Recommended Procedures
Class 6

Tough, Hard Non-Ferrous Metals

Hardness 120–490 HV Typical Ti · Inconel · Hastelloy · Co-Cr Key challenge Work hardening & oxide

Class 6 covers high-performance non-ferrous alloys that share a common preparation profile: rapid work hardening, tenacious oxide formation, and multiple phases that must be preserved without relief. The class spans commercially pure titanium through precipitation-hardened nickel superalloys (Inconel 718, Waspaloy) and carbide-bearing cobalt-chromium alloys (Stellite 6, CoCrMo medical grades). Titanium and zirconium alloys add the complication of extremely rapid surface oxidation and low thermal conductivity that concentrates heat during sectioning. Nickel superalloys require careful polishing to preserve fine gamma-prime precipitates, while cobalt alloys demand controlled grinding to retain primary carbides. All of these materials need specialized etchants tailored to their specific phase chemistry.

Ti-6Al-4V titanium alloy microstructure showing typical Class 6 material structure
Ti-6Al-4V: typical Class 6 microstructure

Overview

Class 6 alloys share several preparation characteristics that distinguish them from other classes. Work hardening, oxide formation, and multi-phase microstructures create challenges at every step from sectioning through etching.

Preparation Challenges

Seven properties drive the prep procedure. Tap a card for full detail.

Rapid Work Hardening Light loads and fresh abrasives prevent a deformed surface layer.

All Class 6 alloys work-harden quickly during grinding and polishing. Excessive pressure or prolonged grinding creates a deformed layer that masks the true microstructure. Light loads (20-25 N), fresh abrasives, and contra-rotation are essential to minimize subsurface damage.

Surface Oxide Formation Titanium and zirconium grow oxide layers within seconds of exposure.

Titanium and zirconium form oxide layers within seconds of exposure to air or water. These thin, tenacious oxides can obscure grain boundaries and phase features. Minimize time between final polishing and examination, and avoid prolonged rinsing.

Gamma-Prime & Delta Phase Preservation Napped cloths rip fine precipitates from the superalloy matrix.

Nickel superalloys (Inconel 718, Waspaloy, Nimonic 80A) contain fine gamma-prime (Ni₃Al/Ni₃Ti) or delta phase precipitates critical to their properties. Aggressive polishing on napped cloths can pull these precipitates from the matrix, creating pits that look like porosity.

Alpha-Beta Phase Relief Alpha and beta phases polish at different rates and create height differences.

Titanium alloys have alpha (HCP) and beta (BCC) phases with different hardness that polish at different rates, creating relief. Napless cloths and short polishing times minimize height differences between phases. Vibratory polishing with colloidal silica produces the flattest surfaces.

Carbide Retention Primary carbides in cobalt alloys pull out on napped cloths under heavy load.

Cobalt-chromium alloys and some Hastelloys contain primary carbides (M₆C, M₂₃C₆, MC) that can pull out during polishing if the cloth nap is too aggressive or if loads are too high. Diamond on napless cloths at moderate pressure retains carbides best.

Low Thermal Conductivity Titanium concentrates cutting heat; slow feeds and generous coolant required.

Titanium's thermal conductivity (~22 W/m·K) is roughly one-third that of carbon steel and comparable to austenitic stainless, concentrating heat at the cutting interface during sectioning. This can cause localized phase transformation, oxidation discoloration, or heat-affected zones. Slow feed rates and generous coolant flow are required.

Alloy-Specific Etching No universal etchant; each alloy family has its own reagent.

Each alloy family requires a different etchant: Kroll's for titanium, Marble's or Kalling's No. 2 for nickel alloys, electrolytic oxalic acid for gamma-prime in superalloys, and Murakami's for carbide distribution in cobalt alloys. Universal etchants do not exist for this class.

Class 6 Materials

Thirty-six alloys across seven families. Each family needs grinding, polishing, and etching adjustments.

Commercially Pure Titanium 4
  • Commercially Pure Titanium Grade 1
  • Commercially Pure Titanium (Grade 2)
  • Commercially Pure Titanium Grade 3
  • Commercially Pure Titanium Grade 4
Titanium Alloys 6
  • Beta C Titanium
  • Ti-3Al-2.5V
  • Ti-5Al-2.5Sn
  • Ti-6Al-2Sn-4Zr-2Mo (Grade 5)
  • Ti-6Al-4V (Grade 5)
  • Ti-15V-3Cr-3Al-3Sn
Inconel (Ni-Cr-Mo Alloys) 7
  • Inconel 600
  • Inconel 601
  • Inconel 617
  • Inconel 625
  • Inconel 690
  • Inconel 718
  • Inconel 725
Hastelloy (Ni-Mo/Cr Alloys) 4
  • Hastelloy B-2
  • Hastelloy C-22
  • Hastelloy C-276
  • Hastelloy X
Other Nickel Alloys 7
  • Haynes 230
  • Incoloy 800
  • Incoloy 825
  • Incoloy 925
  • Monel 400
  • Nimonic 80A
  • Waspaloy
Cobalt-Chromium Alloys 4
  • Cobalt-Chromium Alloy (Stellite 6)
  • CoCrMo Cast (ASTM F75)
  • CoCrMo Wrought (ASTM F1537)
  • MP35N
Zirconium Alloys 1
  • Zircaloy-4
Additive Manufactured 2
  • AM Inconel 718 (SLM)
  • AM Ti-6Al-4V (SLM/EBM)

Recommended Procedure

Five-stage workflow. Adjust pressure, abrasive, and etchant to the specific alloy family.

