Microstructure Changes and Steady State Creep Characteristics in the Superplastic Sn-5wt.% Bi alloy During Transition

Creep tests of Sn-5wt.%Bi alloy have been studied at different creep < br />temperatures between (308-388K) and at three constant applied stresses of
values 20.5, 21.7 and 23 MPa. Superplastic flow was found to be based on the
steady state law of creep with the incorporation of grain size effect. In the
steady state deformation, the flow stress (σ ) was found to depend on strain
rates ( st ε& ) and temperatures. The steady state creep of Sn-5wt.%Bi has been
investigated within the temperature range from (308-388K), and the strain rate
sensitivity parameters (m=∂ lnσ / ∂ lnε& ) were calculated at different
temperatures; they indicated two transition temperatures at "328 and 361K.
The strain sensitivity parameters were found to be shifted to higher values with
increasing temperatures or decreasing grain diameters. The energies activating
creep calculated in the three ranges of temperatures were found to have the
values “37.16 ± 3KJ/mole” in the first range of temperature (308-328K) and
“50.41 ± 5KJ/mole” in the intermediate range of temperature (338-361K) and
"72.02 ± 7KJ/mole in the third range of temperature (361-388K) respectively.
The values of the activation energies in the first and the intermediate ranges of
temperatures suggest the grain boundary sliding with partially diffusion
mechanisms while in the third range one it might be due to the cross slipping
mechanism. Metallurgical observations and grain diameter measurements
confirmed these mechanisms as well as the above two transition temperatures.
The x-ray structure analysis through the measurements of the lattice constants
(a and c), the ratios (c/a), the half line widths (Δ2θ ), the integral intensities (I)
and the residual lattice strains ( a
a
Δ and c
c
Δ ) for the Sn-matrix, exhibited peaks
at the two transition temperatures.