements and alloy elements.     

Formation,thermal stability and electrical resistivity of quasicrystalline phase in rapidly quenched Al-Cr alloys

The icosahedral quasicrystal has been found to appear in a wide composition range from about 5 to 16 at % Cr in rapidly quenched Al-Cr alloys, but the formation of the quasicrystal line single phase was limited only in the vicinity of about 15.5at% Cr. Analytical solute concentrations in the quasicrystalline phase are not always constant and increase continuously from 9.0 to 15.4 at% Cr with increasing nominal solute concentration from 6 to 15.4%. The quasicrystal can be approximately formulated to be Al₁₁ Cr₂ with a maximum deviation of about 6% Cr from the stoichiometric ratio to lower concentration side. Vickers hardness and electrical resistivity increase gradually with increasing chromium content and rapidly at about 14.5% Cr, and their values of Al_84.6Cr_15.4 quasicrystal are 710 DPN, 2.38m at 4.2 K, and 2.72m at 293 K. On the other hand, the onset transformation temperature of quasicrystal to crystalline phase,T _t, and the heat of transformation, H _t show maximum values of 644 K and 1805 J mol^–1 at 14.5% Cr and decrease to 625 K and 550 J mol^–1 at 15.4% Cr. Al_84.6Cr_15.4 quasicrystal trans forms at two stages to a stable orthorhombic Al₁₁Cr₂ compound through a metastable intermediate phase with unidentified structure, while the quasicrystal + Al structure in Al-Cr alloys containing less than 15% Cr changes directly to stable phases of compounds and aluminium. The distinct difference in transformation behaviour of the quasicrystal is thought to be the reason for the abrupt changes inT _t and H _t at a composition between 14.5 and 15.4% Cr.