Off-eutectic composite solidification and properties in Al-Ni and Al-Co alloys

To investigate the range of coupled eutectic growth in Al-Co alloys from 1 to 4 wt% Co and Al-Ni alloys from 5 to 10 wt% Ni directional solidification, using rates from 0.8×10^–2 cm/sec to 10.6×10^–2 cm/sec, was employed. Both alloy systems exhibited coupled zones skewed towards the hypereutectic compositions. Fully eutectic structures were obtained in the ranges Al-1 wt% Co to Al-3 wt% Co and Al-5.7 wt% Ni to Al-9.2 wt% Ni.The off-eutectic alloys which exhibit a fully eutectic structure behave as reinforcing composite materials, with the tensile strength and microhardness increasing as the volume fraction of the strengthening phase increases.     

Stress corrosion cracking mechanisms in ductile FCC materials

Recent results on the SCC behaviour of ductile fcc materials are reviewed. Critical experiments are presented to test the corrosion enhanced plasticity model proposed some years ago by one of the present authors to describe the SCC of austenitic stainless steels in Cl^- solutions. Slow strain rate tests on 110 and 100 316L alloy single crystals clearly confirm that the macroscopically brittle fracture is in fact achieved by microcracking on {111} microfacets in zig-zag. Moreover the corrosion deformation interactions on which the model is based are quantitatively analysed through softening effects observed in cyclic plastic deformation in the corrosive solution. The conditions for hydrogen entry in the material are described, which leads to the notion of critical surface defects for hydrogen effects. New developments of the model are then discussed and a numerical simulation of the corrosion deformation interactions is presented.Presented at Fourth Greek National Congress on Mechanics, 26–29 June 1995, held at Xanthi, Greece.     

Thermal analysis of copper-tin alloys during rapid solidification

A series of solidification experiments using a mirror furnace and a levitation technique were performed on different Cu-Sn alloys. Cooling curves during solidification were registered using a thermocouple of type K connected to a data acquisition system. The undercooling, cooling rates of the liquid and of the solid state, solidification times and temperatures were evaluated from the curves. The samples were found to solidify far below the liquidus temperature. The cooling curves for different samples and alloys were simulated using a FEM solidification program. The heat transfer coefficient, heat of fusion and specific heat were evaluated. It was found that the calculated values of the heat of fusion were much lower than the tabulated ones. The calculated values of the specific heat in the solid state were also found to be much higher than those quoted in the literature, especially for the mirror furnace experiments. The effect of rapid cooling on the thermodynamic state and the solidification process of the alloys has been evaluated. The effect of cooling rate on the formation and condensation of vacancies is discussed. It is proposed that a large number of vacancies form during rapid solidification and that they condense during and after the solidification. The influence of these defects on the thermodynamics and solidification of the alloys has been evaluated.     

Corrosion of nickel,iron, cobalt and their alloys in molten salt electrolytes

The processes of high-temperature corrosion, anodic dissolution and passivation of nickel, iron, cobalt and their alloys are reviewed to reveal the progress in understanding the reaction mechanisms defined in the last two decades. In the first part, the procedures of thermodynamical analysis of corrosion processes by potential — pO^2– diagrams are outlined. The second part is devoted to the electrochemical corrosion, anodic dissolution and passivation of the metals studied, the reaction mechanisms and composition of the corrosion layers formed. The effect of the alloying elements on the corrosion resistance and anodic behaviour of the base metal is treated in the third part. A brief summary of the kinetics of the so-called hot corrosion of the studied metals and their alloys in contact with thin molten salt films and aggressive atmospheres is then given. Finally, some conclusions are drawn and some future trends of investigation are indicated.     

