*3.1. Evolution of the Microstructure Along the Gradient*

The microstructure features of the different graded materials in their as-deposited states has been detailed previously [34,35]. The main features are just recalled in Figure 1b,c for the sake of clarity. The evolution of the microstructure, and in particular the parent grain morphologies after a reconstruction from the martensitic ' variants, is illustrated in Figure 1b,c. The evolution of the microstructure for the Ti-Nb and TA64-Mo walls shared a number of aspects and the features shown in Figure 1b,c for TA64-Mo pertain for the other FGM sample. Because of the fast cooling rate [34,35], the initial layers of Ti or Ti6Al4V were characterized respectively by a Widmanstätten or a martensitic α' microstructure, issued from the high temperature parent phase nucleated from the melt (Figure 1c). With the addition of Nb or Mo, the microstructure became beta equiaxed with a decreasing grain size as the alloying element amount increased (Figure 1b). In the pure Niobium (Nb) or pure Molybdenum (Mo) layers, the beta grains were columnar parallel to the z axis (deposition axis).

Figures 2 and 3 give SEM images showing details of the microstructures after the application of the SMAT process. In Ti and Ti6Al4V as well as at the lowest amount of additions (25% of Mo), the top surface microstructure was refined over a depth of the order of 5 to 15 μm. This is particularly visible in Figure 3a,b where the grains are refined and equiaxed near the surface while elongated grains are visible at the sub-surface. In addition, for the initial structure, kink bands appeared in the subsurface over significant thicknesses (>200 μm). This type of kink band has already been observed in highly alloyed beta titanium alloys during deformation at high speed rate [41]. The width of the zone where these kink bands were present was thicker for the case of the Nb addition than for the Mo one (Figures 2b and 3b). Indeed, the kink bands reached the surface for the Ti-25Nb alloy, whereas for the Ti6Al4V-25Mo alloy, the nanostructured layer was present from the surface towards 15 μm below the surface (Figure 3b). From 50% of alloying elements, the kink bands disappeared and the contrast visible within the beta grains at the SMATed surface witnessed strong internal misorientations. Figures 2c–e and 3c–e show that the thickness of this layer depends on the amount in alloying element. A comparison of the blue and red arrows in Figures 2c–e and 3c–e indicates, however, that its thickness is equivalent for both the Ti-Nb and Ti6Al4V-Mo alloys at a given amount of alloying element.

**Figure 2.** (**a**–**e**): cross section pictures of the surface mechanical attrition treatment (SMAT)ed TiNb sample focused on the severe plastic deformed region (the SMATed edges are always at the bottom of the pictures); (**b**): in the top right corner, focus on the kink bands at 20 μm from the surface in the Ti-25Nb; (**c**–**e**): the blue arrow represents the affected depth.

**Figure 3.** (**a**–**e**): Cross section pictures of the SMATed TiMo sample focused on the severe plastic deformed region (the SMATed edges are always at the bottom of the pictures); (**b**): in the top right corner, focus on the kink bands at 20 μm from the surface in the Ti6Al4V-25Mo; (**c**–**e**): the red arrow represents the affected depth.
