Microscale Testing and Characterization Techniques for Benchmarking Crystal. . .
97
the morphology of the subgrain structure in order to subsequently homogenize and
develop a single crystal model. This single crystal model is then brought up to the
polycrystalline scale at which individual grains and twins are organized together to
represent the microstructure of the material. The simulation predictions observed at
this scale can be directly compared and benchmarked, with the experimental results
captured at the mesoscale.
Alongside the ability to machine and test samples at an expanding number
of sizes and for a multitude of materials, characterization methods such as DIC
to observe strain and local deformation behavior, as well as 3D techniques for
characterizing microstructure, have led to the ability to interrogate material behavior
at extremely fine length scales. In addition, these advances in property and
microstructural characterization have supported the development of more informed
models, and modeling techniques such as CPFEM have allowed for the explicit
representation of microstructure in silico. Though significant progress has been
made in terms of ICME to date, most work has been done on pure materials;
structurally complex materials such as René 88DT and other Ni-base superalloys
remain a challenge. As such, the continued development of techniques to machine,
test, and characterize microtensile samples of commercially relevant materials is
needed to promote model development.
3 Machining Methods for Microscale Samples
Miniaturized mechanical testing is fast becoming a widespread technique for
capturing the influence of microstructure and measurements of local properties
[43, 55, 56]. Capturing these scale-specific properties is critical to benchmarking
and development of multiscale CPFEM models. However, at smaller length scales,
sample quality has a significant effect on its measured response. Defects and surface
roughness can lead to premature failure during tensile loading and inaccurate results,
which can be especially troublesome when there is uncertainty in the expected
outcome of an experiment. ASTM standards give some guidelines in terms of a
target of quality and dimensions of tensile samples, but do not necessarily provide a
methodology and best practices for fabricating samples, especially at the microscale
[57]. At this length scale, traditional machining methods are not applicable because
of inherent limits in the dimensional tolerance of the surface finish.
The three microtensile sample preparation techniques that were utilized in the
current study of René 88DT are: focused ion beam (FIB) milling, femtosecond laser
machining, and wire electric discharge machining (EDM). Before machining of the
final microtensile sample geometries, thin foils were excised from a bulk piece of
René 88DT and polished to the proper thickness and surface finish. The foils were
prepared to have a specific final thickness that would allow for machining of samples
with a square cross section. The initial slices were machined using wire EDM. Due
to the recast layer created by the wire EDM, foils were cut to be 100 μm thicker
than the final desired thickness and subsequently polished on both sides using SiC
97
the morphology of the subgrain structure in order to subsequently homogenize and
develop a single crystal model. This single crystal model is then brought up to the
polycrystalline scale at which individual grains and twins are organized together to
represent the microstructure of the material. The simulation predictions observed at
this scale can be directly compared and benchmarked, with the experimental results
captured at the mesoscale.
Alongside the ability to machine and test samples at an expanding number
of sizes and for a multitude of materials, characterization methods such as DIC
to observe strain and local deformation behavior, as well as 3D techniques for
characterizing microstructure, have led to the ability to interrogate material behavior
at extremely fine length scales. In addition, these advances in property and
microstructural characterization have supported the development of more informed
models, and modeling techniques such as CPFEM have allowed for the explicit
representation of microstructure in silico. Though significant progress has been
made in terms of ICME to date, most work has been done on pure materials;
structurally complex materials such as René 88DT and other Ni-base superalloys
remain a challenge. As such, the continued development of techniques to machine,
test, and characterize microtensile samples of commercially relevant materials is
needed to promote model development.
3 Machining Methods for Microscale Samples
Miniaturized mechanical testing is fast becoming a widespread technique for
capturing the influence of microstructure and measurements of local properties
[43, 55, 56]. Capturing these scale-specific properties is critical to benchmarking
and development of multiscale CPFEM models. However, at smaller length scales,
sample quality has a significant effect on its measured response. Defects and surface
roughness can lead to premature failure during tensile loading and inaccurate results,
which can be especially troublesome when there is uncertainty in the expected
outcome of an experiment. ASTM standards give some guidelines in terms of a
target of quality and dimensions of tensile samples, but do not necessarily provide a
methodology and best practices for fabricating samples, especially at the microscale
[57]. At this length scale, traditional machining methods are not applicable because
of inherent limits in the dimensional tolerance of the surface finish.
The three microtensile sample preparation techniques that were utilized in the
current study of René 88DT are: focused ion beam (FIB) milling, femtosecond laser
machining, and wire electric discharge machining (EDM). Before machining of the
final microtensile sample geometries, thin foils were excised from a bulk piece of
René 88DT and polished to the proper thickness and surface finish. The foils were
prepared to have a specific final thickness that would allow for machining of samples
with a square cross section. The initial slices were machined using wire EDM. Due
to the recast layer created by the wire EDM, foils were cut to be 100 μm thicker
than the final desired thickness and subsequently polished on both sides using SiC