Metastable microstructures in rapidly solidified zirconium alloys

Zirconium alloys exhibit a wide variety of phase transformations. The present review outlines some aspects of phase transformations, which are encountered in rapidly solidified zirconium alloys. The possibility of solidification of unalloyed zirconium directly into the low temperature phase is examined and the role that solidification microstructure plays in modifying the to martensitic transformation in rapidly solidified material is discussed. Zirconium alloys undergo two types of displacive transformations, namely, the martensitic transformation and the transformation. The influence of rapid solidification on the transition from the martensitic to the transformation is also discussed. Addition of transition metals are known to depress the melting point of zirconium alloys very drastically. As a consequence, glass forming abilities of a number of binary and ternary zirconium alloys are quite strong. A number of zirconium based metal-metal amorphous alloys have been synthesized using rapid solidification. In recent years, this work has been extended to bulk metallic glasses, which usually contain a larger number of alloying elements. Crystallization of these glasses and quasi-crystalline phase formation in these systems is also discussed.     

Corrosion of Aircraft Structures Made of Aluminum Alloys

We analyze corrosion damages of the airframe load-bearing unit, made of aluminum alloys, of Antonov airplanes for 20–40 year of operation. We describe some examples of corrosion manifestations, its kinds, and ways of removal. The applicability of a series of aluminum alloys in Antonov airplanes is considered with regard for the influence of corrosion factor.     

The influence of Cu and Mg additions on the decomposition of Al-(40 to 50) wt% Zn alloys during isothermal and continuous heating

The influence of 1 wt% copper and 0.2wt% magnesium additions on the heat evolution and structural changes of Al-40 and Al-50wt% Zn alloys during isothermal ageing at 30° C and continuous heating up to 400° C was studied using microcalorimetry, differential scanning calorimetry and transmission electron microscopy. Isothermal studies indicate rapid formation of Guinier-Preston (GP) zones at initial stages of ageing followed by discontinuous precipitation of-phase competing with the formation of transition phases also containing third elements. DSC curves show that the amount of metastable phases formed during room temperature ageing increases with ternary additions, especially to Al-50% Zn alloys. Up to 75° C, GP zones undergo composition but no size changes: during further continuous heating,' (Mg₂Zn) appears, the dissolution of which proceeds parallel to the continuous precipitation of at medium temperatures. Addition of copper reduces the discontinuous-precipitation to a lesser extent than magnesium.     

Bone formation induced by calcium phosphate ceramics in soft tissue of dogs: a comparative study between porous α-TCP and β-TCP

Two kinds of tri-calcium phosphate ceramics (Ca/P = 1.50), -TCP and -TCP, which has the same macrostructure and microstructure, but different phase composition, were implanted in dorsal muscles of dogs. The samples were retrieved at 30, 45 and 150 days, respectively, after implantation, and were analyzed histologically. There were critically different tissue responses between -TCP ceramic and -TCP ceramic. Higher cell populations were observed inside the pores of -TCP than those of -TCP, bone tissue was found in -TCP at 45 and 150 days, but no bone formation could be detected in any -TCP implants in this study. On the other hand, the bone tissue in -TCP seemed to degenerate at 150 days. The results indicate that porous -TCP can induce bone formation in soft tissues of dogs; while the rapid dissolution of the ceramic and the higher local Ca²⁺, PO ₄ ^3- concentration due to the rapid dissolution of -TCP may resist bone formation in -TCP and the less rapid dissolution of -TCP may be detrimental to already formed bone in -TCP.     

