Wettability of SiC by aluminium and Al-Si alloys

The variations with time of the contact angle formed by molten pure aluminium or Al-Si alloys with single crystalline SiC were measured by the sessile drop method in a vacuum of 10^–4 to 10^–5 Pa at temperatures ranging from 933 to 1200 K. In the Al/SiC system, a non-wetting-wetting transition was observed at a temperature that decreases as time increases. After holding times of about 2 h, contact angles were stabilized to acute angles even at the aluminium melting point. Although additions of silicon to aluminium were in such amounts as to prevent Al₄C₃ formation at the interface, wettability in both pure Al/SiC and Al-Si alloys/SiC systems was not observed to differ appreciably.     

Wettability and phase formation in TiC/Al-alloys assemblies

The effect of alloying elements on the wetting behavior of TiC substrates by commercial aluminum alloys (1010, 2024, 6061, 7075) and its relation to phase formation at the metal–ceramic interface was investigated at 900 °C using a sessile drop technique. It was found that wetting behavior in Al-alloys/TiC is typical of reactive systems, furthermore, wettability of TiC by pure Al-1010 was better than the alloys. Interface examination revealed the formation of Al₄C₃ in all the cases; the thickness of the reaction layer varied within the samples and was discontinuous in nature, particularly for the 7075/TiC and 6061/TiC systems, which exhibited poor wetting. The formation of alloyed phases in the ceramic surface decreased the amount of the undesirable Al₄C₃ at the metal/ceramic interface.     

Wettability of graphite by liquid aluminum under molten potassium halide fluxes

This study is a continuation of our previous work (Baumli et al. J Mater Sci 45:5177–5190, 2010) in which the wettability of graphite by liquid aluminum was studied under different molten chlorides containing K₂TiF₆ as function of the cation of the molten chloride. In the present paper, the same was studied as function of the anion in different potassium halides (fluxes). The fluxes consisted of different potassium halides (KX) + 10 wt% potassium hexafluoro-titanate (K₂TiF₆). The potassium halides studied were potassium iodide (KI), potassium chloride (KCl), and their equimolar mixture. For perfect wettability of graphite by liquid Al under different molten KX-10 wt% K₂TiF₆ systems at 800 °C, certain critical values of the salt:Al mass ratio should be achieved, the value of which increases from KI toward KCl. Comparing the present results with the results of our previous study, we found that the critical value of the salt:Al mass ratio increases with the cation size and decreases with the anion size of the salt.     

Corrosion protection of aluminum alloys by inhibiting pigments

We show that calcium monohydrophosphate can serve as an efficient inhibiting component of anticorrosive coatings applied on an aluminum-copper alloy. It has been established that the charge-transfer resistance of D16T alloy under the influence of CaHPO₄ solution approaches the value characteristic of a medium with strontium chromate. Calcium monohydrophosphate simulates the behavior of strontium chromate inhibitor, retarding the cathodic and anodic corrosion reactions on the surface of aluminum alloys. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 5, pp. 7–11, September–October, 2006.     

Wettability of transition metal boride eutectic alloys to graphite

The wetting of nickel, platinum and palladium boride eutectics (Ni₅₅B₄₅, Ni₆₂B₃₈, Pd₇₂B₂₈, and Pt₇₂B₂₈) to polycrystalline graphite has been studied by sessile drop and surface analytical techniques. The wettability is critically influenced by surface segregation of low-level bulk impurities in the alloys. At the melting temperature, the impurities segregate to the surface of the liquid alloy and inhibit wetting. A poorly-wetted droplet with a large contact angle results. Good wetting results by introducing materials into the contact system which possess high chemical reactivity to the major segregant species, either as a surface coating or by incorporation into the alloy during manufacture. The materials act to suppress segregation by tying up the alloy impurities through compound formation. The results indicate that the use of surface sensitive techniques in the study of wetting is essential.     

Mo添加对Ni3Al-TiC润湿特性的影响机制研究

采用2AP-LEITZ高温显微镜对TiC-Ni₃Al的润湿接触角进行了实验测定,着重探讨了Ni₃Al对TiC的润湿特性以及Ni₃Al中添加少量Mo的影响机制.结果表明,TiC与Ni₃Al之间具有良好的润湿性能.5wt％Mo的添加使Ni₃Al向TiC基板浸渗的深度增大,并与TiC颗粒发生固溶反应置换出部分Ti,在其周围形成一个含Mo的壳层.在这个壳层里,Ti、Mo进一步与Ni₃Al固溶,这些反应降低了液-固表面张力,导致了润湿接触角的下降,从而改善了TiC-Ni₃Al的润湿性.     

Fabrication of in situ TiC reinforced aluminum matrix composites

In the present work, the room and elevated temperature mechanical behavior of Al/TiC, high-strength Al-Si/TiC and the elevated temperature-resistant Al-Fe(-V-Si)/TiC composites has been evaluated. The microstructural characteristics of ingot metallurgy (IM) or rapid solidification (RS) Al-Si/TiC and Al-Fe(-V-Si)/TiC composites could be thought of as a combination of the related alloy matrix microstructures and the IM or RS Al/TiC composites. The IM Al/TiC and the Al-Si/TiC composites show superior strength and ductility to the relevant aluminum based composites.The RS Al/TiC and the Al-Fe-V-Si/TiC exhibit high Young's moduli and substantial improvements in room and elevated temperature tensile properties compared to those of rapidly solidified alloys and conventional composites.The Young's modulus values of RS Al/TiC and Al-Fe-V-Si/TiC composites are well within Hashin-Shtrikman limits in keeping with the strong interfacial bonding. In the micromechanics approach, the principal strengthening mechanisms for the present dispersed particle-hardened RS in situ Al-TiC composites would include Orowan strengthening, grain-size and substructure strengthening, and solid-solution strengthening. The RS technique was used in the present work to maximize strength and ductility for a particular volume fraction, and influence the degree of flexibility available to meet these requirements: a fine, uniform particle size distribution; a high interfacial strength; control of particle shape; and a ductile matrix.     

