  Fretting wear is a surface degradation process caused by oscillatory motion
and contact slipping. During gross slip, high local stresses and plastic
deformation in the surface and subsurface can lead to the creation of a
nanosized grained structure called Tribologically Transformed Structure (TTS).
The current paper studies the formation of TTS in an alpha-beta Ti-6Al-4V alloy
under fretting loading while changing the contact pressure and the number of
fretting cycles.Cross-sections of wear scars are observed after polishing and
chemical etching. Above a threshold pressure of 300 MPa, TTS appears early in
the contact (before 1000 cycles) along with two other structures: a Third Body
Layer (TBL) made of compacted debris and a General Deformed Layer (GDL) which
is the plastic zone under the TTS. TTS first appears as islands and merges in
the middle of the contact after enough cycles. Below 200 MPa, only TBL and GDL
are formed. At 200 MPa, only small, localized TTS is found. All structures have
the same chemical compositions as the initial bulk material except for the
nitrided TBL. TTS has a very high hardness compared to the bulk. TTS was
carefully extracted using a Focused Ion Beam (FIB) and its microstructure was
observed with a Transmission Electron Microscope (TEM). It shows extreme grain
refinement and is composed of two alternated zones. The first zone I is
composed of $\alpha$ grains with a size of 20 to 50 nm with crystallographic
texture. Zone II comprises nanosized equiaxed grains whose sizes range from 5
to 20 nm without texture. The results made it possible to establish a scenario
of the appearance of the TTS according to the conditions of contact pressure
and number of fretting cycles.
