The microstructure of a material is usually studied by using metallographic observation
methods by which it is possible to qualitatively or semi-quantitatively investigate the
changes in the state of the microstructure. Currently, researches on digital characterization of
the microstructure and machining-induced material-altered layer for different difficult-to-cut
materials are rarely reported. Many researches had mainly focused on the formation of
hardened white layer on the surface of steels [55-59]. Chou et al investigated the factors that
lead to the formation of white layer by using both theoretical analysis and experimental
observation [56]. They concluded that abrupt temperature change, excessive deformation,
original grain size and material properties are the key factors for the formation of the white
layer. Barry studied the hardened white layer of high-strength steel of the surface and
analyzed the reasons for the formation of the white layer [60]. Han made a similar study and
proposed the thickness of the white layer as an objective of interest, through which the
formation mechanism of the white layer and the corresponding variation in microhardness
cutting and investigated the performance of the white layer and the grey layer by analyzing
the variation of microhardness [62-63]; this laid the foundation for microhardness and
microstructure analyses by using the finite element method.
Nowadays, existing studies on material-altered layers are numerous, but they are not
systematically organized; some key issues and the correlation between formation mechanism,
dislocation and phase transformation within the material-altered layers are still not well
identified. Ezugwu compared the surface damage and corresponding subsurface
microstructures of G-17 steel and nickel-based high temperature alloy Inconel 718 which are
machined with different tools [64]; the result suggests that the tool selection and workpiece
material properties are vital to the surface integrity characteristics of machined parts. Zou
investigated the machinability of nickel-based superalloy NiCr20TiAl using a Polycrystalline
cubic boron nitride (PCBN) cutting tool; the machining-induced surface voids, inclusions
and slip-lines being observed in detail [65]. Further, the machining-induced burrs and its
chemical composition were analyzed using energy spectral density (ESD). Obikawa [48]
investigated the relationship between cutting force, surface residual stress and microstructure
for dual-phase steel using finite element numerical simulation. Fox-Rabinovich studied the
difference of the microstructure in the material-altered layer of machined parts which are
processed using a coated and an uncoated tool respectively [66]. They used an optical
microscope, a scanning electron microscope (SEM), an x-ray diffractometer (XRD), an x-ray
photoelectron spectroscope (XPS) and a high resolution electron energy loss spectroscope
in the subsurface microstructure of parts machined by the two types of tools. Although these
were not relating to quantitative analysis of the microstructure, this research could still help
to further investigate and characterize the microstructure of the material-altered layer with
multiple means of experimental observation.
Lehnert analyzed the changes in the microstructure of aluminum and copper materials
processed by hot rolling, but their experimental work was based on light microscopy analysis
and was still not concerned with the quantitative description of the microstructure [67]. Tang
et al studied and obtained the changes of phase volume fraction in the microstructure using
numerical simulation method; the correlation between stress, strain and microstructure are
also established based on the different phase volume fraction. However, characterization of
microstructure with volume fraction does not apply to the case of single-phase structure [68].
Li proposed and summarized three numerical methods for microstructure simulation: namely
the Monte Carlo method, cellular automata method, phase-field method [69]. These
numerical methods are implemented in different ways and are mainly used to characterize
the grain size of microstructure.
The surface stress state of machined parts will vary when undertaking fatigue loading; the
subsurface microstructure will also experience variation due to the dramatic changes in
temperature during the machining process. After machining, the rough surface and
nonuniform microstructure below the surface will cause uneven distribution of stress within
the material. Stress concentration is easily generated at these locations and are the main
loading for dual-phase steel [70]. Simultaneously, they also investigated and compared the
microstructure of 2Cr13 steel using the transmission electron microscope (TEM) when the
samples experienced different cycles of fatigue. The microscopic explanation for damage
evolution under low cycle fatigue process is also given.
From the perspective of micromechanics, Yuan studied the relationship between dislocation
and the minimum depth of cut when using the rounded cutting tool by using TEM [71]. It is
found that the main dislocation density will rise with the increase of tool radius; the larger
the tool radius, the more mechanical deformation occurs and the greater the resultant
dislocation generates. Yashiro analyzed the dislocation motion at the interface for
precipitation hardening type nickel-base superalloy; numerical simulation being adopted to
study the dislocation accumulation and grain boundary during the nucleation of the / '
interface [72]. Three dislocation motion models are used to reveal the dislocation formation
at the / ' interface. Tang studied the properties of silicon by means of molecular dynamics
(MD) simulation technology and atomic force microscopy (AFM) [73]. The result showed
that the shear strength of the dislocation is much less than the yield strength of the silicon.
Dlouh studied the dislocation variation of heat resisting nickel-base alloy
16Cr-10W-4Mo-TiA1 during creep using TEM [74]; the results showed that the dislocation
movement could be an alternative to well explain the material deformation and
microhardness change.