3.1. Indentation Deformation of the Hot-Rolled CP-Ti
Figure 1 exhibits the original indentation load-displacement curves of the hot-rolled CP-Ti with various maximum indentation loads for the sample with 60% thickness reduction. The lager indentation load, as expected, the deeper indentation depth. It can also be seen that the five loading curves overlap with various indentation loads, which means the different loading rate has a negligible effect on the microindentation behavior for the hot-rolled CP-Ti under the various loading conditions. There exists a mild increase in unloading curve slope with the increasing of the maximum load, which can be explained by the elastoplastic recovery of sample. There exists strong interaction among the dislocations and the pile-up of dislocations in plastic deformation zones during the loading-unloading progress. During the unloading process, the mobile dislocations continuously move to other slip planes under the influence of plastic deformation. After reaching a certain state, these slip planes will combine with each other and annihilate, which can reduce strain energy and improve material stability. At the same time, dimensional deformation is also significantly reduced, which will have a certain impact on material properties [
25].
Figure 2 exhibits the typical indentation loading-unloading curves of the hot-rolled CP-Ti sheets with different thickness reductions under the maximum load of 100 mN. The five samples have a similar indentation curve. The 0% thickness reduction specimen without rolling has the largest indentation depth, and the 80% thickness reduction one has the least indentation depth. The reason is that there appears strain hardening related to the plastic deformation of CP-Ti sheets in hot rolling process which will be discussed later. The strain hardening enhanced with the increasing of the thickness reduction which is positive related to plastic deformation. This suggests that the specimen with maximum deformation (80%) experienced the largest resistance to indentation deformation and has the highest dislocation density.
Generally speaking, the relation between micro indentation load and depth for a homogeneous material can be expressed as follows [
26]
In which
F represents indentation load,
Hm and
Hr are maximum indentation depth and residual indentation depth, respectively.
Km and
Kr is a constant related to the material elastoplastic feature, and
n is a constant index. From
Figure 1, the maximum depth can be obtained for the corresponding maximum micro indentation load. As a result, the relation of maximum micro indentation depth and maximum indentation load for CP-Ti sheets can be obtained in
Figure 3. As expected, the indentation displacement enlarged with the increase of the load. Otherwise, the much severer plastic deformation (the larger thickness reduction), the shallower of the indentation depth. Based on Equation (1) to curve-fit the data in
Figure 3, the power index can be acquired,
n = 1/
k ≈1.8, which is different from 1.5 for the microindentation results of CP-Ti treated via ECAP using the same diamond Vickers indenter [
25]. This means there is a little surface microstructure difference between the hot-rolling and the ECAP process after plastic deformation for the same Ti sheets.
Similarly,
Figure 4 exhibits the relation between the residual depth and the maximum micro indentation load for CP-Ti sheets. The larger micro indentation load, the much severer the plastic deformation and the deeper residual indentation depth, as expected. The hot-rolled CP-Ti samples of thickness reduction of 80% exhibits minimum residual depth and the 0% ones have the largest residual depth under the same micro indentation load, suggesting the influence of plastic deformation after hot rolling. The exponential indices,
m =1/
k ≈1.8, in Equation (1) can also be obtained, which represents the association of the indentation load and the residual indentation depth. Here,
m is compatible with
n for power relation. Both these two exponential indexes are less than 2 as obtained Chen et al. [
25,
27], suggesting that the micro indentation size influences the indentation behaviors for the CP-Ti sheets. Specially, the reasons of indentation size effect are complex. The deviation of power index from 2 or the indentation size effect can be attributed to the influence of surface interaction, the blunting of the indenter tip, and the local residual stress gradient [
28].
3.2. Vicker Hardness of the Hot-Rolled CP-Ti
As everyone knowns that there exists strain hardening which is related to the dislocation multiplication after hot rolling, leading to the enhanced resistance to dislocation motion. From the previous work [
29], the relationship between the Vickers hardness,
H, and strain, ε
eff, can be expressed as
In which
H0 represents the Vicker hardness at ε
eff =0, α corresponds to a constant, μ refers to the sear modulus,
b refers to Burger’s vector,
t corresponds to the thickness of the hot-rolled CP-Ti plate, ∆
t/
t0 is the thickness reduction, and
n equals to the strain exponent.
Figure 5 exhibits the relation between the Vickers hardness and the effective strain for CP-Ti samples with indentation load of 200 mN. Through the Equation (2) to fit the data in
Figure 5, it can be obtained that
n=0.5 which is the same value of dislocation movement mechanisms during the plastic deformation, confirming the dominant role of the dislocation motion in the indentation deformation for the hot-rolled CP-Ti sample.
To explore the influence of the micro indentation load on indentation Vickers hardness, which was defined as the micro indentation load divided by the projected area of the indentation.
