MP35N

MP35N is a very hard (45-50 HRC, ~450 HV in cold-worked + aged condition), ductile cobalt-nickel alloy with a density of 8.43 g/cm³ and a melting range of 1350-1440°C. The extreme hardness combined with high ductility and work-hardening tendency makes this one of the more challenging alloys to prepare metallographically. The microstructure features an FCC matrix with HCP platelets and fine Co3Mo precipitates that are only resolvable by TEM. At optical magnifications, the solution-annealed condition shows equiaxed FCC grains with twins; the cold-worked condition shows elongated grains with heavy deformation bands.

Use an abrasive cut-off wheel rated for hard alloys (cobalt/nickel superalloys) with continuous heavy coolant flow. The extreme hardness and toughness make sectioning slow. For standard cut-off: 2500-3500 RPM with moderate to firm feed pressure. Avoid stalling, which causes local overheating and additional work hardening. For wire or rod specimens (pacemaker leads, fasteners), a precision wafering saw with a diamond blade at 200-400 RPM is recommended. Leave 2-3 mm allowance for grinding. Expect blade wear rates 2-3 times higher than for standard stainless steels.

Compression mounting with standard phenolic or epoxy compounds is appropriate. The high melting range (1350-1440°C) is unaffected by mounting temperatures. For wire cross-sections (pacemaker leads), mount multiple wires together in a single mount for efficient preparation. Ensure wires are perpendicular to the mount surface for true transverse sections. Cold mounting with castable epoxy is acceptable; use a mount with hardness close to the specimen to avoid preferential grinding of the mount.

MP35N in the cold-worked + aged condition is very hard. Diamond grinding discs are strongly preferred over SiC paper. The alloy will rapidly load and dull SiC papers. Disc speed: 250-300 RPM. Apply 25-30 N per sample.

Grinding sequence:

• 240 grit: Remove sectioning damage (45-90 seconds). Firm pressure. Expect slow removal.
• 320 grit: Remove previous scratches (30-60 seconds).
• 400 grit: Continue refinement (30-45 seconds).
• 600 grit: Fine grinding (30-45 seconds).
• 800 grit: Final grinding (20-40 seconds).

Rotate specimen 90° between steps. Use complementary rotation. Abundant water lubrication. The high hardness means longer grinding times at each step compared to the CoCrMo alloys. Fresh abrasive surfaces are critical; worn discs generate heat without removing material, increasing work hardening artifacts.

Use napless pads throughout. The homogeneous single-phase microstructure (at optical magnification) means relief is minimal, but the work-hardened surface layer from grinding must be fully removed.

Diamond polishing sequence:

• 9μm diamond: 4-6 minutes on a napless composite pad at 20-25 N per sample. This step removes the work-hardened layer from grinding.
• 3μm diamond: 3-5 minutes on a napless synthetic pad at 20-25 N.
• 1μm diamond: 2-4 minutes on a napless pad at 15-20 N.

Final polishing:

• 0.05μm colloidal silica: 3-5 minutes on a porous chemical-mechanical pad at 10-15 N. Extended colloidal silica polishing or vibratory polishing (6-12 hours) is recommended for revealing the fine deformation structure, grain boundaries, and twin boundaries in this alloy. The chemical-mechanical action of colloidal silica is particularly effective at removing the tenacious work-hardened layer.

MP35N responds to electrolytic and chemical etchants common for cobalt-nickel alloys.

Electrolytic 10% Oxalic Acid (Electrolytic Etching) - Primary choice:

• Composition: 10 g oxalic acid (H2C2O4), 100 ml water
• Application: Electrolytic at 3-6 V DC for 5-20 seconds. Stainless steel cathode.
• Reveals: Grain boundaries, twin boundaries, deformation bands in cold-worked specimens. In solution-annealed material, reveals equiaxed grains with prominent annealing twins.
• Rinse: Water, then ethanol. Dry with warm air.

Kalling's No. 2 (Chemical Etching) - Alternative:

• Composition: 5 g CuCl2, 100 ml HCl, 100 ml ethanol
• Application: Immerse for 10-30 seconds or swab. Monitor closely.
• Reveals: General microstructure, grain boundaries, and cold work deformation structure. Provides good contrast between deformed and undeformed regions.

Safety: Use fume hood. DC power supply for electrolytic etching. Kalling's No. 2 contains CuCl2, which is toxic; wear appropriate gloves and eye protection.