Magnesium Metallographic Preparation

Preparation of magnesium and its alloys is rather difficult due to the low matrix hardness and the higher hardness of precipitate phases that lead to relief problems, and from the reactivity of the metal. Mechanical twinning may result during cutting, grinding, or handling if pressures are excessive. Final polishing and cleaning operations should avoid or minimize the use of water and a variety of solutions have been proposed. Pure magnesium is attacked slowly by water while Mg alloys may exhibit much higher attack rates. Some authors state that water should not be used in any step and they use a 1 to 3 mixture of glycerol to ethanol as the coolant even in the grinding steps. Always grind with a coolant, as fine Mg dust is a fire hazard. Because of the presence of hard intermetallic phases, relief may be difficult to control, especially if napped cloths are used.
Table 6.3: 5-Step Methods for Magnesium Alloys
Sectioning Precision saw, with no water and a 15HC blade recommended for use on Metal Matrix Composites, PCBs, Bone, TI, TSC
Mounting Castable, typically with PhenoCure, EpoxiCure, EpoThin or SamplKwick
Surface Abrasive / Size Load - lbs [N]
Specimen
Base Speed [rpm] Relative Rotation Time [min:sec]
CarbiMet 320[P400] grit Sic*
water cooled
3[13] 300 Complimentary Rotation Until Plane
TexMet C 9µm MetaDi Oil-based
Diamond
3[13] 150 Contra Rotation 6:00
TexMet C 3µm MetaDi Oil-based
Diamond
3[13] 150 Complimentary Rotation 5:00
TexMet C 1µm MetaDi Oil-based
Diamond
3[13] 150 Complimentary Rotation 4:00
ChemoMet 0.05µm MasterPolish 4[18] 150 Contra Rotation 1:30
Platen = Platen Specimen Holder = Specimen Holder *SiC surface were coated with wax to minimize ebedment
Image & Analysis Dendritic spaciing, Porosity Assessment, Grain Size
Hardness Testing Brinell
MasterPolish is nearly water free and yields excellent results as the final abrasive (See figures 6.3 - 6.4). After the last step, wash the specimens with ethanol. Cleaning after the last step, without using water, is difficult. Holding the specimen under running water for about a second eased the cleaning problem and did not appear to harm the microstructure. Cosmetic cotton puffs can scratch the surface when swab etching. For best results, etch-polish-etch cycle may be needed. Magnesium has a hexagonal close packed crystal structure and will respond to polarized light. To enhance the response, use a brief vibratory polish with the materials used in the last step.

HELPFUL HINTS FOR LIGHT METALS

For polishing, apply diamond paste and use lapping oil, or use oil-based MetaDi diamond suspension, available down to a 0.05µm size, with a medium nap cloth.

For more information on metallographic specimen preparation for magnesium and other metals, refer to the Buehler SumMet Guide.