Influence of pre-existing configurations of dislocations on the initial pop-in load during nanoindentation in a CrCoNi medium-entropy alloy

ABSTRACT

The origin and mechanisms responsible for incipient plasticity in metals are still poorly understood. Moreover, the reasons for the recently reported large scattering of the initial pop-in load remain unclear. Hence, this study addresses these issues through a combination of nanoindentation tests and electron channelling contrast imaging characterisation considering a CrCoNi medium-entropy alloy. Experimental findings were also supported by elastic calculations that consider both the indentation and dislocation stress fields. A wide scatter in the maximum shear stress underneath the indenter, as expected, was observed for the analysis based on dislocation density. As a consequence, the spatial arrangement of dislocations within the indented region or local dislocation configuration is introduced as a new parameter to overcome overly simple analysis based on the dislocation density. The maximum shear stress underneath the indenter increased from 6 GPa for dislocation closer to the indentation axis to 11 GPa at 600 nm for dislocation far away from it. Additionally, elastic calculations revealed that the response to the incoming nanoindenter was different for dislocations with different configurations. Thus, the complex interactions of stress fields due to configurations of dislocations and indentation account for the large scatter of the maximum shear stress beneath the indenter.

KEYWORDS:

• High-entropy alloy
• nanoindentation
• electron channelling contrast imaging
• plasticity
• density of dislocations

Disclosure statement

No potential conflict of interest was reported by the authors.

List of abbreviations

CrCoNi alloy	=	Chromium-Cobalt-Nickel alloy
HEA	=	High entropy alloy
MEA	=	Medium entropy alloy
ECCI	=	Electron channelling contrast imaging
SFE	=	Stacking fault energy
g	=	Diffraction vector
P-h curve	=	Load-displacement curve
R	=	Nanoindenter tip radius
h	=	Displacement depth during nanoindentation
P	=	Applied load during nanoindentation
${\tau }_{max}$	=	Maximum shear stress underneath the indenter
$P_p$	=	Pop in load
G	=	Shear modulus
${\tau }_{\mathrm{t}\mathrm{h}}$	=	Theoretical shear stress
HomND	=	Homogeneous nucleation of dislocation
HetND	=	Heterogeneous nucleation of dislocation
ρ	=	Global dislocation density
${\rho }_{\mathrm{l}\mathrm{o}\mathrm{c}}$	=	Local dislocation density
PED	=	Pre-existing dislocation
Dnd	=	Distance between the nanoindentation axis and the nearest dislocations
$a_c$	=	Contact radius
RSS	=	Resolved shear stress
MRSS	=	Maximum resolved shear stress
$\overline{L}$	=	Mean distance between dislocations
n	=	Number of dislocations
A	=	Area of interest in the micrograph
${\tau }_c$	=	Applied resolved shear stress necessary to nucleate a dislocation loop on the active slip plane
b	=	Burgers vector
$r_c$	=	Critical loop radius
$r_o$	=	Dislocation core radius
$p_o$	=	Maximum pressure
$E_r$	=	Reduced modulus
$E_s$	=	Young’s modulus of the sample
$E_i$	=	Young’s modulus of the diamond nanoindenter
${\nu }_i$	=	Poisson’s ratio of the diamond nanoindenter
${\nu }_s$	=	Poisson’s ratio of the sample
$\mathrm{\Delta }{h}_p$	=	Pop-in width
$h_p$	=	Pop-in depth
$\mathrm{\Delta }{h}_{hkl}$	=	Projection of $\mathrm{\Delta }{h}_p$ along the slip direction
$N_{hkl}$	=	Number of induced dislocations emitted below an indenter
T	=	Temperature
e	=	Euler number
${\tilde{\sigma }}_{ij}$	=	stress components of a point load applied on an elastic half-space
$\tilde{p}\left(x^{\mathrm{\prime }},y^{\mathrm{\prime }}\right)$	=	Surface pressure distribution
FCC	=	Face centered cubic

Additional information

Funding

G. L. acknowledges funding from the Deutsche Forschungsgemeinschaft through project B8 of the SFB/TR 103.