Think about this a minute before peeking.
|
When tough pitch copper is annealed in hydrogen in order to
maintain a bright surface free of oxide, cavities form near copper
oxide particles in the grain boundaries as shown at left at 500X. |
|
Hydrogen diffuses rapidly (as atomic hydrogen, also known as
protons, because the electrons are mixed with the copper's free
electrons) in copper and reacts with the copper oxide during annealing
to form steam bubbles. How would these steam bubbles affect
ductility ? How would you avoid this situation ? Think about this a minute before peeking. |
| Use of an inert atmosphere of
argon or nitrogen would not cause the embrittling effect, but might not
retain the bright surface. OFHC copper does not exhibit this
difficulty and would be a better choice for bright annealed,
mechanically tough articles. |
| The cavities
form in hydrogen annealed tough pitch copper because the steam can't
escape; it is not soluble as a molecule in the copper, and the
extremely high pressures contained within the otherwise ductile copper
matrix are not able to stop the chemical reaction between the hydrogen
and the copper oxide. The specimen is macroscopically embrittled
because ductile fracture (by growth and linkage of voids) is greatly
facilitated by the preexisting cavities during subsequent
deformation. |
| Specimen 9 to follow is an introductory failure analysis of a broken centrifuge strap. |