  Thin foils of the double solid solution
$(Zr{_0.5},Ti{_0.5})_{2}(Al_{0.5},Sn_{0.5})C$ MAX phase were in situ irradiated
in a transmission electron microscope (TEM) up to a fluence of
$1.3\times10^{17} ions$ $cm^{-2}$ (${\sim}$ 7.5 dpa), using 6 keV $He^{+}$
ions. Irradiations were performed in the 350-800 {\deg}C temperature range. In
situ and post-irradiation examination (PIE) by TEM was used to study the
evolution of irradiation-induced defects as function of dose and temperature.
Spherical He bubbles and string-like arrangements thereof, He platelets, and
dislocation loops were observed. Dislocation loop segments were found to lie in
non-basal-planes. At irradiation temperatures ${\geq}$ 450 {\deg}C, grain
boundary tearing was observed locally due to He bubble segregation. However,
the tears did not result in transgranular crack propagation. The intensity of
specific spots in the selected area electron diffraction patterns weakened upon
irradiation at 450 and 500 {\deg}C, indicating an increased crystal symmetry.
Above 700 {\deg}C this was not observed, indicating damage recovery at the high
end of the investigated temperature range. High-resolution scanning TEM imaging
performed during the PIE of foils previously irradiated at 700 {\deg}C showed
that the chemical ordering and nanolamination of the MAX phase were preserved
after 7.5 dpa $He^{+}$ irradiation. The size distributions of the He platelets
and spherical bubbles were evaluated as function of temperature and dose.
