Comparative study of spinodal decomposition in symmetric and asymmetric Cu-Ni-Cr alloys

Spinodal decomposition in three alloys of nominal compositions 71Cu-27Ni-2Cr, 45Cu-45Ni-10Cr and 33Cu-52Ni-15Cr were studied by X-ray diffraction technique and transmission electron microscopy. It was found that the first and third alloys are asymmetric in nature while the second is symmetric. The symmetric alloy was found to decompose faster than the asymmetric ones. The asymmetry of the side bands was found to be related to the proportion of phases in the alloy. Electron microscopic studies revealed that during coarsening the major phase increases its connectivity by isolating the minor phase.     

An electron diffraction study of quasicrystals in Ti-37 at% Mn and Ti-24 at% Mn-13 at% Fe alloys

Electron diffraction studies were carried out to establish the icosahedral phase formation in rapidly quenched Ti-37 at% Mn and Ti-24 at% Mn-13 at% Fe alloys. Distortions in the diffraction spots and diffuse intensities in the diffraction patterns were investigated. The existence of a rational approximant structure and a decagonal like phase are also reported.     

Effect of Prior Deformation on Aging Process in a Cu-30Ni-25Fe Spinodal Alloy

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In-situ electrical resistivity measurements: study of magnetic and phase transitions and solid-HDDR processes in Nd-Fe-B-type alloys

Nd-Fe-B-type alloys have been characterized by means of in-situ electrical resistivity measurements. The potential of this technique for monitoring various phenomena relevant to the hydrogenation, disproportionation, desorption and recombination (HDDR) processing of Nd-Fe-B-type alloys is assessed, together with an evaluation of its capacity for delineating magnetic and phase transitions. The effects of external parameters, such as hydrogen pressure and processing temperature, and of intrinsic parameters, such as alloy composition and initial microstructure, on the kinetics of the solid-HDDR process have been investigated. It was found that the amount of neodymium-rich intergranular phase present in the material had a significant influence on the rates of disproportionation and recombination reactions. At 620C, the recombination process takes place as a solid-solid reaction, and this has a marked effect on the reaction rate. It was also found that the disproportionation process is very sensitive to the hydrogen pressure and the dependence of the overall process on the processing temperature between 620 and 900C has been determined.     

Precipitation hardening and recrystallization in Cu-4% to 7% Ni-3% Al alloys

The effects of microstructure on mechanical properties in three cold-worked Cu-4% to 7% Ni-3% Al alloys have been investigated by changing ageing time at 500 °C. Hardness and strength in the Cu-7% Ni-3% Al and Cu-5.5% Ni-3% Al alloys increase with ageing time and have maximum values at an ageing time of 10³–10⁴s at 500 °C, then decrease. During ageing of Cu-7% Ni-3% Al at 500 °C, the coherent Ni₃Al phase was first precipitated out and later incoherent NiAl phase was formed. Ni₃Al formed during the initial stage of ageing is likely to be a transient phase. The increases in hardness and strength are due to the precipitation of coherent Ni₃Al phase. Coherent Ni₃Al particles are effective in increasing the strength and retarding the recrystallization process. On the other hand, the hardness and strength in the Cu-4% Ni-3% Al alloy gradually declined with ageing time. Only incoherent NiAl phase was formed during ageing at 500 °C. Decreases in hardness and strength in the Cu-4% Ni-3% Al alloy are attributed to softening during recovery and recrystallization, because incoherent NiAl particles have an insufficient effect to increase the strength.     

Structural relaxation in FeNiCrPB amorphous alloys by joint isothermal and tempering measurements of the electrical resistivity

Primak's analysis of kinetic processes distributed in activation energy has been applied to discuss electrical resistivity measurements performed on two FeNiCrPB amorphous alloys. The structural relaxation has been studied both in isothermal and in tempering conditions. From a comparison of the kinetic analysis of these different measurements, the attack frequency of the activated processes has been computed and turns out to be 10⁹ sec^–1. Reversible and irreversible phenomena have been observed and studied separately. In these amorphous alloys, a higher Cr content leads to a higher thermal stability against structural relaxation, whereas the kinetic behaviour seems to be quite similar.     

A device for contactless measurement of electrical resistivity of metals and alloys

A simple apparatus for contactless mesurement of resistivity is described. The method is based on the change in penetration depth of electromagnetic fields (due to the skin effect) by the change of the conductivity of a foil.     

