  We report the mechanical properties of different two-dimensional carbon
heterojunctions (HJs) made from graphene and various stable graphene
allotropes, including {\alpha}-, {\beta}-, {\gamma}- and 6612-graphyne (GY),
and graphdiyne (GDY). It is found that all HJs exhibit a brittle behaviour
except the one with {\alpha}-GY, which however shows a hardening process due to
the formation of triple carbon rings. Such hardening process has greatly
deferred the failure of the structure. The yielding of the HJs is usually
initiated at the interface between graphene and graphene allotropes, and
monoatomic carbon rings are normally formed after yielding. By varying the
locations of graphene (either in the middle or at the two ends of the HJs),
similar mechanical properties have been obtained, suggesting insignificant
impacts from location of graphene allotropes. Whereas, changing the types and
percentages of the graphene allotropes, the HJs exhibit vastly different
mechanical properties. In general, with the increasing graphene percentage, the
yield strain decreases and the effective Young's modulus increases. Meanwhile,
the yield stress appears irrelevant with the graphene percentage. This study
provides a fundamental understanding of the tensile properties of the
heterojunctions that are crucial for the design and engineering of their
mechanical properties, in order to facilitate their emerging future
applications in nanoscale devices, such as flexible/stretchable electronics.
