Determining the Mechanical Strength of Ultra-Fine-Grained Metals

Summary

The protocol presented here describes the high-pressure radial diamond-anvil-cell experiments and analyzing the related data, which are essential for obtaining the mechanical strength of the nanomaterials with a significant breakthrough to the traditional approach.

Abstract

The mechanical strengthening of metals is the long-standing challenge and popular topic of materials science in industries and academia. The size dependence of the strength of the nanometals has been attracting a lot of interest. However, characterizing the strength of materials at the lower nanometer scale has been a big challenge because the traditional techniques become no longer effective and reliable, such as nano-indentation, micropillar compression, tensile, etc. The current protocol employs radial diamond-anvil cell (rDAC) X-ray diffraction (XRD) techniques to track differential stress changes and determine the strength of ultrafine metals. It is found that ultrafine nickel particles have more significant yield strength than coarser particles, and the size strengthening of nickel continues down to 3 nm. This vital finding immensely depends on effective and reliable characterizing techniques. The rDAC XRD method is expected to play a significant role in studying and exploring nanomaterial mechanics.

Introduction

The resistance to plastic deformation determines the materials' strength. The strength of the metals usually increases with the decreasing grain sizes. This size strengthening phenomenon can be well illustrated by the traditional Hall-Petch relationship theory from the millimeter down to submicron regime^1,2, which is based on the dislocation-mediated deformation mechanism of bulk-sized metals, i.e., dislocations pile up at grain boundaries (GBs) and hinder their motions, leading to the mechanical strengthening in metals^3,4.

In contrast, mechanical softening, often referred to as the inverse Hall-Petch relationship, has been reported for fine nanometals in the last two decades^5,6,7,8,9,10. Therefore, the strength of the nanometals is still puzzling as continuous hardening was detected for grain sizes down to ~10 nm^11,12, while the cases of size softening below 10 nm regime were also reported^7,8,9,10. The main difficulty or challenge for this debated topic is to make statistically reproducible measurements on the mechanical properties of ultrafine nanometals and establish a reliable correlation between the strength and grain size of the nanometals. Another part of the difficulty comes from the ambiguity in the plastic deformation mechanisms of the nanometals. Various defects or processes at nanoscale have been reported, including dislocations^13,14, deformation twinning^15,16,17, stacking faults^15,18, GB migration¹⁹, GB sliding^5,6,20,21, grain rotation^22,23,24, atomic bond parameters^25,26,27,28, etc. However, which one dominates the plastic deformation and thus determines the strength of nanometals is still unclear.

For these above issues, traditional approaches of mechanical strength examining, such as tensile test²⁹, Vickers hardness test^30,31, nano-indentation test³², micropillar compression^33,34,35, etc. are less effective because the high quality of large pieces of nanostructured materials is so difficult to fabricate and conventional indenter is much larger than single nanoparticle of materials (for the single-particle mechanics). In this study, we introduce radial DAC XRD techniques^36,37,38 to material science to in situ track the yield stress and deformation texturing of nano nickel of various grain sizes, which are used in the geoscience field in previous studies. It has been found that the mechanical strengthening can be extended down to 3 nm, much smaller than the previously reported most substantial sizes of nanometals, which enlarges the regime of conventional Hall-Petch relationship, implying the significance of rDAC XRD techniques to material science.

Protocol

1. Sample preparation Obtain 3 nm, 20 nm, 40 nm, 70 nm, 100 nm, 200 nm, and 500 nm nickel powder from commercial sources (see Table of Materials). The morphology characterization is shown in Figure 1. Prepare 8 nm nickel particles by heating 3 nm nickel particles using a reaction kettle (see Table of Materials). Put ~20 mL of absolute ethanol and ~50 mg of 3 nm nickel powder into the reaction kettle. NOTE: T…

Representative Results

Under hydrostatic compression, unrolled X-ray diffraction lines should be straight, not curved. However, under non-hydrostatic pressure, the curvature (ellipticity of the XRD rings, which translates into the non-linearity of the lines plotted along the azimuth angle) significantly increases ultrafine-grained-nickel at similar pressures (Figure 4). At a similar pressure, the differential strain of the 3 nm sized nickel is the highest. The mechanical strength results (stress-strain curves) are…

Discussion

Computational simulations have been widely employed to study the grain size effect on the strength of nanometals^{5,6,16,17,27,42}. Perfect dislocations, partial dislocations, and GB deformation have been proposed to play decisive roles in the deformation mechanisms of the nanomaterials. In a molecular dynamics simulation, Yamak…

Materials

20 nm Ni	Nanomaterialstore	SN1601	Flammable
3 nm Ni	nanoComposix		Flammable
40, 70, 100, 200, 500 nm Ni	US nano	US1120	Flammable
Absolute ethanol			as the solution to make 8 nm Ni
Absolute isopropanol			as the solution to make 12 nm Ni
Amorphous boron powder	alfa asear
Copper mesh	Beijing Zhongjingkeyi Technology Co., Ltd.		TEM grid
Epoxy glue
Ethanol			clean experimental setup
Focused ion beam	FEI
Glass slide
Glue tape	Scotch
Kapton	DuPont		Polyimide film material
Laser drilling machine			located in high pressure lab of ALS
Monochromatic synchrotron X-ray	Beamline 12.2.2, Advanced Light Source (ALS), Lawrence Berkeley National Laboratory		X-ray energy: 25-30 keV
Optical microscope	Leica		to mount the gasket and load samples
Pt powder	thermofisher	38374
Reaction kettle	Xian Yichuang Co.,Ltd.		50 mL
Sand paper			from 400 mesh to 1000 mesh
Transmission Electron Microscopy	FEI		Titan G2 60-300
Two-dimension image plate	ALS, BL 12.2.2		mar 345