Observations on the formation of β-Al5FeSi phase in 319 type Al-Si alloys

In Al-Si alloys, the properties are influenced by the shape and distribution of the eutectic silicon particles in the matrix, as also by the iron intermetallics and copper phases that occur upon solidification. The -Al₅FeSi iron intermetallic phase, in particular, is known for its detrimental effect on the properties, and is controlled variously by the iron content and the melt/solidification conditions of the alloy. The formation of the iron intermetallics has been observed in commercial 319 alloy end-chilled castings, obtained from non-treated and treated melts, where the effects of cooling rate, strontium modification and grain refinement have been studied. The volume fraction of -phase formed was seen to increase with the decrease in cooling rate (i.e. with increasing distance from the chill) in the untreated alloy. Sympathetic (preferential) nucleation of the -iron needles was also observed, in which the branching of -needles from a parent needle resulted in the formation of large -needle entities that can cover distances of 1200 m across the matrix surface. The beneficial effect of modification, i.e. strontium addition to the melt, was manifested through its influence on the fragmentation and dissolution of the -needles. The strontium poisons the sites where sympathetic nucleation takes place. Dissolution was accelerated with increasing strontium content, the optimum level being 300 p.p.m. Grain refining, on the other hand, negated the beneficial effect of modification, in that the -needles underwent thickening and the sympathetic nucleation/branching also occurred. The modified alloy was found to possess the lowest volume fraction of -Al₅FeSi phase among the unmodified, modified, grainrefined, and modified/grain-refined alloys.     

Mechanisms of uniform corrosion under corrosion deposits

The liquid-phase transport phenomena which occur at the surface of iron-base alloys during corrosion have been analysed. These mechanisms determine either the maintenance of bare metal or the precipitation of solid corrosion products, the build-up of a corrosion deposit and the control of its thickness, and finally, the kinetics of the electrochemical reactions under the deposit. Although it is shown that pure precipitation-redissolution or direct formation reactions are impossible, the only conceivable mechanisms are nevertheless closely related, because the transport of iron between the metal and the external corrosive medium occurs chiefly either via the solid phase of the deposit (for soluble deposits), or via the liquid phase permeating its porosities (for insoluble deposits). It is also shown that, depending on the precipitation conditions, any given solid compound Fe _n X₂ can lead to three types of deposit with quite different properties. (i) Soluble deposits: moderately protective, steady-state corrosion insensitive to potential, but highly sensitive to turbulence; (ii) Insoluble cationic deposits (controlled by the removal of Fe²⁺ cations by liquid-phase diffusion): highly protective, corrosion rate slightly sensitive to potential, and insensitive to turbulence. (iii) Insoluble anionic deposits (controlled by the diffusional supply of the precipitatable anion X ^n– : slightly or unprotective, corrosion slight or insensitive to the presence of the deposit; possibly profuse deposit if steady state corrosion is not attained. This theoretical analysis can retrospectively explain numerous experimental observations reported in the literature, such as the incubation time before the drop in corrosion rates, the multiple forms of CO₂ and H₂S corrosion, the role of Ca²⁺ ions, erosion-corrosion and bacterial corrosion. This analysis also paves the way for the reliable laboratory prediction of real corrosion rates under deposits.     

Effect of alloying elements and dendrite arm spacing on the microstructure and hardness of an Al-Si-Cu-Mg-Fe-Mn (380) aluminium die-casting alloy

The microstructure of aluminium alloys based on 380 die-casting alloy was studied in detail, as a function of the alloying elements iron, magnesium, copper and manganese, and the solidification rate. Three methods of solidification were employed to simulate cooling rates obtained from investment, permanent, and die-casting processes, corresponding to 0.4, 12 and 260 °C s^–1, respectively, with emphasis on the highest cooling rate. Hardness measurements were carried out on samples obtained from the latter, in the as-cast and T5 tempered conditions (4 h at 25, 155, 180, 200 and 220 °C). The results have been discussed and the correlation between the hardness and microstructure as a function of alloying elements is presented. The effect of solution heat treatment on the variations in the microstructure and hardness has also been discussed.     

Production of Ni-Pd-Si and Ni-Pd-P amorphous wires and their mechanical and corrosion properties

