1 Dynamic Hardness Evolution in Metals from Impact Induced Gradient Dislocation Density

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Dynamic Hardness Evolution in Metals from Impact Induced Gradient Dislocation

Density

Jizhe Cai, Claire Griesbach, Savannah G. Ahnen, Ramathasan Thevamaran*

Department of Engineering Physics, University of Wisconsin-Madison, Madison, WI 53706.

*Corresponding author: thevamaran@wisc.edu

Abstract

A clear understanding of the dynamic behavior of metals is critical for developing superior structural

materials as well as for improving material processing techniques such as cold spray and shot peening.

Using a high-velocity (from ∼120 m/s to 700 m/s; strain rates >107 1/s) micro-projectile impact testing and

quasistatic (strain rates: 10-2 1/s) nanoindentation, we investigate the strain-rate-dependent mechanical

behavior of single-crystal aluminum substrates with (001), (011), and (111) crystal orientations. For all

three crystal orientations, the dynamic hardness initially increases with increasing impact velocity and

reaches a plateau regime at hardness 5 times higher than that of at quasistatic indentations. Based on

coefficient of restitution and post-mortem transmission Kikuchi diffraction analyses, we show that distinct

plastic deformation mechanisms with a gradient dislocation density evolution govern the dynamic behavior.

We also discover a distinct deformation regime—stable plastic regime—that emerge beyond the deeply

plastic regime with unique strain rate insensitive microstructure evolution and dynamic hardness. Our work

additionally demonstrates an effective approach to introduce strong spatial gradient in dislocation density

in metals by high-velocity projectile impacts to enhance surface mechanical properties, as it can be

employed in material processing techniques such as shot peening and surface mechanical attrition treatment.

Keywords: dynamic behavior, single crystal, aluminum, hardness, impact, ultra-high strain rate.

1. Introduction

A fundamental understanding of the mechanical behavior and the deformation mechanisms of materials

subjected to high-strain-rate dynamic loading is crucial for developing high-performance structural

components in armored vehicles, spacecraft [1], and sports gears [2] that withstand impacts as well as for

improving modern processing techniques such as cold spray [3], surface mechanical attrition treatment [4],

and shot peening [5]. A strain-rate-dependent mechanical behavior is observed in metals when tested across

quasistatic (strain rate =10-3 1/s) to dynamic (strain rate =103 1/s) regimes, because of thermally activated

dislocation interactions with obstacles [6]. The yield and flow stresses of metals at constant strain typically

increase linearly with the logarithm of strain rate. When the strain rate is increased further beyond a

threshold (strain rate > 103-104 1/s), the strain rate sensitivity of the material’s mechanical properties

increases dramatically, which is interpreted as the result of deformation mechanism transition to dislocation

drag controlled process [7–9]. Pressure-shear plate impact tests that reach ultra-high strain rate (strain rate~

105-108 1/s) [10,11] showed that the flow stress of pure metals increases strongly with increasing strain rate

[12,13]. The microstructural origins of metallic behavior at extreme loading conditions (strain rate >106

1/s), however, has still remained elusive because of the experimental challenges in conducting ultra-high-

strain-rate dynamic tests with holistic microstructural diagnostics of the entire deformation zone on metal

samples with well-defined microstructure that enables us to clearly identify structure-property relations.

Recent developments in laser-induced microprojectile impact testing (LIPIT) provide effective

approach for studying the dynamic behaviors of various materials [14–18] at ultra-high strain rates with the

ability to investigate the entire deformation zone and mechanisms down to atomistic resolutions. In LIPIT,

a microparticle selectively launched at high-velocity (100 m/s to over 1 km/s) generates extreme loading

conditions onto test materials—employed either as a projectile [14,19,20] or as target [15–17,21–24]—

deforming them at far from equilibrium conditions, including ultra-high strain rates (up to ~108 1/s) and

adiabatic heating induced local temperature increase (~100s oC) in a very short time (~a few ns). For

example, face centered cubic (FCC) single-crystal silver (Ag) microcubes impacted at ~400 m/s against a

rigid target has been shown to create a crystal symmetry dependent martensitic phase transformation to

hexagonal close-packed (HCP) phase and a gradient nano-grained structure [14,20]. A 9R phase formation

has been demonstrated in nanocrystalline aluminum (Al) thinfilms impacted by rigid microprojectiles [22].

A nanotwinning-assisted dynamic recrystallization mechanism resulting in finer nanocrystalline grain sizes

was also observed in LIPIT of polycrystalline copper microparticles [24]. Hardness of polycrystalline

metals (copper and iron) measured by LIPIT at ultra-high strain rate (>106 1/s) has shown distinctly different

response beyond 103 to 104 1/s strain rates [15].