Figure 6 shows the dependence of the average indentation Vickers hardness on the maximum micro indentation load. It can be seen that the average Vickers hardness decreases with the increase of the micro indentation load. The value of indentation Vickers hardness reduced drops slightly from 1.92 to 1.52 GPa for the specimen without hot rolling with the maximum load in range of 100 to 500 mN. Similar trend appeared for the other four hot-rolled samples demonstrate the indentation size effect [
30]. From the curve-fitted spot lines in the
Figure 6, the slope of the line representing for the relationship exponential index between average Vickers harness and indentation load can be obtained from 1.83 to the maximum value 2.0, which is the same range as the results by Chen et al. [
27], determining the close relation between the indentation Vickers hardness and indentation load for CP-Ti alloys.
Figure 7 displays the dependence of average Vickers hardness on the residual micro indentation depth. With the increase of the residual micro indentation depth, the average indentation Vickers hardness decreases slightly for all the hot-rolled CP-Ti sheets, indicating indentation size effect, which is as expected from the exponential index of
m in Equation (1). Also, combined with the results from
Figure 4 and
Figure 6, the hot-rolled CP-Ti sheets with 80% thickness reduction which owns the shallowest indentation depth exhibited maximum hardness, and the CP-Ti sheets for 0% thickness without hot-rolling has the smallest indentation hardness. This demonstrated that, the average Vickers harness enhanced with the increase extent of plastic deformation due to the work hardening caused by dislocation motion.
3.3. Energy Dissipation of the Hot-Rolled CP-Ti
For the indentation loading-unloading curves, the plastic energy was determined by using the total energy (
Eloading) stored and the elastic energy dissipated (
Eunloading) in the sample as:
Figure 8 shows the
Eplastic (plastic energy dissipated) in the indentation of all the hot-rolled CP-Ti samples with indentation load of 100 mN. It can be observed clearly that the plastic energy dissipated can be expressed as a nonlinear function of the thickness reduction after the hot-rolling treatment. The 0% thickness reduction specimen has the highest energy dissipation and maximum ductility, and the hot-rolled sample with 80% thickness reduction exhibits lowest energy dissipation. Such a result can be explained that more thickness reduction causes the increased dislocation density during the plastic deformation processing. The larger the thickness reduction, the higher the dislocation density for the severe distorted CP-Ti sheets. The
Eplastic in the micro indetation deformation of crystallized metals is a function of material microstructure, it should be addressed in the analysis.
To analyze the relationship between plastic energy dissipated and micro indentation depth,
Figure 9 displays the influence of residual indentation depth on plastic energy dissipated for different hot-rolled samples. Combined with
Figure 8, it can be seen that the higher plastic energy dissipated, the lower indentation residual depth, corresponding the smaller effective strain for the CP-Ti sheets. Through the data points fitting, straight line with a slope,
k, of 0.8127 is obtained, which suggest that the plastic energy dissipated is proportional to the third power of indentation residual depth.
The ratio of plastic energy dissipated to the total energy,
Eplastic/
Etotal, can be determined via the typical indentation load-displacement curves. The effect of the plastic strain on the hot-rolled CP-Ti sheets on energy ratio can be seen from
Figure 10. The error bars in the figure represent the different results for various indentation loads. It can be indicated that the energy ratio is independent with micro indentation load for both the as-annealed and hot-rolled Ti alloys, which is consistent with Malzender’s work [
31]. Obviously, the ratio is nonlinear relationship of the plastic strain after hot-rolling treatment, which is inversely related to the plastic strain. The 0% thickness reduction sample without hot rolling exhibits the maximum energy ratio. This can be explained by the higher dislocation density for the hot-rolled CP-Ti sheets after plastic deformation, which increases nominal hardness. The result indicated that the energy ratio is related to material microstructures. We should conduct in-depth research on this relationship and clarify its mechanism to provide support for improving the performance of materials, which is also one of the main research hotspots in this field.
Several works [
31,
32] have proposed a linear relationship between the micro indentation energy ratio,
Eplastic/
Etotal, and the micro indentation depth retio,
Hr/
Hm, for
Hr/
Hm > 0.4.
Figure 11 exhibits the variation of
Eplastic/
Etotal, as a function of
Hr/
Hm, for different CP-Ti samples at the indentation load of 100 mN. The value of
Eplastic/
Etotal increase with the increase of
Hr/
Hm. The results indicated that the linear relation can also be used for hcp Titanium alloys materials (seen the dotted line in
Figure 11), which is independent of the deformation history. Thus, a reasonable conclusion can be drawn that the linear association between the
Eplastic/
Etotal and
Hr/
Hm is suitable for all undeformed and deformed crystalline ductile materials.