Nanoindentation study on the creep resistance of SnBi solder alloy ...

Fig. 8(a) shows the SEM image of SnBi with 4 wt% Ni deformed
at 354 MPa. The indentation imprint displayed a regular triangle
with evaginated boundaries indicating discernible pile-up of the
material during the indentation process. It is believed that the
indentation induced deformation is accomplished by the bulk
deformation inside Sn-rich and Bi phases. As a result, the
hardened individual phases push the surrounding material outwards
causing significant pile-up around the indented area.
However, when material is deformed at a lower stress of
116 MPa where a lower strain rate is applied, significant number
of pores are formed at the boundaries of the adjacent phases as
shown in Fig. 7(b). Prominent delamination at phase boundaries
is formed at the surrounding area of the indent. It is believed that
at a lower stress condition, the deformation of the material is
mainly channelled through slipping or shifting at the phase
boundaries between Sn-rich and Bi phases. Comparing to the
indentation creep results, the stress exponent n 2 at lower stress
region (o170 MPa) is in the range from 2.42 to 3.85, which
corresponds well with the phase boundary sliding mechanism.
As shown above, the creep resistance reaches a maximum at
3 wt% Cu and 1 wt% Ni filler concentration, and then starts
decrease when more filler is added. With the nano-filler incorporation,
it is believed there are two major strengthening
mechanisms from the stiff metal particles. One comes from the
L. Shen et al. / Materials Science & Engineering A 561 (2013) 232–238 237
Fig. 7. Strain rates of SnBi(Cu) alloys deformed at (a) 200 MPa and (b) 100 MPa. Strain rates of SnBi(Ni) alloys deformed at (c) 200 MPa and (d) 100 MPa.
pinning of the dislocation movement through effective particle–
dislocation interactions. Rosler et al. [20] demonstrated that such
interactions are primarily responsible for the excellent creep
resistance enhancement of carbide dispersion-strengthened aluminum
alloys. Secondly, the obvious microstructural refinement
due to filler addition strengthens the matrix due to effect such as
Hall–Petch hardening. However, such microstructure refinement
promotes diffusional creep (e.g. Nabarro-Herring, Coble) and
grain boundary sliding, adversely affecting creep resistance especially
at lower stress regions. With the two effects functioning
simultaneously, it is believed that at certain low filler concentrations,
the metal filler pinning effect dominants the creep deformation
until an optimum filler concentration is reached, after that
creep rate starts to increase due to significant amount of diffusional
creep and phase boundary sliding is involved, which causes
the stress exponent shift to a lower value.
4. Conclusions
Pores
Fig. 8. SEM images of Sn–58Bi with Ni 4% deformed at stress of (a) 354 MPa; and (b) 116 MPa.
The elastic, plastic and creep properties of Sn–58Bi alloy have been
studied with varying amount of nano-metal filler additions. Moderate
stiffness enhancement is found in the two types of composite alloys.
The formed hard particles and the microstructure refinement due to