Wire-shaped nickel-based amorphous alloys exhibiting high strength and good ductility combined with a high corrosion resistance were produced for Ni-Pd-Si and Ni-Pd-P alloys by melt spinning in rotating water. The amorphous wires were formed over a relatively wide range from 29 to 82 at % palladium for (Ni-Pd)₈₂Si₁₈ alloys and from 12 to 52 at % palladium for (Ni-Pd)₈₀P₂₀ alloys. The Ni-Pd-Metalloid amorphous wires had a circular cross-section and smooth surface, and their diameters were 80 to 150m. With increasing nickel content, their tensile strength, _f, increased from 1340 to 1710 MPa and the elongation to fracture, _f, decreased slightly from 2.2% to 1.9%. Cold-drawing the wires was an easy technique to reduce their diameter and to increase _f and _f up to an appropriate value of reduction in diameter. In addition, it is also effective in smoothing the wire surface. Their corrosion resistance was assumed to be sufficiently high since their polarization behaviour in 1 N H₂SO₄ solution was similar to palladium metal. Cold-drawing did not enhance corrosion and rather decreased apparently the active dissolution current density of some alloys owing to smoothing of the surface.     

The structure of the γ′ extended solid solution in a splat-cooled Ag-50 at.% Cu alloy

The structure of the extended solid solution in electron transparent areas of a splatcooled Ag-50 at.% Cu alloys was examined by transmission electron microscopy. This phase was usually found to be spinodally decomposed at large grain sizes (–1m in diameter), in contrast to X-ray diffraction data indicating that the solid solution was undecomposed. A solidification model for rapidly quenched eutectic alloys is proposed to account for the observed structure of the splat-cooled alloy. A transformation curve for the spinodal decomposition of is also calculated and related to predictions derived from the solidification model.     

Investigation of energy-gap anisotropy in superconducting aluminum,cadmium, and zinc

Measurements of the low-temperature specific heat of the pure metal and dilute nonmagnetic alloys of Al, Cd, and Zn are described. The washing out of the energy gap anisotropy with increasing impurity as predicted by Anderson was observed in aluminum-magnesium alloys, and the mean-squared anisotropy parameter a ² was measured to be a ²0.010. Values deduced from measurements on Zn and Cd and their corresponding alloys with 1₀ gave a ²0.02 for Zn and a ²0.05 for Cd. Values of the gaps determined by fitting to a two-gap model are given and compared to those deduced from previous work.     

Microstructural characterization of as-cast high-nitrogen Fe-15Cr-15Ni alloys

A base composition of Fe-15Cr-15Ni was melted in a hot-isostatic-pressure furnace under nitrogen pressures from 0.1–150 MPa (1–1500 atm) to produce materials with nitrogen concentrations from 0.04–3.6 wt%. Microstructural characterization of the as-cast materials was completed using optical microscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy techniques. The phases present in the materials were austenite, nitrides (Cr₂N and CrN), and martensite. All of the materials solidified with primary austenite dendrites. Dendritic solidification of CrN occurred in interdendritic regions of the austenite in alloys containing between 0.6 and 1.9 wt % N. Cellular precipitation of Cr₂N occurred in alloys with intermediate nitrogen concentrations (0.6 and 1.0 wt %). The orientation relationship between the Cr₂N and austenite, expressed as {1 1 1}y{0 0 0 1}Cr₂N and 1 1 0 –1 1 0 0Cr₂N, was confirmed. Precipitates of CrN with both lamellar and disc morphologies formed in the austenite. Both had a cube-cube orientation relationship with the austenite. Only about 10% of the 3.6 wt% N material solidified as primary austenite. The remainder of the material solidified as a eutectic according to the reaction L +CrN. All of the austenite in the eutectic region subsequently transformed to martensite. Increasing volume fractions of martensite formed with increasing bulk nitrogen concentrations in the materials due to alloy depletion of austenite caused by nitride formation.     