Examination of precipitation in the aluminum alloy AA6111 using electrical resistivity measurements

The evolution of electrical resistivity during the aging of aluminum alloy AA6111 has been measured over a wide range of annealing temperatures. From these measurements, the solubility product for the Q phase in aluminum has been proposed and using a new approach, the contribution of precipitates to the overall resistivity is quantified.     

Dependence of magnetic properties on atomic order in 77% Ni-14% Fe-5% Cu-4wt% Mo alloys

We have studied the effect of atomic order on the lattice parameter,a, Curie temperature,T , and the initial permeability, _i, of a series of molybdenum permalloys with varied Ni-Fe ratios and molybdenum concentrations. The ordering temperature range was between 380 and 600° C. The results of the effect of short-range order and long-range order on the lattice parameter after annealing for about 5 h between 420 and 470° C indicated a decrease of between 0.15 and 0.22 pm (0.04 to 0.06%) in the lattice parameter. No superlattice lines were detected. This may probably be due to the similarity in the atomic scattering factor of nickel and iron. It was also noted that both long-range and short-range ordering increased the Curie temperature of the ordered materials by about 1.4 to 6.6% due to the production of stronger and shorter Ni-Fe bonds. The number of Ni-Fe bonds, which controls the exchange force, was found to depend on the amount of order and molybdenum content in each material. The Curie temperature, which is a measure of the exchange force, is also an indirect means of measuring the degree of lattice ordering because the exchange integral is affected by metallurgical variables such as atomic ordering, composition, etc. Results of the isothermal annealing time on the initial permeability in the temperature range (380 to 600° C) indicated that between 380 and 460°C, maximum permeability was obtained at a critical degree of short-range ordering which is thought to correspond to a state when both the magnetostriction and anisotropy constants are close to zero. The maximum permeability was independent of ordering temperature in this range, although the time to reach this maximum decreases with increasing temperature. On annealing in the temperature range 460 to 500° C, the permeability reaches a maximum, the maximum permeability in this temperature range decreases with increasing temperature. As in the lower temperature range, the time to reach this maximum decreases with increasing temperature. Annealing between 500 and 600° C produced no maximum permeability. The permeability levelled off after an initial gradual increase. The activation energies for short-range order formation were found to be smaller (between 135 and 142 kJ mol^-1) in alloys with molybdenum concentrations above 4 wt% Mo (>2.5 at.% Mo) and higher activation energies (between 196 and 210 kJ mol^–1) in alloys with molybdenum concentrations below 4 wt% Mo (<2.5 at.% Mo) which suggests the formation of different ordered structures and mechanisms below and above 4 wt% Mo.     

Dispersion of dielectric constant and resistivity of Cu x Zn1−x Fe2O4 samples

Cu _x Zn_1−x Fe₂O₄ samples exhibit dispersion of dielectric constant, tanδ and resistivity in the frequency range of 1 kHz to 50 MHz. The dispersion exhibited is in general accord with Koops’ model. However, the details of the conducting and non-conducting regions must be taken into account when composition tends to change interrelationship between the elementary capacitor resistor circuits. On quenching these samples from 800°C the dielectric constantε ¹ showed an increase for CuFe₂O₄ and Cu_0·8Zn_0·2Fe₂O₄ samples. The dielectric constant of the remaining samples showed no influence on quenching. The compositional variation showed that the dielectric constant has higher value for the ferrite Cu_0·4Zn_0·6Fe₂O₄ The results are explained on the basis of cation transfer.     

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.     

A comparative study of COD measurements for novel metal matrix composites and alloys

The behaviour of novel metal matrix composites and their respective matrix alloys is studied in the present work from the fracture mechanics point of view. A series of experiments were executed in single edge notched specimens with various initial crack inclinations and the crack opening displacement was recorded versus externally applied load. An empirical criterion was established permitting easy, in-situ inspection of the safety and further loading capacity of cracked structural members, already in function.Presented at Fourth Greek National Congress on Mechanics, 26–29 June 1995, held at Xanthi, Greece.     

TEM study and magnetic measurements of precipitates formed in Cu-Fe-Ni alloys

The precipitation behavior of Cu-Fe alloys with Ni addition on isothermal annealing at 878 K was investigated by means of transmission electron microscopy (TEM), electron dispersive X-ray spectroscopy (EDS), and field-emission scanning electron microscopy (FE-SEM). Magnetic element atoms were segregated from the solid solution in supersaturated state, and nano-scale magnetic particles were randomly formed in the copper matrix at the initial stage of annealing at 873 K. With increasing the isothermal annealing time, however, the striking feature that two or more nano-scale magnetic particles with a cubic shape aligned linearly along (100) directions were observed upon the isothermal annealing at 873 K. To investigate the relationship between micro-structures and magnetic properties of the heterogeneous Cu-Fe-Ni alloys, magnetic measurements such as M-H measurements were also carried out, using a superconducting quantum interference device (SQUID) magnetometer. In this study, it was revealed that the magnetic properties of the specimen presented the ferromagnetic behavior, during the precipitation process in a Cu-Fe-Ni alloy.     

