The annealing and re-oxidation of oxidized CuNi alloys

A study has been made of the effects of an intermediate, isothermal annealing treatment in argon on the oxidation kinetics in dry oxygen of Cu10%Ni and Cu24%Ni at 800°C and Cu80%Ni at 1000°C using a semi-automatic microbalance. Changes in scale morphology and composition have been investigated using various techniques.Oxidation of Cu10%Ni and Cu24%Ni produces external scales consisting of a thick, inner Cu₂O layer and a thin, outer CuO layer, together with nickel-rich internal oxide in the adjacent alloy. Oxidation of Cu80%Ni yields an external scale consisting of a thick, inner NiO layer and a thin, outer CuO layer, together with a few nickel-rich internal oxide particles in the adjacent alloy. During annealing, the outer CuO layers on the three alloys dissociate to give Cu₂O and oxygen. With Cu80%Ni only, further dissociation to give copper metal takes place after long annealing times. In all cases, the annealing treatment leads to a reduction in the cation vacancy gradient across the scale and a reduction in the total vacancy level in the scale due to the reduction in oxygen activity at the oxide-gas interface.In Cu10%Ni and Cu24%Ni, internal oxide formation during annealing stimulates dissociation of the Cu₂O scale near the oxide-alloy interface to give copper metal and oxygen. Similarly, internal oxide formation stimulates dissociation of the NiO layer on Cu-80%Ni, although to a lesser extent than for the copper-rich alloys.During re-oxidation, the kinetics are partly determined by the extent of scale thinning during the prior annealing treatment, giving more rapid weight gains than expected from those recorded during the initial oxidation period. Nevertheless, particularly for Cu-80%Ni, where the scale thinning is relatively small, the reduction in the cation vacancy gradient across the scale and the reduction in the total vacancy level in the scale do result in a reduced oxidation rate compared with the rate recorded during the initial oxidation period.     

The dissolution and passivation of Fe and FeCr alloys in acidified sulphate medium: Influences of pH and Cr content

The steady state polarization curves of ferritic FeCr alloys, containing 7–12% Cr and immersed in 1M H₂SO₄, exhibit two current maxima, i.e. two passivating processes take place at the electrode interface within two different voltage ranges. The origin of these two passivation phenomena was investigated by plotting the steady state polarization curves and also by measuring the electrode impedance along with different polarization points. These results are then compared to those obtained with Fe electrode immersed in sulphate solutions of various pH value (0–5) since the steady state polarization curves of this electrode exhibit also two current maxima in a weakly acid medium, i.e. at solution pH greater than 4. However, even in a strongly acid solution in which only one current maximum is observed for the Fe electrode, the electrode impedance showed two passivation processes are occurring at the electrode interface. Therefore, the Fe dissolution involves at least two dissolution paths and the current at which one dissolution path overtakes the other is dependent upon the solution pH. The impedance diagrams of FeCr alloy electrode are similar to those of Fe electrode especially when the comparison is made with the Fe electrode immersed in a less acid solution, i.e. the addition of Cr in Fe enhances the appearance of two passivation phenomena with increase of solution pH.     

Oxidation of FeCr alloys in O2/3% H2O

A range of FeCr binary alloys containing 2–100%Cr was exposed in O₂/3%H₂O at 1023–1223K to determine whether the presence of the water had a significant effect on oxidation. Oxidation rates were not significantly different from those in dry oxygen and decreased with increasing time of oxidation. The kinetics of oxidation and the oxide structures both suggested that the early stages of oxidation were controlled by lattice diffusion through layers of FeCr spinel for Fe-2 1/4Cr and Fe-8 1/2Cr, or through (Cr, Fe)₂O₃ scale for alloys containing 16%Cr or more.     

