Microstructures
by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2005 by George Langford
Non Ferrous Alloys - Lesson 1 - Ninth specimen
Bronze centrifuge strap at 50X etched
 The three photomicrographs on this page describe the failure of a bronze centrifuge strap.

Always be sure to scan the entire specimen for evidence of differences in microstructure.

In the first image shown at 50X at left, the structure is undeformed.  The annealing twin boundaries are all straight.
Bronze centrifuge strap at 200X etched
 In the second photomicrograph, shown at 200X at left, there is evidence of strain markings and oxide precipitation at the grain boundaries.  The specimen was annealed at too high a temperature, causing grain boundary melting, also known as burning, because oxygen diffused readily into the molten metal and formed oxides deep inside.  The molten metal naturally wets its own grain boundaries, facilitating transport of the oxygen.
Fracture of bronze centrifuge strap at 50X etched
 In the last frame, shown here at 50X, the localized failure reveals heavy deformation in the necked-down metal as well as intercrystalline cracking due to the oxide and subsequently formed solidification shrinkage pores along the affected grain boundaries.




Summary, below.





SUMMARY:  This group of microstructures illustrates in a series of simple alloys the kinds of microstructural features to which you should be attuned, because they indicate the processing and environmental history of the metal.  Some failures originate in processing, others , in service.

You can also tell something about the time and temperature history from the extent, scale, and depth of the microstructural changes.  Be careful to look over the entire specimen for evidence of cracks, local heating, surface morphology, and other subtle evidence of its previous history.
Go on to Lesson 2.       Return to Non Ferrous Metals Introduction.