dc.description.abstract |
In recent years, considerable concern has been shown about the
effects SPF cavitation has on the mechanical properties of superplastic
alloys. This investigation was undertaken to ascertain
whether Hot Isostatic Pressure (HIP) eliminated this cavitation in
Supral 150 and correspondingly brought about an improvement in the
mechanical properties.
It was found that the density increased with various isothermal
anneals; the activation energy for this process was close to that
for grain boundary diffusion in aluminium (61.93 KJ mol-1).
The rate of cavity sintering was seen experimentally to be
enhanced by the application of pressures greater than 7 HPa, and had
an activation energy of 62.42KJ mol-1.
Complete cavity closure occurred when the external pressure was
greater than the flow stress of the material at thaý temperature
and strain-rate; the ratio of external pressure (Pe) to flow stress
(of) increased with falling HIP temperature. The activation energy
at constant strain-rate (Q-) associated with plastic flow under
conditions of hole closure
was found to be 53.54KJ mol-1; giving
a corresponding activation energy at constant stress (Qa), which is
close to that for lattice diffusion in aluminium.
The alloy used contained a high level of hydrogen which caused
blistering on heat treatment, and was also responsible for the
reappearance of porosity in subsequently heat-treated material
previously returned to theoretical density; the extent of which was
decreased by the use of higher temperatures and pressures or by
vacuum degassing the material prior to HIP.
Post SPF room temperature ductility was enhanced by HIP. The
scatter in the 0.2% PS and UTS values found in as-received SPF
specimens was not altered by the use of low pressure HIP (up to
35 MPa), although higher pressures (100 NPa) did slightly enhance
these values and drastically reduced the scatter.
Room temperature fracture of as-received Supral was by a 450
ductile shear mechanism.
In the SPF cavitated material, the external characteristics Of
fracture had a more jagged appearance, as the cavitation alters the
route of the propagating crack.
SPF material which has been HIPped to remove cavitation, fails
in a manner similar to the as-received material. |
en_UK |