Metallography and Microstructures of Heat-Resistant Alloys

HEAT-RESISTANT ALLOYS cover a wide range of chemical compositions, microstructural constituents, and mechanical properties. This article summarizes metallographic techniques and microstructural constituents for three types of cast and wrought heat-resistant alloys: iron-base, nickel-base, and cobalt-base. The metallographic methods discussed also are suitable for preparing both cast and wrought heat-resistant alloys; microstructural constituents are quite similar except for obvious differences in homogeneity and porosity.
The procedures used to prepare metallographic specimens of cast or wrought heat-resistant grades are quite similar to those for ironbase alloys, especially stainless steels (see the Section “Metallographic Techniques” in this Volume). Aspects particularly significant to the preparation of cast or wrought heat-resistant alloys are emphasized. Tables 1 to 3 list the nominal compositions of Fe-Ni-Cr Alloy Casting Institute H-series alloys and other iron-nickel,
nickel-, and cobalt-base cast and wrought heatresistant alloys, respectively.


Metallographic Specimen Preparation for Electron Backscattered Diffraction


Electron backscattered diffraction (EBSD) is performed with the scanning electron microscope (SEM) to provide a wide range of analytical data; e.g., crystallographic orientation studies, phase identification and grain size measurements. A diffraction pattern can be obtained in less than a second, but image quality is improved by utilizing a longer scan time. Grain mapping requires development of diffraction patterns at each pixel in the field and is a slower process. The quality of the diffraction pattern, which influences the confidence of the indexing of the diffraction pattern, depends upon removal of damage in the lattice due to specimen preparation. It has been claimed that removal of this damage can only be obtained using electrolytic polishing or ionbeam polishing. However, the use of modern mechanical preparation methods, equipment and consumables does yield excellent quality diffraction patterns without use of dangerous electrolytes and the problems and limitations associated with electropolishing and ion-beam polishing. Basically, if mechanical preparation results in quality polarized light images of noncubic crystal structure elements and alloys (e.g., Sb, Be, Hf, -Ti, Zn, Zr), or color tint etching of cubic, or non-cubic crystal structure elements or alloys produces high-quality color images, then the surface is free of harmful residual preparation damage and EBSD patterns with high pattern quality indexes will be obtained. Because of the acute angle between the specimen and the electron beam (70 – 74 ), exceptional surface flatness is also necessary for best results.


Metallographic Preparation of Orthopedic Medical Devices


Metallographic sample preparation methods for porous coated implant devices can be difficult due to inadequate fill of the mounting materials into the porous metallic structures. Inadequate fill of the mounting material during sample preparation leads to problems such as edge rounding, uneven etching, and metal smearing during polishing. These problems make proper microstructural identification and analysis difficult and/or inaccuate.


Metallographic Preparation of Medical Devices

Specimen Mounting Program

Implant Specimens

  • Acetabular cup: Ti-6Al-4V substrate, CP Ti wire
  • Acetabular Cup: Ti-6Al-4V substrate, CP Ti Powder, Ta Beads
  • Femoral Hip Stem: Co-Cr-Mo with Co-Cr-Mo Beads
  • Femoral Knee: Co-Cr-Mo with Co-Cr-Mo Wire

Mounting Procedures

  • PhenoCure™thermosetting phenolic resin
  • Electroless Ni-plate/EpoMet® thermosetting resin
  • Vacuum impregnate with low-viscosity EpoThin® epoxy resin
  • Vacuum impregnate with EpoHeat™epoxy resin
  • SamplKwick® cast acrylic resin


Low-Load Vickers Microindentation Hardness Testing

In 1925 in the UK, Smith and Sandland developed an indentation test that used a squarebased pyramidal-shaped indenter made of diamond[1]. The test was developed because the Brinell test (introduced in 1900), which (until recently) used a round hardened steel ball indenter, could not test steels harder than ~450 HB (~48 HRC). They chose this shape with an angle of 136° between opposite faces to obtain hardness numbers that would be as close as possible to Brinell hardness numbers for the same specimens over the usable Brinell range. This made the Vickers test easy to adopt, and it rapidly gained acceptance. The Vickers test has the great advantage of one hardness scale being used to test all materials, unlike the 30 different Rockwell test scales, each yielding numbers between ~20 and ~100.


Light Microscopy

Nature of Light in Microscopy

Amplitude – light intensity
Wavelength – color;
Phase displacement;
Polarization – one plane of vibration

Laws of Refraction and Reflection

Part of the incident light ray is reflected and part is
refracted; the angle of reflection equals the angle of
incidence, i.e., θ = Φ


Introduction to Metallography

What is Metallography

Metallography is the study of microstructure, how it is produced by composition and processing control, and the relationship between microstructure, mechanical and physical properties, and performance or service behavior. To examine the microstructure, we must prepare specimens properly so that the true structure, free of artifacts due to preparation, can be observed and properly identified, measured and interpreted.

What is Microstructure

Microstructure is the structure of a suitably prepare specimen as revealed by a microscope Microstructure consists of phases and/or constituents.
A phase is a physically homogeneous, mechanically separable portion of a material system. A constituent is a phase or combination of phases, which occurs in a characteristic configuration in an alloy microstructure.


Microstructure of Nickel, Cobalt and their Alloys

Lucas’ Reagent

  • 50 mL lactic acid
  • 150 mL HCl
  • 3 g oxalic acid

Lucas’s reagent for Fe-Ni, Ni- and Co-base superalloys. Use electrolytically at 1-2 V dc, for 10-20 s. Electrolyte can be made as a stock solution.

Molybdic Acid Reagent

  • 100 mL water
  • 100 mL HCl
  • 100 mL HNO3
  • 3 g molybdic acid

Molybdic acid reagent to reveal the dendritic structure of cast Ni-base superalloys. Mix and alloy to stand >1 h before use. Immerse specimen for several seconds. Can be stored.


Metallography of Superalloys


Superalloys are complex alloys of Fe-Ni, Ni, or Co-base compositions. Their microstructure can be quite complex due to the potential for a variety of phases that can be formed by heat treatment or service exposure conditions. The paper presents the use of new metallographic materials to prepare these alloys with emphasis on modern, four- and five-step practices. Different etchants are required to reveal the structure of these alloys properly. Examples will be presented showing the use of different etchants as a function of alloy composition, heat treatment, and microstructural phases.


Metallographic Techniques for Superalloys


Preparation of superalloys for microstructural examination is not exceptionally difficult. The procedures are similar to those used to prepare stainless steels. Because they are facecentered cubic “austenitic” alloys with exceptionally good toughness, machinability is poorer than for steels and the age hardened alloys, especially the cast alloys, can be more difficult to section than most steels when they have a very high ‘ content. FCC metals readily deform and work harden, consequently aggressive sectioning methods (e.g., power-hacksawing or band sawing) will introduce considerable damage which can be very difficult to remove in the subsequent preparation steps. If these procedures must be used, it is advisable to re-section the material with the correct abrasive cutoff wheel (consumable type) with abundant cooling.