Superplasticity and anelasticity in fine-grained Sn-Pb alloys

Thesis



Mechanisms which may play a role in superplastic deformation (grain strain mechanisms, grain boundary sliding (GBS) mechanisms) are reviewed. Two well-known lattice dislocation mechanisms are re-evaluated for grain boundary dislocations (GBDs). The manner in which the deformation mechanisms interact, or are inhibited or obscured, is discussed.

Mechanisms of anelastic deformation are outlined, with particular reference to fine-grained materials. Expressions for anelastic recovery caused either by grain boundary (GB) tension or by the relaxation of GBD pile-ups are derived. The plastic properties of Sn-38.1w/o Pb and Sn-2w/o Pb are measured. They are similar in both alloys. No threshold stress for plastic deformation is detected, for stresses and strain rates as low as 0.IMPa and 10^-10s^-1 respectively. The presence of GB diffusion creep (Coble creep) is established experimentally in Sn-2w/o Pb with grain sizes ≥ 50μm. Coble creep is inhibited for small grain sizes (~10μm). The inhibition is explained by GBS caused by GBDs. In disagreement with the measurements, high threshold stresses are predicted for Sn-38.1w/o Pb. This implies that GBD line tensions are lower than those of lattice dislocations.

The anelastic properties of Sn-2w/o Pb and Sn-38.1w/o Pb are determined from the elastic after-effect (anelastic recovery after unloading). They are remarkable: anelastic contractions larger than 0.2% and relaxation strengths (= ratio of anelastically recovered to elastically recovered strain) in excess of 100 are found. The anelastic strains are approximately proportional to the stress and the inverse grain size. A wide range of relaxation times (~ 6 decades) is observed.

A mechanism based on the relaxation of GBD pile-ups is in qualitative agreement with the measured anelasticity. The high measured relaxation strengths, however, imply that the interaction between GBDs is much weaker (~ 2 orders of magnitude) than that between lattice dislocations. This could be due to a relatively low self-energy of GBDs and would be in qualitative agreement with the low GBD line tensions suggested above.

The influence of anelasticity on transients (e.g. stress relaxation, dip test) is investigated using a rheological model with three Voigt elements (anelasticity) and a nonlinear dashpot (plasticity). Using independently determined plastic and anelastic parameters the 4-th order differential equa tion corresponding to the model is solved numerically for several examples. Measured transients are much more accurately predicted with the present model than with models neglecting anelasticity.

Authors: Schneibel, J; Joachim H. Schneibel

• Publication date: 1979
• DOI: 10.5287/ora-1zvoyxzy5
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Lead-tin alloys; Superplasticity