The abrasive corrosion behavior of plasma-nitrided alloy steels in chloride environments

Both corrosion and abrasive corrosion behavior of plama-nitrided type 304 and 410 stainless steels and 4140 low alloy steel were investigated in 3% NaCl solution (pH = 6.8) by electrochemical corrosion measurements. Surface morphology and alloying elements after corrosion and abrasion corrosion tests were examined by scanning electron microscopy and energy dispersive analysis of X-rays. The results indicated that the plasma-nitrided SAE 4140 steel containing -(Fe,Cr)_{2 – 3}N and -(Fe,Cr)₄N surface nitrides which produce a thick and dense protective layer exhibited a significant decrease of corrosion currents by inhibition of the anodic dissolution of iron, whereas the plasma-nitrided type 304 and 410 stainless steels containing the segregation of chromium nitride CrN exhibited a extensive pitting corrosion by acceleration of the anodic dissolution of iron. It is concluded that the susceptibility to pitting is consistent with the degree of chromium segregation, and decreases as follows: 304 stainless steel > 410 stainless steel > 4140 steel. Also, the results of abrasive corrosion testing for the plasma-nitrided alloys are strongly related to the subtleties of the nitrided microstructures resulting in a pitting and spalling type of abrasive corrosion of type 304 and 410 stainless steels, and excellent abrasive corrosion resistance for SAE 4140 steel.     

Titanium for aerospace: Rationale and applications

Titanium and titanium alloys are excellent candidates for aerospace applications due to their high strength to weight ratio and excellent corrosion resistance; titanium and its alloys are immune to almost every medium to which they would be exposed in an aerospace environment. Titanium usage is, however, strongly limited by its higher cost relative to competing materials, primarily aluminum alloys and steels. Hence the advantages to using titanium must be balanced against its added cost. The titanium alloys used for aerospace applications, some of the characteristics of these alloys, the rationale for utilizing them, and some specific applications of different types of actual usage will be discussed herein.This is an extension of References 1 and 2, which reviewed applicaiions of alloys. These references will provide more details on applications of the alloys, while the and / alloys are added herein.     

Oxidation and corrosion behaviour of mild steel laser alloyed with nickel and chromium

Surface alloys were made on mild steel, coated with nickel and chromium using laser surface alloying. Mild steel was coated with a composite layer of nickel and chromium using the plasma technique. This was followed by laser irradiation using a continuous carbon dioxide laser. Oxidation and corrosion behaviour of these alloys was then determined by carrying out oxidation in air at 800 °C and corrosion tests at room temperatures in 1 n H₂SO₄. With a 75 m layer of nickel and chromium each, it was possible to make surface alloys on mild steel, which had a chromium concentration of 6–7 wt%, but the nickel concentration varied from 10–20 wt%. Oxidation behaviour improved significantly over the as-coated specimen and aqueous corrosion improved considerably.     

Transient liquid-phase sintering of ceramic-reinforced Fe-based composites

The microstructural development of ceramic-reinforced iron-based composites has been studied. The composites were fabricated via powder metallurgy and liquid-phase sintering, a processing route which achieves near-net-shape with good ceramic particulate dispersion. Two matrix alloys were used, Fe-1 wt% C-1 wt% Si and Fe-2 wt% Cu; up to 30 wt% (36 vol%) yttria-stabilized zirconia in the form of 20 m particles was added to these alloys. The microstructural evolution of these composite materials was studied by examining the densification rate and volume fraction of liquid phase as a function of time. Different particle/matrix interfaces developed in the two composites. A glassy silicon-rich layer formed in the Fe-1C-1Si-YSZ composites and a more limited crystalline layer was found in the Fe-2Cu-YSZ composites.     

Rapidly solidified titanium alloys for high-temperature applications

The application of rapid solidification technology (RST) to titanium alloy systems is relatively new and became the subject of active research since it was demonstrated that novel titanium alloys of higher temperature capability can be synthesized through new alloy design based on rapid solidification processing. The effects of rapid solidification on the occurrence of metastable phases, microstructures and mechanical properties in binary and ternary titanium alloys are reviewed. In particular, earlier results from RS-Ti alloy research have shown that many different novel dispersoids, some of which are coarsening-resistant at elevated temperatures (600 to 800° C), can be created in the matrix through RST. The alloys containing novel dispersoids also exhibit good creep resistance at elevated temperatures. Further studies on/- and-Ti alloys through RST, in conjunction with the development of various processing technologies for bulk alloy manufacturing, are clearly desirable.