Investigation of graded strengthened hyper-deformed surfaces by impact treatment: micro-percussion testing

In the industry, mechanical surface treatments could improve the mechanical behaviour of materials by the means of local hyper-deformation and graded strengthening. Micro-percussion test represents an interesting case scenario to emulate these kinds of conventional treatments (shot-peening, SMAT, roller-burnishing, etc) and go further on microstructural and mechanical characterization at local and global scales. For this technique, every impact is made at the same position by a rigid conical indenter, controlling the number, angle and velocity of impacts. The main issue of this work is to establish a complete description of the transformed microstructures; to understand the mechanisms involved on the formation and growth of refined structures; to make a parametric sensitivity analysis of different impact conditions.


Introduction
The mechanical surface treatments confer better local mechanical properties against wear or fatigue service conditions. In the case of conventional impact treatments (shot-peening [1]- [2], SMAT [3]), the material is exposed to repeated mechanical loadings, producing a severe plastic deformation (SPD) in the near-surface. It leads to a local progressive refinement of the microstructure over a few tens of microns [2]- [3], commonly known as Tribologically Transformed Structure (TTS) [4]. In this work, a model surface treatment (micro-percussion [5]- [6]) is used to explore several impact conditions (angle and amount of impacts) and its effects on the microstructural transformation. In this work, three main goals are considered: (i) characterize the transformed surfaces and evaluate the possibility to emulate industrial techniques (ii) make a parametric analysis, (iii) describe the microstructural transformation process and the mechanisms involved on it. This investigation is carried out in pure α-iron.

Material: α-iron
The micro-percussion tests are done in a high-purity α-iron with less than 15 ppm of carbon. This material is produced by the cold crucible melting method. The resulting metallic bar is thermomechanically treated by forging and annealing at 800 ºC during 60 minutes. This treatment produces a homogeneous microstructure with equi-axed grains (average size in the order of 1 mm). No inclusions have been observed in this model material.

Micro-percussion tests
The principle of micro-percussion tests [5]- [6] is to carry out several impacts in the same position of the sample with a rigid conical punch (carbide tungsten). For metallic materials, the SPD produces a remaining print of millimetric diameter. The indenter tip has a spherical shape with a radius of 0.5 mm and an angle of 60º (Figure 1-a). A schematic representation of the test is shown in Figure 1-b. The impact set-up permits to control the number and velocity of impacts [7]. The impact angle is introduced directly on the sample geometry (tilted plane). Three angles were explored for the impact conditions: 0º, 15º and 30º. At each angle, the prints are done for different amounts of impacts: 10, 30, 100, 300, 1000, 3000 and 10000. Prints are arranged in a rectangular matrix (2x4), spaced of 2 mm for each line and 3 mm between both lines (Figure 1-b). Only the 10000 impacts print is spaced of 4 mm because of its high diameter. Two main conditions were considered to make a parameter sensitivity analysis: (i) the angle and (ii) the number of impacts. Concerning the velocity of impacts, some preliminary tests pointed out that the set-up working range for α-iron is enclosed between 80 mm/s and 200 mm/s [7]. Then, the velocity used for all the tests was in the order of 150 mm/s. three angles (0º, 15º, 30º) and seven impact amounts (10, 30, 100, 1k, 3k, 10k) at 150 mm/s.

Near-surface microstructural characterization
The characterisation of the resulting prints was done using scanning electron microscopy (SEM) and electron back-scattered diffraction (EBSD) mapping. Both techniques were carried out on the print cross-section in order to observe the near-surface microstructural transformation. For both methods, the samples were cutted with a metallic wire saw and then mechanically polished up to the middle plane of each print. The sample surfaces were polished with several abrasive papers (P240 to P1200), two diamond suspensions (3 μm and 1 μm) and finished with colloidal silica. The SEM images were done in a Zeiss Supra 55 VP at 20 kV using a back-scattered detector (BSE). The EBSD maps were carried out in a Tescan Lyra3/XMU (FEG/FIB) using an indexation step of 2 μm.

Characteristics of a print: different deformed regions
An example of a characteristic print is shown in Figure 2 mechanically affected zone (MAZ). On the one hand, the mechanically attrited structure (MAS) [6] is essentially characterized by the presence of high local misorientations and new well-defined grain boundaries [9]. On the other hand, the low angle boundaries (LAB's) zone corresponds to a nonmicrostructured region in spite of the plastic deformation (Figure 2-a) [10]. Indeed, some previous works [11]- [12] revealed that this kind of region has some slight crystal orientation nuances and it could be considered as a single crystal zone due to the initial grain sizes in the bulk material.
Furthermore, the MAS region (total thickness of 150 μm) is defined by the TTS and the transition zones (Figure 2-b). The tribologically transformed structure (TTS) corresponds to the nearest surface layer with a progressive grain size refinement in depth [4]. The transformed microstructure in this case is considerably similar to the TTS regions obtained by other surface treatements (shot peening [2], [12], [14], SMAT [3], [15], roller-burnishing [16]- [17]). Beyond the TTS layer, the transition zone is characterized by the presence of shear bands (white arrow) [13] and sub-grain boundaries (not wellformed grains) [9]. This zone is an intermediary region between the well-refined microstructure (TTS) and the "single crystal" region deformed at low strain rates (LAB's zone).