The annealing and re-oxidation of oxidized CuNi alloys

A study has been made of the effects of an intermediate, isothermal annealing treatment in argon on the oxidation kinetics in dry oxygen of Cu10%Ni and Cu24%Ni at 800°C and Cu80%Ni at 1000°C using a semi-automatic microbalance. Changes in scale morphology and composition have been investigated using various techniques.Oxidation of Cu10%Ni and Cu24%Ni produces external scales consisting of a thick, inner Cu₂O layer and a thin, outer CuO layer, together with nickel-rich internal oxide in the adjacent alloy. Oxidation of Cu80%Ni yields an external scale consisting of a thick, inner NiO layer and a thin, outer CuO layer, together with a few nickel-rich internal oxide particles in the adjacent alloy. During annealing, the outer CuO layers on the three alloys dissociate to give Cu₂O and oxygen. With Cu80%Ni only, further dissociation to give copper metal takes place after long annealing times. In all cases, the annealing treatment leads to a reduction in the cation vacancy gradient across the scale and a reduction in the total vacancy level in the scale due to the reduction in oxygen activity at the oxide-gas interface.In Cu10%Ni and Cu24%Ni, internal oxide formation during annealing stimulates dissociation of the Cu₂O scale near the oxide-alloy interface to give copper metal and oxygen. Similarly, internal oxide formation stimulates dissociation of the NiO layer on Cu80%Ni, although to a lesser extent than for the copper-rich alloys.During re-oxidation, the kinetics are partly determined by the extent of scale thinning during the prior annealing treatment, giving more rapid weight gains than expected from those recorded during the initial oxidation period. Nevertheless, particularly for Cu80%Ni, where the scale thinning is relatively small, the reduction in the cation vacancy gradient across the scale and the reduction in the total vacancy level in the scale do result in a reduced oxidation rate compared with the rate recorded during the initial oxidation period.     

High temperature oxidation of CoMn alloys

The oxidation of cobalt-manganese alloys in the range 0–45 wt%Mn corresponding to the stability of the α-phase of cobalt has been studied in the range 750–950°C as an example of binary alloys producing solid solution scales. The alloys oxidize according to a parabolic rate law with a rate constant intermediate between those of the pure metals. The scales were formed mainly by a CoOMnO solid solution with a spinel phase present either as dispersed particles or a continuous layer in the cubic oxide, particularly for the Mn-rich alloys: some degree of internal oxidation has also been observed. The scale properties show a time dependence at constant temperature which is unusual in these systems and is probably related to the presence of the spinel phase. The internal oxidation and the structure and composition of the external scale are discussed with reference to the phase diagram for this system and to the recent theories for the growth of solid solution scales.     

The effect of internal stable nitride and oxide dispersion on the high temperature oxidation of FeCr alloys

The behaviour of FeCr alloys containing either a dispersed nitride or oxide phase is examined. Fe-12, 14 or 18% Cr containing 0.5 or 1 wt% Ti were used. The oxide dispersion was introduced by an internal oxidation treatment using a 50/50 Cr/Cr₂O₃ mixture in a sealed quartz capsule at 1100 or 1200°C. Internal nitrides were produced by nitridation in either an 80/20 or a 90/10 H₂/N₂ mixture at 1150°C. Subsequent oxidation tests of the treated alloys were carried out isothermally at 900, 1000 and 1100°C, or by thermal cycling (3 h cycles) at 1100°C.The dispersed nitride phase is not as effective in improving the oxidation resistance as corresponding dispersed oxides. Considerable particle coarsening is observed and the nitrides tend to dissociate and are converted to oxide near to the alloy/scale interface.     

A thermometric study of the dissolution of some CuZn alloys in acid solutions

The dissolution of Cu and Zn, and of three CuZn alloys in solutions of different concentrations of HNO₃ was studied by the thermometric technique. The variation of ΔT (i.e. T_m ? T_i) and the reaction number (R.N.) with the concentration of the attacking acid followed the relations:

$\Delta T=A_1(C?C_{\mathit{0}})$

and

$R.N.=A_2{\mathit{C}}_n,$

successively. Based on the values of the constants A₁, C_o, A₂ and n, the rate of dissolution of the different materials was found to increase in the order: Cu < Alloy I (70.5%Cu) < Alloy III (58.00%Cu) < Alloy II (62.00%Cu) < Zn.A break was noted in the thermometric curves of Alloy I. This was due to the primary attack on the Zn-component of the alloy, and was confirmed by studying the effect of the initial temperature, T_i, and the area/volume ratio on the shape of the curves, as well as by chemically analysing the corroding solution for both Cu²⁺ and Zn²⁺.The effect of the concentration of HCl, H₂SO₄, H₃PO₄ and of their salts on the reaction number of the alloys in 4N HNO₃ was examined in detail. The adsorption of the foreign acid anion on the metal surface retarded dissolution. Inhibition increased in the succession: Cl^? < HSO₄^? < H₃PO₄^?.The alloys were not affected by cold 4N HCl, H₂SO₄ or H₃PO₄. Attack could be initiated in the case of the first two acids through the addition of KNO₃. The thermometric curves were characterized by long incubation periods, by sharp temperature maxima and by the rapid decrease of temperature thereafter. This supported the conclusion that attack was of the pitting rather than of the general type. Because of stronger anion adsorption, attack in 4N H₂SO₄ was much lower than in 4N HCl. No attack was recorded in 4N H₃PO₄.Parallelism between corrosion assessment by the thermometric technique and by the weight loss method was established.     

