Copper Alloy Microstructures and Common Etchants

Copper and copper-alloy microstructures are examined by matching the etchant to the material and feature of interest. Brass and bronze show different phase patterns; the solutions below reveal grains, phase boundaries, inclusions, and weld structures under specified swabbing or immersion conditions.

Microstructural Characteristics

Brass and bronze are the most widely used industrial copper alloys.

Brasses can be divided into single-phase and two-phase alloys. Cast single-phase α brass often exhibits dendritic segregation. Deformation followed by annealing removes the segregation and produces uniform grains with annealing twins. In α+β’ two-phase brass, the α phase precipitates from the β phase during cooling, and its morphology depends on the processing conditions. In the as-cast state, α forms a coarse Widmanstatten structure. Slow cooling after hot working produces uniform equiaxed α grains, whereas rapid cooling produces a fine acicular Widmanstatten structure. Except for NH₄OH and H₂O₂ aqueous solutions, the etchants listed below preferentially attack the β phase, turning it dark while the α phase remains bright.

Tin bronze, aluminum bronze, and beryllium copper share a common structural feature: an α solid-solution matrix with a small amount of eutectic or eutectoid constituent. Tin bronze contains α+(α+δ) eutectoid; aluminum bronze contains α+(α+γ₂) eutectoid; beryllium copper contains α+(α+β) eutectoid; and phosphor bronze contains α+(α+Cu₃P) eutectic plus (α+δ+Cu₃P) eutectoid. Tin bronze also exhibits severe segregation and can differ substantially from its equilibrium phase diagram because the liquidus-solidus interval is wide and tin diffuses slowly. Aluminum bronze segregates only slightly. At low magnification its structure resembles two-phase brass, while the (α+γ₂) eutectoid becomes visible at high magnification. Beryllium copper is generally used after solution treatment and aging. Solution treatment produces a uniform single-phase structure. Aging precipitates very fine CuBe second-phase particles within grains or along grain boundaries. Optical microscopy may faintly reveal particles aligned along slip lines, but their details require electron microscopy.

Common Etchants

The following solutions are commonly used to reveal copper and copper-alloy microstructures.

Inclusion Examination

Identifying inclusion types and features is increasingly important in microscopic examination because inclusions directly affect copper formability and machinability. Inclusions may be examined in the polished condition. Alternatively, briefly swab the polished surface with ammonium hydroxide solution, Etchant 1 above, to make them more visible.

Sulfur and oxygen can form Cu+Cu₂S and Cu+Cu₂O eutectics in copper. Both Cu₂S and Cu₂O are brittle compounds that fracture readily during cold working, so copper is subject to strict oxygen and sulfur limits. Metallography can provide an approximate indication of impurity content in addition to chemical analysis. Under ordinary illumination, copper oxide and copper sulfide cannot be distinguished because both appear blue-gray. Under polarized light, copper oxide appears ruby red and can therefore be distinguished from copper sulfide. In the as-cast state, oxygen content can be assessed by comparison with standard micrographs representing different oxygen levels.

Trace bismuth and lead form Cu+Bi or Cu+Pb eutectics with copper. Both elements separate from the eutectic, but their distribution differs. Bismuth forms a network along grain boundaries and causes pronounced hot shortness. Lead has less tendency to form a continuous grain-boundary distribution; stirring before solidification can disperse Pb as fine particles in the matrix. Small lead additions can therefore improve machinability.