Microstructures
by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2005 by George Langford
Non Ferrous Alloys - Lesson 2 - Third specimen
Naval brass at 100X etched
 This is a naval brass composed of copper, 39% zinc, and 1% tin.  It failed during deformation processing.  Why?

Look at all four photomicrographs before answering.

The upper pair show the outer edge at 100X ...
Naval brass at 500X etched
 ... and at 500X.
Naval brass at 100X etched
 The lower pair, also at 100X ...
Naval brass at 500X etched
 ... and at 500X, were taken near the center of the piece.

The manufacturer was "stuck" with many pieces which were too brittle to be used.  He wanted to "rescue" them from the scrap barrel.  What would you have done ?





Again, pause a while before looking at his solution.










Explanation
 The microstructure shows alpha and beta plus a finely dispersed phase which appears to be concentrated at the alpha-beta interfaces.  There are small grains near the edge of the piece, coarse grains in the interior.  There was more sigma precipitated where there were more alpha-beta interfaces, i.e., in the fine grained exterior regions.  The third phase is sigma, which is less soluble at lower temperatures.  The manufacturer simply solutionized the sigma by reheating and then quenching to avoid its reprecipitation.

The fourth specimen demonstrates an instance of chemically induced failure.