The mechanism of high-temperature oxidation of directionally solidified Ni3AlNi3Nb eutectic

The isothermal oxidation behaviour of directionally solidified Ni₃AlNi₃Nb eutectic in 1 atm flowing air at high temperature has been examined. The alloy oxidizes relatively rapidly and develops an NiO external scale which thickens with time. At the same time, internal oxidation takes place, penetrating deeper into the Ni₃Nb lamellae than the Ni₃Al lamellae. Eventually, the alloy at the internal oxide/alloy interface becomes sufficiently depleted in aluminium and niobium that a single phase, nickel-rich alloy zone develops. Further internal oxide no longer has a direct relationship with the alloy lamellar structure. After long periods, sufficient aluminium and niobium diffuse to the internal oxide/alloy interface to enable complete, healing Al₂O₃ layers to develop. These layers retard further penetration of internal oxide into the alloy although they do eventually break down and allow this to occur. This oxidation behaviour is found to be reasonably independent of lamellar orientation.     

Corrosion behaviour of the extruded and unidirectionally solidified AlAl4Ca eutectic alloy in a neutral sodium chloride solution

The corrosion behaviour of extruded and unidirectionally solidified AlAl₄Ca eutectic alloy in a neutral sodium chloride solution has been compared with the behaviour of commercially pure Al and AlCu alloy. From both anodic and cathodic cyclic linear potentiodynamic measurements and from S.E.M. observations it was possible to point out that the corrosion behaviour of unidirectionally solidified alloy is substantially equal to that of the extruded alloy, slightly different from that of commercially pure Al and better than AlCu alloy. Therefore, the use of this new lamellae-reinforced AlAl₄Ca composite is promising, even though from the corrosion point of view further studies involving different techniques and conditions (solutions, temperature, surface state, stress, etc.) should be undertaken.     

The initiation of stress corrosion cracks and pits in austenitic CrNi steel in MgCl2 solutions at 40–90°C

The processes of pit and crack initiation have been studied for 18Cr11Ni type steel in 5 and 35%MgCl₂ solutions at 40, 60 and 90°C using both unstressed test specimens or specimens submitted to uniaxial tensile stress (constant loads equal to 50, 64 and 80% of the tensile strength measured at the test temperature).The results show proportionality between the logarithm of induction time of pitting on both stressed and unstressed specimens and the logarithm of induction time of stress corrosion cracking. In a majority of cases (in 5%MgCl₂ at 60–90°C and in 35%MgCl₂ at 40–90°C) the induction time of stress corrosion cracking proved to be longer than that of pitting, but in 35%MgCl₂ at 90°C the opposite was observed. In the last case the stress corrosion cracks originated from a smooth metal surface, whereas in all other cases cracks started from pits. A possible mechanism for these processes is given.     

The effect of pressure on the reaction between Fe(s)Zn(l) at 501 and 554°C

The effect of pressure on the nature and kinetics of the reactions between Fe_(s)Zn_(l) at 501 and 554°C have been investigated.It was found that pressure stabilized the δ₁ and ζ phases. At 501°C pressure causes a change in the reaction kinetics from linear to parabolic due to the coherence of first, the δ₁ phases and then the ζ phase. At 554°C pressure increases the rate of dissolution of iron but does not alter the type of reaction kinetics.The composition ranges of the different alloy layers are given for different pressures and the mechanisms of the reactions are discussed.     

Solidification microstructure of laser remelted Al2O3ZrO2 eutectic

Surface resolidification experiments using a high power CO₂-laser have been performed on an Al₂O₃ZrO₂ containing 36.8 at.% ZrO₂ eutectic alloy at beam velocities between 0.3 and 8 mm·s⁻¹. The local growth rate has been measured by observation of the orientation of the microstructure using scanning electron microscopy. In the whole range of velocities, the structure is essentially a regular lamellar eutectic and the value of the growth productλ²V was found to be ≈ 9.6·10⁻¹⁷ m³·s⁻¹. The measured eutectic spacings were compared with Jackson and Hunt model. Using thermophysical properties from the literature, the measured spacings were more than four times larger than the calculated ones. Assuming all parameters of the growth relationship except the diffusion coefficient to be of the right order of magnitude or to have a negligible influence, agreement is found when using a larger liquid diffusion coefficient,D_L≈5·10⁻¹⁰m²·s⁻¹.     