In contrast to polycrystalline metals, from the near perfect crystal structure without defects associated

with grain boundaries, single-crystal metals exhibit remarkable high-temperature resistance to mechanical

and thermal degradation [25], as well as strong physical property anisotropy [26]. This anisotropy stems

from the variation of active slip systems and hardening behavior, depending on the dislocation interactions

in different crystal orientations [27,28]. In dynamic regime, the anisotropic mechanical behavior of single-

crystal metals with different crystal structures i.e. HCP [29,30], BCC [27] and FCC [28,31,32], are governed

by the interplay between effects of strain rate and crystal orientations [14,20,28,30]. For example, a strong

strain rate dependency of flow stress was observed in single-crystal FCC metals, e.g. copper, with different

crystal orientations, stemming from the thermally activated motion of dislocations [31,32]. The flow stress

of metals is also significantly affected by the crystal orientations with respect to the loading direction, for

example, the highest flow stress is obtained at [111] direction and the lowest at [110] direction primarily

governed by the characteristic slips that ensue during loading [31]. Beyond a threshold strain rate (~103

1/s), flow stress of single-crystals exhibit increased strain rate sensitivity because of the viscous drag

induced resistance on dislocation motion [33]. Dynamic hardness provides a direct quantitative measure of

the resistance of metals to plastic deformation [34]. Although the quasistatic hardness and anisotropic

behavior of single-crystal metals has been studied extensively using nanoindentation [35,36], the dynamic

hardness, and the mechanistic origins of its strain rate sensitivity at ultra-high strain rate deformations

remain elusive.

Here, we use single-crystal Al substrates as a model material to study the effects of crystal orientation

and strain rate on its dynamic behavior and deformation mechanism under ultra-high strain rate impacts.

Al is a lightweight FCC metal (space group 𝐹𝑚3

̅𝑚; No.225) that is widely used in structural applications

in its polycrystalline and nanocrystalline forms and as alloys. Using LIPIT, we selectively launched

individual rigid spherical silica microparticles (diameter ~21 μm) at a wide velocity range (from ∼120 to

700 m/s, strain rate ∼107 1/s) to impact the single-crystal Al substrates of three principal crystal orientations,

(001), (011), and (111). We characterized the post-impact topography of impact-induced craters by optical

profilometer and scanning electron microscopy (SEM) and the microstructural evolutions using

transmission Kikuchi diffraction (TKD). Compared to the quasistatic hardness measured from

nanoindentation, the dynamic hardness of single-crystal Al measured by LIPIT shows dependency on

strain-rate and crystal orientation. The interplay between impact-induced dislocation nucleation and

thermally-enhanced dislocation annihilation leads to a strain-rate-sensitive dynamic hardness evolution and

a spatial gradient in dislocation density in the sample. Beyond these strain-rate-dependent plasticity

regimes—elasto-plastic, fully plastic, deeply plastic regimes—we discover a distinct plastic deformation

regime with strain rate insensitivity on dynamic hardness and microstructure, which we refer to as stable

plastic regime.

2. Materials and methods

We performed the high-strain-rate impact tests in a laser-induced microprojectile impact testing

system [16,17,21], shown in Fig.1(a). Single-crystal Al substrates (size: 10×10×0.5 mm and purity>99.99%,

purchased from MTI Corporation) with different crystal orientations, i.e. (001), (011), and (111), were

placed ∼500 μm away from the microprojectile launch pad, which is made of a thin metal film (60 nm thick

gold for low-velocity impacts (Vi<550 m/s) or 60 nm thick chromium for high-velocity impacts (Vi>550

m/s)) deposited on glass substrate followed by a 80 μm thick cross-linked polydimethylsiloxane (PDMS)

layer. Monodisperse spherical silica projectiles (diameter ∼21 μm, purchased from Cospheric) were drop-

cast and air-dried on the launch pad and individual projectiles were selectively launched by the rapid

expansion of PDMS layer from ablation of the gold layer underneath by a pulse from an Nd:YAG laser (5–

8 ns pulse duration, 1064 nm). By tuning ablation laser energy, projectile velocity was varied between 120

m/s to 700 m/s. The process of projectile impacting the substrate and its rebound was imaged by a

microscope camera (Allied Vision Mako G-234B) illuminated by continuous pico-second white-laser (NKT

Photonics SuperK EXR-20) pulses at interval of 256.5 ns and gated by an acousto-optic modulator

(ISOMET 1250C-848). The impact and rebound velocities of the projectile were calculated by dividing the

distance measured between adjacent projectile snapshots in the multi-exposure image by the time interval

between consecutive imaging laser pulses. Nominal strain rate of material upon impact is estimated by

dividing the projectile impact velocity (120 m/s to 700 m/s) by its diameter (∼21 μm, measured by optical

microscope for each projectile), resulting in ∼106-107 1/s strain rates [15].

To investigate the effect of strain rate on the mechanical properties, we performed quasistatic spherical

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1DynamicHardnessEvolutioninMetalsfromImpactInducedGradientDislocationDensityJizheCai,ClaireGriesbach,SavannahG.Ahnen,RamathasanThevamaran*DepartmentofEngineeringPhysics,UniversityofWisconsin-Madison,Madison,WI53706.*Correspondingauthor:thevamaran@wisc.eduAbstractAclearunderstandingofthedynamicbehavi...

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