In order to further under stand the mechanism of theYPE
phenomenon,the microstructures of Ti samples after annealed
at 500 1C for30min and tensile tested for a plastic strain of3%
were investigated by TEM observations. As for the 500 1C/30 min
annealed samples,the plastic strain of 3% corresponds to the end
of the YPE(Fig.1).The typical TEM image of the tensile tested
samples for a plastic strain of 3% is given in Fig.3. There were
relatively larger and smaller grains,and a large number of
dislocations at or near grain boundary or within the grain,but
no twins and stacking faults were observed.Thus,the YPE
phenomenonin this FG HCP Ti was not associated with twins.
In the study by Barnett eta l. [8] on HCP magnesium alloy,twins
initiate twinning events in neighboring grains and twinning
spreads its way over the sample during YPE.Therefore,YPE of
FG Ti in the present study is different from that of the
magnesium alloy.Generally,YPE in BCC iron and steels was explained
by the effect of carbon or nitrogen atmospheres around dislocations . However,in the present study,the solubility of
carbon or nitrogen in pure Ti was too limited to cause this
effect.In addition,the content of oxygen in tensile tested Ti
samples was expected to have no change after short time
annealing at 500 1C;since Ti possesses excellent oxidation
resistance due to the highly protective surface oxide film and
the surfaces of Ti samples were polished before tests.Furthermore,it is impossible that the content of oxygen has the same
variation regularity with the length of YPE,i.e.increase with the
growth of grain(corresponding to annealing time)and then
decrease with the further enlargement of grain size.Therefore,
the YPE phenomenon in this study was expected to have no
relation with oxygen as well as other impurity atoms,such as
carbon and nitrogen in pureTi