130
A. Cruzado et al.
3.1 Micromechanical Characterization
3.1.1 Experimental Procedure
Compression tests were performed on micropillars machined out of grains of a
coarse-grained Inconel 718 alloy. The micropillars were milled in the center of
selected grains to avoid grain boundary effects, using FIB (FEI Helios Nanolab
600i) following an annular milling strategy, with a final polishing step using a
current of 230 pA to minimize FIB surface damage. The micropillar aspect ratio
(length/diameter) was selected to be 2.4 to avoid buckling during compression [54].
The annular milling parameters used resulted in a minimum taper (<1.5 ◦ ) of the
pillars. The pillars were compressed inside an instrumented nanoindentation system
(Hysitron TI950) using a circular diamond flat punch of 10 μm in diameter.
Tests were carried out under displacement control at three different strain rates
(10 −4 , 10 −3 , and 10 −2 s −1 ), and the diameter was varied between 1 and 7.5 μm
to determine the effect of the diameter on the mechanical response. Inconel 718
crystals deform according to the lattice structure of the γ phase, along 12 octahedral
{111} }110 slip systems. Therefore, different crystallographic orientations were
selected for testing in order to promote single, double (coplanar and noncoplanar),
and multiple slip systems. The crystallographic orientation of the pillars was
determined by EBSD, using an Oxford AZTEC system. The Sneddon correction
[66] was applied in the obtained load displacement curves to account for the extra
compliance associated with the elastic deflection of the matrix at the base of the
pillar.
3.1.2 Results
The effect of the pillar diameter in the mechanical behavior was first assessed by
performing micropillar compression on pillars with diameters ranging between 1
and 7.5 μm built in grains with similar crystallographic orientation. In this study
grains favorably oriented for single slip, in either 245 or 235, with Schmid
factors (SF) of 0.445 and 0.4512, respectively, were selected.
The corresponding resolve shear stress (RSS) vs. strain curves are shown in
Fig. 2a. The pillars ranging between 3 and 7.5 μm present similar response, while
the micropillar with diameter of 1 μm presents stiffer and stronger response. A small
difference is also presented in the initial slope, which shows a much stiffer behavior
as the pillar diameter decreases. This is consequence of the initial contact between
the punch and the micropillar, affected by the surface asperities or the incorrect
alignment between the flat punch and the head of the pillar, as described in [67].
These results confirm that, opposite to what is found when single crystals of pure
metals are tested [73], the micropillar behavior is size independent for diameters
above 3 μm. The independence of the pillar response with the diameter size is the
consequence of the γ + γ precipitate spacing (≈50 nm), which is much smaller
A. Cruzado et al.
3.1 Micromechanical Characterization
3.1.1 Experimental Procedure
Compression tests were performed on micropillars machined out of grains of a
coarse-grained Inconel 718 alloy. The micropillars were milled in the center of
selected grains to avoid grain boundary effects, using FIB (FEI Helios Nanolab
600i) following an annular milling strategy, with a final polishing step using a
current of 230 pA to minimize FIB surface damage. The micropillar aspect ratio
(length/diameter) was selected to be 2.4 to avoid buckling during compression [54].
The annular milling parameters used resulted in a minimum taper (<1.5 ◦ ) of the
pillars. The pillars were compressed inside an instrumented nanoindentation system
(Hysitron TI950) using a circular diamond flat punch of 10 μm in diameter.
Tests were carried out under displacement control at three different strain rates
(10 −4 , 10 −3 , and 10 −2 s −1 ), and the diameter was varied between 1 and 7.5 μm
to determine the effect of the diameter on the mechanical response. Inconel 718
crystals deform according to the lattice structure of the γ phase, along 12 octahedral
{111} }110 slip systems. Therefore, different crystallographic orientations were
selected for testing in order to promote single, double (coplanar and noncoplanar),
and multiple slip systems. The crystallographic orientation of the pillars was
determined by EBSD, using an Oxford AZTEC system. The Sneddon correction
[66] was applied in the obtained load displacement curves to account for the extra
compliance associated with the elastic deflection of the matrix at the base of the
pillar.
3.1.2 Results
The effect of the pillar diameter in the mechanical behavior was first assessed by
performing micropillar compression on pillars with diameters ranging between 1
and 7.5 μm built in grains with similar crystallographic orientation. In this study
grains favorably oriented for single slip, in either 245 or 235, with Schmid
factors (SF) of 0.445 and 0.4512, respectively, were selected.
The corresponding resolve shear stress (RSS) vs. strain curves are shown in
Fig. 2a. The pillars ranging between 3 and 7.5 μm present similar response, while
the micropillar with diameter of 1 μm presents stiffer and stronger response. A small
difference is also presented in the initial slope, which shows a much stiffer behavior
as the pillar diameter decreases. This is consequence of the initial contact between
the punch and the micropillar, affected by the surface asperities or the incorrect
alignment between the flat punch and the head of the pillar, as described in [67].
These results confirm that, opposite to what is found when single crystals of pure
metals are tested [73], the micropillar behavior is size independent for diameters
above 3 μm. The independence of the pillar response with the diameter size is the
consequence of the γ + γ precipitate spacing (≈50 nm), which is much smaller