Motion of 1 0 0-tilt grain boundaries

The motion of planar symmetrical 1 0 0-tilt boundaries in high-purity aluminum was investigated in situ under the influence of an external mechanical stress field. It was observed that the motion of planar 1 0 0-tilt boundaries could be activated by an applied shear stress which acts perpendicular to the grain boundary plane. The motion of the grain boundaries is thermally activated so that we could derive the activation parameters for the motion from the in situ data of the mobility at different temperatures. A sharp transition between low angle grain boundaries and high angle grain boundaries was observed for the planar tilt boundaries at 8.6° which was apparent from a step change in the activation enthalpy for the stress induced grain boundary motion. The observed activation parameters for the stress-induced motion can be correlated to the activation parameters of self-diffusion for the motion of low angle boundaries and to the activation parameters of grain boundary diffusion for the motion of high angle grain boundaries. From these correlations, it seems obvious that the motion of planar low and high angle grain boundaries under the influence of a mechanical stress field can be attributed to the movement of the grain boundary dislocations which comprise the structure of the boundary.     

Determination of the crystal structure of the phase in the Fe–Al system by high-temperature neutron diffraction

The crystal structure of the high-temperature phase of the Fe–Al system has been determined by in-situ high-temperature neutron diffraction for the first time and the crystallographic parameters have been established by Rietveld refinement of the data. The phase was found to have a body-centered cubic structure of the Hume-Rothery Cu₅Zn₈-type (space group I3m (No. 217), Z = 4, Pearson symbol cI52, Strukturbericht designation D8₂). Accordingly, the ε phase has the formula Fe₅Al₈. Its lattice parameter is a = 8.9757(2) Å at 1120 °C, which is 3.02 times that of cubic FeAl (B2) at the same temperature. All fractional atomic coordinates, site occupation factors and interatomic distances were determined and it was examined how this phase can meet the rules for Hume-Rothery-type phases.     

Complex -phases in the Al–Pd-transition–metal systems: Towards a combination of an electrical conductor with a thermal insulator

-Phases in the Al–Pd–(Mn,Fe,Co,Rh,…) alloy systems form in wide compositional ranges and belong to the interesting class of complex intermetallics that are characterized by giant unit cells with quasicrystals-like cluster substructure. In order to see how the exceptional structural complexity and the coexistence of two competing physical length scales affect the physical properties of the material, we performed investigation of the magnetic, electrical, thermal transport and thermoelectric properties of the -phases in the Al–Pd–Fe, Al–Pd–Co and Al–Pd–Rh systems. Magnetic measurements reveal that the materials are diamagnetic, containing tiny fractions of magnetic transition–metal atoms (of the order 10–100 ppm). Electrical resistivity is moderate, of the order 10² μΩ cm, and shows weak temperature dependence (in most cases of a few %) in the investigated temperature range 4–300 K. An interesting feature of the -phases is their low thermal conductivity, which is at room temperature comparable to that of thermal insulators amorphous SiO₂ and Zr/YO₂ ceramics. While SiO₂ and Zr/YO₂ are also electrical insulators, -phases exhibit electrical conductivity typical of metallic alloys, so that they offer an interesting combination of an electrical conductor with a thermal insulator. The reason for the weak thermal conductivity of the -phases appears to be structural: large and heavy atomic clusters of icosahedral symmetry in the giant unit cell prevent the propagation of extended phonons, so that the lattice can no more efficiently participate in the heat transport. The thermoelectric power of the investigated -phase families is small, so that these materials do not appear promising candidates for the thermoelectric application.     

Crystal, electric transport properties and electronic structures of the Ti5Me1−xSb2+x series of compounds (MeCr, Mn, Fe, Co, Ni, Cu)

The Ti₅Me_1−xSb_2+x compounds where MeCr, Mn, Fe, Co, Ni, Cu, were synthesized and their crystal structure was determined (W₅Si₃ structure type, space group I4/mcm). The transport properties were investigated by means of electrical resistivity and Seebeck coefficient measurements in the temperature range 80–380 K. All the investigated compounds exhibit metallic-like type of conductivity confirmed by electronic structure calculations based on Density Functional Theory.     

Mechanochemical synthesis of activated Me–BN (MeAl, Mg, Ti) nanocomposites

Mechanochemical synthesis in mixtures of light metals (Al, Mg, Ti) with hexagonal boron nitride under ball milling was studied. Well-pronounced stage behavior of the synthesis process was established and described in terms of dose of mechanical energy supplied to the material under milling (energy approach). At the initial stage the porous composite with nanometer-sized metal crystallites and high specific surface forms. The composite exhibits high reactivity with respect to external reactants, such as atmosphere and water. Mechanical activation also strongly facilitates the solid-state reaction between components, decreasing the temperature required for its initiation down to 450 °C. Solid-state reaction under milling takes place only after prolonged milling and results in formation of poorly crystallized metal borides and nitrides.