Selecting aluminum alloys to resist failure by fracture mechanisms

Useful information and guidelines are presented for engineers who wish to minimize fracture problems in aluminum engineering structures through better application of materials knowledge and optimum alloy choice. Consideration is given to methods and examples of characterizing fracture resistance of aluminum alloys including interpretations of data addressing basic design approaches. Specific attention is given to trade-offs among properties of strength, fracture toughness, corrosion, stress corrosion cracking, and fatigue in structural aluminum materials.     

Wettability and spreading kinetics of molten aluminum on copper-coated ceramics

The purpose of this study was to investigate the influence of Cu-coating on the spreading kinetics and equilibrium contact angles of aluminum on ceramics using a sessile drop technique. Al₂O₃ and SiC plates were coated by electroless plating. The copper film overcomes the low wetting of the uncoated samples by dissolution in the drop at 800 °C in argon, showing an intrinsically favorable effect on the adhesion energy. Just after 2 min, the contact angle decreased to 12.6° and 26°for Al/Cu–Al₂O₃ and Al/Cu–SiC, respectively. However, a de-wetting behavior was observed, reaching equilibrium contact angles of 58.3° and 45.5° for the couples. The dissolution reaction rate at the triple junction was so high that the spreading process was controlled by local diffusion rather than chemical reaction kinetics.     

Wettability of AlN with different roughness, porosity and oxidation state by commercial Ag-Cu-Ti brazes

The effects of porosity (about 27%), of roughness (Ra = 0.17–0.20 μ m versus Ra = 0.02–0.03 μm) and of pre-oxidation (air, 1250^°C, 30 min) on wettability and contact interaction of AlN with commercial brazing alloys of Ag-Cu-Ti composition were studied. Wettability was determined by the sessile drop method. The interface interaction was identified by SEM and microprobe analysis. Experimental data for porous, pre-oxidized and rough samples are compared with data for dense samples polished to Ra = 0.02–0.03 μ m not subjected to pre-oxidation. The results show that for these systems surface roughness does not influence the contact angle value significantly. Pre-oxidation of the AlN, however, tends to reduce wettability as a result of the replacement of braze/AlN interaction by braze/surface aluminium oxide interaction. Contact angles for porous samples are higher by about 20–30° than for dense samples.     

In situ synthesis of Al–TiC in aluminum melt

In situ technology for the preparation of Al–TiC composite by self-propagating high-temperature synthesis is considered in this paper. It involves the synthesis of the reinforcement phase TiC from elemental powders directly in aluminum melt. Based on thermodynamic calculations, the composition and molar ratio of powders were chosen for the experiments. The effects of initial melt temperature and fluxes on the character of SHS reaction and TiC recovery were studied. The synthesis was shown to result in the formation of Al and TiC phases. The composites Al–TiC with the uncontaminated macrofracture, with the best TiC recovery and with the smallest TiC particles, can be produced at 1000 °C with fluxes.     

Structure of rapidly solidified aluminum alloys

This paper is a summary of an extensive research program carried out by the authors on the structure of rapidly solidified aluminum alloys; and a comparison with the work of others also involved in this field. The paper discusses the changes in the dendritic and non-dendritic structure of the matrix at cooling rates from 10^–3 to 10¹⁰ K/s and discusses the hetergeneity of the structure caused by interdendritic-segretion during solidification.     

Hydrogen-enhanced stress-corrosion cracking of aluminum alloys

We present the results of the investigation of Al-Zn-Mg and Al-Zn-Cu-Mg alloys in NaCl solutions at low strain rates. The contribution of hydrogen to the process of stress-corrosion cracking is analyzed by taking into account the influence of the admixtures of arsenic trioxide and residual hydrogen (remaining after the processes of release and cathodic polarization) on the susceptibility of metals to this kind of cracking. A mechanism of hydrogen-assisted stress-corrosion cracking taking into account the time dependence of the microstructure of grain boundaries, concentration of hydrogen, and its distribution is suggested on the basis of the concept of critical concentration of hydrogen. Technical University of Gdansk, Poland. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 4, pp. 20–26, July–August, 1998.     

Characterization of close-celled cellular aluminum alloys

The deformation behaviour of two different types of aluminium alloy foam are studied under tension, compression, shear and hydrostatic pressure. Foams having closed cells are processed via batch casting, whereas foams with semi-open cells are processed by negative pressure infiltration. The influence of relative foam density, cell structure and cell orientation on the stiffness and strength of foams is studied; the deformation mechanisms are analysed by using video imaging and SEM (scanning electronic microscope). The measured dependence of stiffness and strength upon relative foam density are compared with analytical predictions. The measured stress versus strain curves along different loading paths are compared with predictions from a phenomenological constitutive model. It is found that the deformations of both types of foams are dominated by cell wall bending, attributed to various process induced imperfections in the cellualr structure. The closed cell foam is found to be isotropic, whereas the semi-open cell foam shows strong anisotropy.