  1. 1

    Sectioning

    Non-ferrous abrasive or precision diamond wafering with slow feed and generous coolant; keep titanium swarf wet.

    More detail

    Use aluminum oxide (non-ferrous) abrasive blades with generous coolant flow. Titanium has very low thermal conductivity, so slow feed rates are critical to avoid heat-affected zones and oxidation discoloration at the cut face. Keep titanium fines wet at all times (dry titanium swarf is a fire hazard). Precision wafering is preferred for additive manufactured samples to preserve the as-built columnar grain structure.

  2. 2

    Mounting

    Compression mounting acceptable for most wrought alloys; castable epoxy with vacuum impregnation for AM or porous samples.

    More detail

    Compression mounting (150-180°C) is acceptable for most wrought titanium and nickel alloys, as these temperatures do not alter their microstructure. Use castable (cold) mounting for additive manufactured parts where porosity must be preserved, for medical implant surfaces, or when edge retention is critical (e.g., oxide scale examination on titanium). Vacuum impregnation with low-viscosity epoxy is recommended for porous AM parts.

  3. 3

    Grinding

    Start 240 grit SiC, progress 320 / 400 / 600 with light pressure (20–25 N) and contra-rotation; replace papers often.

    More detail

    Start at 240 grit SiC and progress through 320, 400, and 600 grit. Use light pressure (20-25 N) and contra-rotation to minimize work hardening and subsurface deformation. Replace SiC papers frequently because these alloys load the abrasive quickly. Diamond grinding discs (75 and 40 µm) are an effective alternative for nickel superalloys and cobalt-chromium alloys, providing more consistent material removal with less deformation.

  4. 4

    Polishing

    9 → 3 µm diamond on napless cloths, finish 0.05 µm colloidal silica; add 30% H₂O₂ for titanium attack polish.

    More detail

    Polish with 9 µm polycrystalline diamond on a napless cloth, then 3 µm diamond on a napless cloth. Final polish with 0.05 µm colloidal silica on a short-nap cloth for 1-2 minutes. For titanium and titanium alloys, add 30% hydrogen peroxide to the colloidal silica (roughly 1 part H₂O₂ to 5 parts silica). This chemo-mechanical "attack polish" removes the deformation layer that titanium maintains stubbornly under conventional polishing and is the standard technique for clean alpha/beta phase contrast. Use napless cloths throughout the diamond steps to minimize relief between alpha/beta phases (titanium) and to retain carbides (cobalt alloys). Vibratory polishing with colloidal silica for 30-60 minutes produces the flattest surfaces and best phase contrast for superalloys.

  5. 5

    Etching

    Each alloy family needs its own reagent: Kroll's for Ti/Zr, Marble's or Kalling's for nickel, Murakami's for cobalt carbides.

    More detail

    Kroll's reagent for all titanium and zirconium alloys; swab 5-15 seconds. Marble's reagent or Kalling's No. 2 for solid-solution nickel alloys; swab 10-30 seconds. Electrolytic etching with 5% chromic acid at 5V DC reveals gamma-prime distribution in precipitation-hardened superalloys (Inconel 718, Waspaloy). Murakami's reagent at room temperature highlights carbide distribution in cobalt-chromium alloys. Swab etching is preferred over immersion for better control across all alloy families.

    Common etchants by alloy family

    Titanium alloys
    Kroll's reagent (1–3 mL HF + 2–6 mL HNO₃ + 100 mL H₂O); Weck's tint; modified Kroll's for α/β contrast
    Nickel superalloys (Inconel, Waspaloy)
    Glyceregia (HCl + HNO₃ + glycerol); Kalling's #2 (CuCl₂ + HCl + ethanol); Marble's reagent; 10% oxalic electrolytic
    Hastelloy
    Glyceregia; aqua regia + glycerol; 10% oxalic electrolytic
    Cobalt-Chromium (Stellite, CoCrMo)
    Murakami's (carbides); Marble's reagent (matrix); electrolytic HCl in methanol
    Zircaloy / zirconium alloys
    Kroll's (HF + HNO₃ + glycerol); polarized light for grain structure

    Titanium etchant guide → Hastelloy etchant guide → Learn about etchants → Shop etchants →

Quality Checks

  • No work-hardening artifacts (scratches or deformation twins) visible at 500×
  • Phase boundaries clearly defined with minimal relief between α/β (Ti) or γ/γ' (Ni)
  • Carbides and precipitates retained in place with no pull-out pits
  • No oxide discoloration on titanium or zirconium surfaces
  • Grain boundaries uniformly revealed without preferential attack or over-etching