SiC particle reinforced aluminum metal matrix composites (SiCp/Al MMCs) are finding increasing applications in aerospace, automotive and microelectronic industries because of their excellent combination of high specific strength, high specific toughness, low coefficient of expansion and excellent wear resistance. However the addition of totally dissimilar reinforcements into the aluminum matrix and the great different in electrochemical properties between the aluminum metal matrix and the SiC reinforcement deteriorate the corrosion resistance and make the SiCp/6061Al MMC prone to corrosion attacks especially when servicing under electrochemical corrosive environments.

     In this dissertation, laser surface melting, laser surface alloying with Ni-Cr-B alloy powders and laser cladding with Cu-Sn-Pb-Zn bronze powders were utilized to improve the corrosion resistance of P/M SiC particles reinforced metal matrix composite (P/M SiCp/6061Al MMC) by producing corrosion resistant surface layers or metallurgical coatings with rapidly solidified and effectively refined microstructure. The microstructure of the laser surface melted, laser surface alloyed and laser clad corrosion-resistant modified surface layers or coatings were characterized by OM, SEM, XRD as functions of laser surface processing parameters and contents of alloying elements. The hardness distribution along the laser surface modified coatings’ depth direction was tested using microhardness tester. The electrochemical corrosion resistance of laser surface melted, laser surface alloyed and the laser clad SiCp/6061Al MMC were evaluated under direct current anodic polarization electrochemical corrosion test conditions in NaCl water solutions and H₂SO₄ water solutions as functions of laser processing parameters and corrosion test conditions.

Results show that:

(1) During laser surface melting of SiCp/6061Al MMC with high power CO₂ laser beam, the SiC particles are completely dissolved and the laser surface melted layer has a rapidly solidified, homogeneous and refined microstructure consisting of aluminum silicon carbide Al₄SiC₄ aluminum carbide Al₄C₃ as well as crystalline silicon particles evenly distributed in the aluminum matrix. The microstructure as well as size and volume fraction of the newly produced phases in the laser surface melted layer on substrate of SiCp/6061Al MMC depend strongly on the laser processing parameters especially the laser linear energy density. Both the hardness and corrosion resistance of SiCp/6061Al MMC under direct current anodic polarization electrochemical corrosion test conditions in 0.1M NaCl water solutions are slightly enhanced after the laser surface melting treatment.

(2) Laser surface alloying of SiCp/6061Al MMC with Ni-Cr-B alloying powders leads to the production of rapidly solidified corrosion resistant surface layers having homogeneous and refined microstructure. The laser surface alloyed SiCp/6061Al MC is composed of fine rapidly solidified nickel aluminides of Al₃Ni_2, Al₃Ni and AlNi, and chromium borides of d-CrB and Cr₂B evenly distributed in the nickel solid solution. Microstructure of the laser surface alloyed layers is strongly dependent on the laser processing parameters and the chemical composition especially boron content of the Ni-Cr-B alloying powders. The laser surface alloyed SiCp/6061Al MMC has high hardness and excellent corrosion resistance to electrochemical corrosion under direct current anodic polarization electrochemical corrosion test conditions in NaCl water solutions.

(3) By laser cladding of SiCp/6061Al MMC with Cu-6Sn-6Pb-3Zn bronze powders a rapidly solidified homogeneous dendritic bronze coating is produced on substrate of SiCp/6061Al MMC with refined microstructure consisting of fine copper-base solid solution and the interdendritic Cu₃₁Sn₈ intermetallic particles_. The laser clad corrosion resistant coating is bonded to the substrate of SiCp/6061Al MMC, metallurgically. The corrosion resistance of SiCp/6061Al MMC to the electrochemical corrosion under direct current anodic polarization electrochemical test conditions in NaCl water solutions is significantly improved after laser cladding treatment with the bronze powders.

(4) Laser surface melting, laser surface alloying and laser cladding are demonstrated to be among the promising surface modification methods for enhancing corrosion resistance of SiCp/6061Al MMC.

Keywords:  SiCp/Al Metal Matrix Composites, Laser Surface Melting, Laser

Surface Alloying, Laser Cladding, Corrosion Resistance, Microstructure

Abstract

Reproductive tracts were collected from 870 ewes slaughtered at an abattoir in County Dublin over a 9-mo period from September 1993 to June 1994. Ovarian activity was evident in 59.3% of the tracts examined. The mean ovulation rate was 1.26 ± 0.045 (mean ± SEM). The spatial relationship between corpora lutea (CL) and dominant follicles was established. Ten percent of the ewes examined were pregnant. Embryonic transuterine migration was observed in 12% of the single ovulating ewes and in all ewes pregnant with twins which had double ovulations on one ovary. Based on the observation of tracts with contemporaneous double ovulations and a single fetus, the prevalence of embryonic loss was 17.2%. Overall, 9.4% of the tracts showed evidence of pathological change, and 5.5% of the tracts had abnormalities likely to impair fertility.

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We have studied the effects of the fine structure of energy levels on the dynamics of the atom and light field. A significant influence is exhibited in the collapse-revival behavior of atomic inversion, the light squeezing, the phase of the field, and the antibunching of a light beam. When the energy difference between the doublet is large (Δ≥20g, where g is the coupling constant), the dynamics of the atomic inversion becomes identical to that of the usual Jaynes-Cummings model, as expected. However, the fine-structure effects on the phase and the antibunching of the light field exist in a much larger range. In particular, for a strong field, the normally ordered variance of the photon-number operator exhibits complicated collapse and revival patterns and the light displays strong antibunching in each revival envelope. Numerical calculations also show that this model does not produce significant squeezing due to the influence of the fine structure. 

In our study of the interaction between few energy level atom and a coherent field, we considered the effect of the excited state's fine structure on the system dynamics. Significant influence exhibited on the collapse-revival behavior of atomic inversion and light squeezing. The numerical calculations show that for weak field, the effects of fine structure may cause disappearance of the collapse and revival phenomena. Our results also show the fine structure has opposite on the light squeezing, this observed on decrease of the amount of squeezing as the average photon number increased.

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We report on the first clear observation of a collisionally induced diffuse band of lithium dimer around 452.0 nm in direct single photon excitation using 337.1 nm (N2) laser radiation from an excimer laser. The band, which is being tentatively assigned to the 2 1Σ+u → X 1Σ+g transition and could only be observed at buffer gas pressures of ⩾ 75 mbar, had been theoretically pre dicted. We also report on the other violet band of Li2 around 458.0 nm, already assigned to the 2 3Πg → a 3Σ+u transition. Parametr ic dependences for the two bands are presented. Together with these continuum emissions, a number of strong atomic lithium lines are also observed. A possible correlation between the atomic and continuum emissions is presented which may help in a better understanding of the kinetics of the system.