Aluminium metallography

Light microscopy aluminum

 

light microscopy

Light microscopy is a fundamental tool for microstructural studies aluminum alloys. It is also always applied before, how to use more time-consuming and expensive electronic optics.

What does an optical microscope see?

Light microscopy of aluminum alloys "works" to a magnification of 1500x, at which details up to 0,1 micrometers. Light microscopy distinguishes most of the particles of the secondary phases with their sufficiently large size (more 1 micrometer), shows:

  • size and distribution of soluble and insoluble particles (Fig. 1)
  • grain or crystal structure of the aluminum solid solution matrix (Fig. 2)

Figure 1 – Alloy 5456-O plate, 13 mm thick, hot rolled, and annealed above the solvus.
Rapid coaling resulted in retention of Mg2AI3 in solid solution.
The light, outlined particles are insoluble (Fe,Mn)Al6;
the dark particles are insoluble Mg2Si.
25%HNO3. Original magnification: 500x [2]

Figure 2 – Alloy 2025-T6 closed-die forging, solution heat treated and artificially aged.
Longitudinal section. Complete recrystallization resulted
from high residual strain in the forging before solution treatment.
Keller’s reagent. Original magnification: 100x [2]

A light microscope reveals such microscopic characteristics, as:

  • coating or diffusion thickness (Fig. 3)
  • type and depth of corrosion damage (Fig. 4)
  • melting of low-melting alloy components during overheating (Fig.. 5)
  • the presence of foreign metal inclusions or unwanted coarse intermetallic particles (Fig. 6).

Figure 3 – Aluminium alloy 2024-T3 sheet clad with aluminium alloy 1230
(5% per side), solution heat treated.
Normal amount of copper and magnesium diffusion
from base metal into cladding (top).
Keller’s reagent. Original magnification: 100x [2]

Figure 4 – Intergranular corrosion in aluminium alloy 7075-T6 plate.
Grain boundaries were attacked, causing the grains to separate.
Keller’s reagent. Original magnification: 200x.
(J.M. VanOrden, E. Wolden) [2]

Figure 5 – Alloy 2014-T6 closed-die forging,
showing rosettes formed by eutectic melting.
Solidus temperature (510 ⁰C) was exceeded during solution heat treating.
Keller’s reagent. Original magnification: 500x [2]

Figure 6 – Aluminium alloy 5454, hot-rolled slab, longitudinal section.
Oxide stringer from an inclusion in the cast ingot.
The structure also shows same particles of (Fe,Mn)Alb (light gray).
As-polished. Original magnification: 500x [2]

What can't the light microscope see?

A light microscope is not able to show particles of precipitation of hardening phases, and he “does not see” dislocations and their structure. Special etching and other sample preparation are sometimes used to exert effects., which make it possible to draw some conclusions about these "invisible" characteristics. Generally speaking, research of these "invisibles" – prerogative electron microscopy.

Aluminum phases under a light microscope

The identification of various phases - chemical elements or intermetallic compounds - is an important task of light microscopy of aluminum alloys. These phases are the products of equilibrium or nonequilibrium reactions and can vary inside this alloy depending on the casting conditions., mechanical or heat treatment.

The chemical composition of these phases is associated with equilibrium and nonequilibrium state diagrams for double, triple, quadruple and even more complex systems. The crystal structure and chemical composition of these phases is known and the challenge is, to identify them by their optical characteristics or by etching behavior by various etchants.

For custom designs or, when there are doubts about the reliability of light microscopy data, it is supplemented or replaced by electron microscopy, which allows more accurate identification of various phases in aluminum alloys.

Industrial light microscopy

Products from wrought aluminum alloys are somehow made from cast ingots. These initial ingots are machined and heat treated., which make changes to the original foundry structure. These changes are usually relatively small for large items., obtained by hot deformation methods, such as forgings, thick plates and massive pressed products. Structural changes become more noticeable with an increase in hood during hot and cold processing of products, as well as using various types of heat treatment, such as annealing or solid hardening.

Visible changes in the microstructure of aluminum alloys include the following [1]:

  • Changes in the chemical composition and crystalline structure of phases due to peritectic reactions, which were suppressed during the casting of the original ingots.
  • Dissolution of the most soluble phases, as well as spheroidization and coalescence of phases due to the desire to reduce their surface energy.
  • Elevation at elevated temperature of elements, which were in a supersaturated solution of the casting structure (Fig.. 7).
  • Mechanical fragmentation of brittle intermetallic particles and elongation of these particles along the main directions of hot or cold machining (Fig.. 8).
  • Deformation of the original cast grain structure and subsequent recovery and recrystallization processes (Fig.. 9).

Figure 7 – Aluminium alloy 7075 heated ingot
etched to reveal the fine dispersoid of the chromium rich phase
that precipitated at elevated temperatures and
reflects the original distribution of chromium
in the as-cast supersaturated solid solution.
0,5% hydrofluoric acid. Original magnification, 460x.
(Courtesy ofKaiser Aluminum & Chemical Corp.) [1]

Figure 8 – Aluminium alloy 5052 sheet showing
the fragmented, more uniform distribution of the constituent particles
consisting of (Fe,Cr)3unlucky12 (light) and Mg2Yes (dark).
0,5% hydrofluoric acid. Original magnification: 455x.
(Kaiser Aluminum & Chemical Corp.) [1]

Figure 9 – Aluminium alloy 7075-T6 large extrusion with the section
near the surface showing a layer of coarse recrystallized grains
overlying fine subgrain structure
that reflects recovery during or subsequent to hot working.
10% phosphoric acid. Original magnification: 455x.
(Kaiser Aluminum & Chemical Corp.) [1]

Sources:

  1. Aluminum: Properties and Physical Metallurgy – ed. John E. Hatch – ASM International – 1984
  2. Aluminum and Aluminum Alloys – ASM Speciality Handbook – 1993