Question

If the lattice is perfectly restored by twinning (simple) shear, then all atom positions are preserved. Explain why this does not happen for bcc twins. What is result for fcc twins ?

Answer 1

Layer-by-layer accumulation of stacking faults is the key to theorizing thetwinning process. Such phenomenon would essentially be sub- jected to overcoming fault energy landscape from the discretecrystal level. the formation of an embry-onic twin in a metallic lattice would encompass rigid shearingof the parent crystal (matrix), leading to the mirrored atomic arrangement. This process is aided by glissile twinning partials.In FCC metals the dependence of the twinning stress on temperature appears to be low with the

twinning stress increasing slightly with increasing temperature[l]. In high ysf FCC pure

metals, such as copper and nickel, deformation twinning only occurs at high stress levels (e.g.,

150 MPa for copper and 300 MPa for nickel), which are normally reached under quasi-static

loading only at low temperatures or during large-strain deformations. FCC metals, which have

a high ysf and do not exhibit twinning at quasi-static deformation rates, twin readily under

stress levels imposed by shock deformation as shown in the 1940's by Smith[3]. Cryogenic

shock studies at 13 GPa on Al-4.8wt.°_ Mg were also found to produce deformation twins[4].

The occurrence of deformation twins in aluminum alloys, which have relatively high ysfs and

thus do not exhibit twins under any other observed loading conditions, illustrates how

extremes in strain rate can increase local stress conditions to the point of activating twinning.

Deformation twins are more frequently observed in lower symmetry crystal structures where

the number of potential active slip systems is reduced. In conventional HCP metals and alloys,

similar to BCC metals, increasing strain rate and/or decreasing temperature are known to

increase the frequency of deformation twinning[5, 6-9]. The propensity for twinning in zinc is

known to be enhanced by high strain rate to such an extent that it could be initiated during non

basal glide by suddenly increasing the strain rate[5]. In addition to temperature and strain rate

the texture and c/a ratio of the particular HCP metal under investigation will determine the

exact type of twins, either tensile or compression, formed. Research on twinning in HCP

metals has shown that the stress for twinning on {1012 }, [ 1121 } and {1122 } planes increases

with increasing temperature while that for {1011} twinning decreases [1,6]. Tensile

deformation studies on polycrystalline Zr showed that while {1121 } twins occur infrequently.