Study On Color Metallography Of Magnesium Alloys

Color metallography is an advanced optical technique which arose in the late development stage of metallography. It involves many other disciplines, such as optics, chemistry, and color science, making the fullest use of the properties of light to distinguish different phases and structures of materials. It has many advantages compared to common black-white metallography. Generally, color metallography has a much higher ability to display grain structure, dendrite structure, second phases, and composition segregation, providing scientists with more accurate and useful information regarding the microstructure of materials. This technique has been applied successfully on steel and aluminum alloys. However, its application on magnesium alloys is still in its early stages. Research into the application of color metallography for magnesium alloys is significant for the meticulous study of its microstructure and development.In the present work, the specimen preparation, forming of films, and observations of staple magnesium alloys were discussed. The affects of surface quality obtained from different preparing methods on color metallography were also examined. Chemical etching was researched, including the effects of different formulations over different periods of time on the formation of color metallography. The Ion sputtering of aurum was tested on specimens in order to obtain color contrast. The effects of the parameters of the instruments on the color metallography were also studied.The results show that optimal color contrast can be achieved through the process of mechanical polishing, since it has the smallest chemical effect on the specimen. However, the main problem with mechanical polishing is that the marking is hard to remove. Fortunately, chemical polishing is useful to remove the scarred and deformed layer. It works well on magnesium alloys with low alloy content, but it attacks the surface of specimens with high alloy content, which ultimately affects the final color contrast significantly. Electrolytic polishing is not suitable for the specimen preparation of magnesium alloys under present experimental conditions, although it cures the scarification. It leads to either a black and pitted surface or a whitish and gridding surface, both of which fail to meet the surface quality requirements of color metallography.Considering the regents, reagent #1 did not work on as-cast or homogenized ?magnesium alloys with low aluminum content, such as AZ31 or as-cast ZK60. However, reagent #1 is useful for revealing color grain contrast and the dendritic crystals of as-cast AZ91 (which has a higher content of aluminum). Reagent #2 works well on all AZ series of magnesium alloys, yet it has little effect on as-cast ZK60. Reagent #3 severely etched the specimens and returned no useable results. The results of using Acetic picral with 5-10% acetic acid content were similar to reagent #2, working well on AZ series magnesium alloys. Only limited second phases information was obtained, and no color metallography was achieved by other reagents including water and/or alcohol solutions of other carboxylic acid, glycol, inorganic acid, chromic salt, pyrosulfite, ammonium salt and potassium permanganate.An enhanced yet limited second phases figure can be obtained by ion sputtering aurum. It has only a limited affect, and is very expensive compared to chemical deposition. Therefore, it is not fit for universal application. Also, no useful color metallography was obtained via the process of heat tinting.Many parameters of the microscope and photography system had a significant effect on the final color contrast. Xenon lamps are much brighter and have a higher color temperature than halogen lamps. They have a continuous spectrum, similar to sunlight, which is quite suitable for color metallography. During this study, optimal contrast could only be achieved by crossed polarized light in most cases. Sensitive tint filters were used to raise the interference level so that the color contrast could be enhanced. The grain color contrast varies with a period of 180°when the objective stage rotates. The white balance of the camera does not affect the contrast between grains, yet it does change the color. A color contrast which is closest to natural colors can be obtained when the white balance of the camera is set to sunlight. Differential interference contrast generated no useful color contrast in this study.