Kinetics of the TTS layer transformation
The wide thickness and homogeneity of the TTS layer observed in Figure 2-a,b is definitively an interesting case scenario to emulate and investigate the transformed surfaces by the means of industrial impact treatments. However, the impact conditions will be decisive to create a significant TTS layer for this purpose. For example, as shown in Figure 3, three different prints were formed at 10000 impacts for different impact angles: (a) 0º, (b) 15º and (c) 30º. The normal impacts (0º) produce a shallow transformed region with less than 50 μm in-depth. On the contrary, the tilted impacts (15º and 30º) can produce TTS layers three times wider (more than 120 μm in-depth). Moreover, the TTS layer at 0º presents more surface heterogeneities, crack formations (white arrows) and material release [18].  The evolution of the TTS layer is shown in Figure 4 as a function of the impact amounts for 30º: (a) 10, (b) 100, (c) 1000 and (d) 10000 impacts. The TTS layer and the print diameter grow progressively with the number of impacts. The microstructural refinement leads sub-micrometric grain sizes (d < 1 μm) in the TTS layer for the highest amounts of impacts (Figure 4-h). On the contrary, the TTS layer is almost invisible beneath 100 impacts and the formation of well-defined grains is essentially nonexistent (Figure 4-g). Indeed, the mechanical deformation of few impacts is not enough to refine the near-surface microstructure [10], [13]. Likewise, considering the same number of impacts (10000) at different angles (0º, 15º, 30º), the refined microstructure and the TTS layer evolves differently (Figure 4-d,e,f). As shown in Figure 3, the tilted impacts (15º, 30º) are more propitious to have wider transformed regions.  Two parametric graphics are presented in Figure 5-a,b in order to quantify the impact condition sensitivity on the TTS layer formation as a function of the number of impact [6]: (a) the thickness of the TTS layer and (b) the mean diameter of the print. The first graphic (Figure 5-a) shows that the thickness of the TTS layer evolves following a power law behaviour. Indeed, the transformed region grows progressively up to 3000 impacts and then it stabilizes in a plateau. As observed in the SEM images presented below, the higher widths of the refined layer are measured for the cases of tilted impacts (15º and 30º). As shown in this graphic (Figure 5a), the thickness of these TTS layers could be more than twice the transformed region with normal impacts (0º). Likewise, it is particularly interesting that the curves for 15º and 30º are juxtaposed. The second graphic ( Figure 5-b) shows the growth of the mean diameter in a log-log scale. The print average diameter evolves in the same manner for all the impact angles (0º, 15º, 30º). It would mean that the impact tilt only affects the width of the refined region and not the global size of the print, in spite of the bulge formed on the side.

Discussion
The parametric analysis pointed out that the impact angle has a relevant local effect on the microstructural refinement without changing the global dimensions of the print. This could mean that the impact tilt is strongly related with the microstructural transformation mechanisms in the material due to the SPD [9], [13]. To disclose this question, two sequences of the TTS layer growth at 15º are presented in Figure 6: (a, b, c) SEM images and (d, e, f) EBSD maps. In both cases, the transformed layer grows from the external side of the print and evolves progressively further below to the center, just underneath the normal impact axis. It suggests that the microstructural transformation starts from a shearing contact process (white arrows) and spreads in-depth (red and black arrows) with a combined phenomenon of shear and normal impact. Considering that shear bands and dislocation cells are quite significant for the refinement process in high stacking fault energy materials (as pure α-iron [3]), it is not surprising that shear contact and normal impact take a major role on the near-surface transformation [9]- [10]. Furthermore, the EBSD maps show that well-defined grains are not formed anymore outside the TTS region. Indeed, the remaining bulk grains beneath the refined layer perceive a low crystal misorientation in spite of the SPD induced in the mechanically affected zone (MAZ).

Conclusions
A parametric investigation had been presented in this work in order to understand the effects of different impact conditions (micro-percussion) on the microstructural refinement of hyper-deformed surfaces. The main results had shown that tilted impacts (θ >> 0º) could produce TTS layers much wider (at least twice) than those formed by normal impacts (θ = 0º). Furthermore, it had been pointed out that the refined layer (sub-micrometric grains) grows progressively with the number of impacts. However, the microstructural transformation is done by a combined effect of shearing contact and normal impact, probably related with the microstructural refinement mechanisms in high stacking fault energy materials (pure α-iron): (i) the shear bands and (ii) the dislocation mobility [10].
From this work, micro-percussion tests could be an interesting method to emulate the transformed surfaces obtained by the means of conventional techniques (as shot-peening or SMAT). Firstly, the refined microstructure in the near surface (for α-iron) is absolutely comparable with the one obtained using industrial procedures [11], [12], [14]. Secondly, the size of the transformed layer is wide enough to be compared with conventional treatments and to analyse the microstructural and the mechanical gradients with several characterization techniques (as SEM, EBSD, nano-indentation, micro-pillar compression, etc.) [11]- [12]. Furthermore, this work highlights that α-iron is an appropriate model material to obtain well-defined TTS and it is favourable for experimental characterization methods. For a future work, it would be interesting to go further on the local mechanical characterization (nanoindentation [19] and micro-pillar compression [15]) and match it with the microstructural characterization presented above. This coupled experimental analysis would reveal the influence of different strengthening effects in surfaces as the Hall-Petch effect or the work hardening [20]- [21].