Structural and electrical study of ZnCuTiO4, ZnCuSnO4 and ZnCuGeO4 synthesised using metallic copper

The structural study of ZnCuTiO₄, ZnCuSnO₄ and ZnCuGeO₄ reveals that while the cooperativeJ-T effect of Sn⁴⁺ is responsible for the orthorhombic symmetry of ZnCuSnO₄, the hexagonal influence of Ge⁴⁺ is more predominant in ZnCuGeO₄. Ti⁴⁺ is uninfluential either in initiating the distortive tendency or cooperating with other ions for the same. The XRD,ρ _RT, ΔE and the nature of the charge carriers, all suggest that the air oxidation of metallic copper leads to the minority concentration of Cu⁺ along with a majority concentration of Cu²⁺, as demanded by the stoichiometric considerations of these compositions.     

Assessment of surface bonding strength in Al clad steel strip using electrical resistivity and peeling tests

Abstract

Aluminium clad steel strip successfully combines the surface properties of an aluminium alloy coating with the satisfactory mechanical properties of the steel substrate. The production of Al clad steel strip by rolling, however, is a more efficient and economical approach compared with other processes. In this investigation, trilayer strips of aluminium/steel/aluminium were produced using the cold roll bonding technique. The bonding strength between the layers and the electrical resistivity of the samples were measured using a peeling test and four point probe test, respectively. The effects of reduction of thickness, the friction condition between the outer layer and rolls, and the rolling velocity on the bonding strength and electrical resistivity of the samples were assessed. Finally, it is shown that the resistivity test can be used as a non-destructive test for the evaluation of the quality of bonding between the layers of aluminium clad steel strip.     

A study of Mg and Cu additions on the foaming behaviour of Al–Si alloys

Aluminium casting alloys containing Mg and Cu in addition to Si were investigated with respect to their potential to be foamed. The kinetics of foam expansion of different alloys was studied and the resulting structures were characterised. Of the stages of evolution of foams, namely (i) pore nucleation, (ii) foam growth in the semisolid state, (iii) further expansion in the fully liquid state, the latter two were explored. Expansion in the semisolid state could be related to the available liquid fraction. Mg-containing Al–Si alloys yielded a less coarse and more uniform pore structure than the other alloys investigated. However, achieving a high volume expansion required restriction to a narrow process window and led to the suggestion of AlMg4Si8 as a practical alloy.     

Compositional range,thermal stability,hardness and electrical resistivity of amorphous alloys in Al-Si (or Ge)-transition metal systems

An amorphous single phase was found to be formed in wide compositional ranges in rapidly solidified Al-Si-transition metal (M) and Al-Ge-M alloys. The compositional ranges are in the range from 12 to 53 at. % Si or Ge and 8 to 23% M and Al-Si-Co and Al-Ge-Fe alloys have the widest glass-formation ranges. Because the interaction between aluminium and silicon or germanium atoms is thought to be repulsive from the immiscible equilibrium phase diagrams, the glass formation is probably due to an attractive interaction of M-Si (or Ge) and Al-M pairs. Hardness, H _v, and crystallization temperature, T _x, increase with increasing M content and the highest values reach 1120 DPN and 715 K, while the change with silicon or germanium content is much smaller for H _v and is hardly seen for T _x. Additionally, the H _v and T _x have maximum values for Al-Si (or Ge)-M (M=Cr, Mn or Fe), decrease with the decrease and increase in the group number of M element and are the lowest for Al-Si (or Ge)-Ni alloys. The compositional dependence is interpreted under the assumption that T _x and H _v of the aluminium-based amorphous alloys are mainly dominated by the attractive interaction of M-(Si or Ge) and Al-M pairs. Room-temperature resistivity, _RT, increases in the range of 220 to 1940 cm with increasing silicon or germanium and M contents. The change in _RT with the group number of M elements shows a maximum phenomenon for manganese. It has thus been clarified that the characteristics of the Al-Si-M and Al-Ge-M amorphous alloys have the different compositional dependence as compared with those for conventional metalmetalloid amorphous alloys, probably because of the unusual interaction among the constituent elements.