Experimental Evidence that the Onset of Mechanical Softening in Nanocrystalline Metals is Strain Rate Dependent

Wang, Li Wei; Prorok, Barton C.

doi:10.4028/www.scientific.net/MSF.633-634.99

Paper Titles

Crack Blunting through Lattice Dislocation Slip in Nanocrystalline and Ultrafine-Grained Materials
p.55

In Situ Transmission Electron Microscopy Investigation of the Deformation Behavior of Cu with Nanoscale Twins
p.63

Microstructural Origin of Superior Compressive Ductility of a Nanocrystalline Metal
p.73

Overview of the Grain Size Effects on the Mechanical and Deformation Behaviour of Electrodeposited Nanocrystalline Nickel − From Nanoindentation to High Pressure Torsion
p.85

Experimental Evidence that the Onset of Mechanical Softening in Nanocrystalline Metals is Strain Rate Dependent
p.99

Deformation Behavior and Plastic Strain Localization of Nanostructured Materials Produced by Severe Plastic Deformation
p.107

Dislocation and Diffusion Deformation Mechanisms of Ultrafine Grained Materials
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Processing, Strength and Ductility of Bulk Nanostructured Metals Produced by Sever Plastic Deformation: An Overview
p.131

Ductility of Nanocrystalline Metals: Intrinsic or Extrinsic
p.151

Home Materials Science Forum Ductility of Bulk Nanostructured MaterialsExperimental Evidence that the Onset of Mechanical...

Experimental Evidence that the Onset of Mechanical Softening in Nanocrystalline Metals is Strain Rate Dependent

Abstract:

This paper reports on the influence of strain rate on the onset of mechanical softening of nanocrystalline gold at room temperature. Micro-tensile testing was performed with applied strain rates on the order of 10−4 s−1 to 10−6 s−1. Our results defined a threshold strain rate, whereby plastic deformation at larger rates was dominated by dislocation processes and at smaller rates by one or more other deformation mechanisms.

Info:

Periodical:

Materials Science Forum (Volumes 633-634)

Main Theme:

Ductility of Bulk Nanostructured Materials

Edited by:

Yonghao Zhao and Xiaozhou Liao

Pages:

99-105

DOI:

https://doi.org/10.4028/www.scientific.net/MSF.633-634.99

Citation:

L. W. Wang and B. C. Prorok, "Experimental Evidence that the Onset of Mechanical Softening in Nanocrystalline Metals is Strain Rate Dependent", Materials Science Forum, Vols. 633-634, pp. 99-105, 2010

Online since:

November 2009

Authors:

Li Wei Wang, Barton C. Prorok

Keywords:

Creep, Hall-Petch Behavior, Nanocrystalline Gold

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Paper TitlePages

The Combined Effect of Grain Size and Strain Rate on the Mechanical Behavior in Ultra-High Purity Aluminum

Authors: Yong Gang Wang, Chun Lei Wang, Hong Wei Liu

Abstract: The effect of grain size on the mechanical properties in ultra-high pure aluminum had been investigated as a function of strain rate. Specimens with average grain diameter sizes of 243, 678 and 1070 m were compressed and elongated at quasi-static and high strain rates by a computer controlled servo-hydraulic testing machine and a Split Hopkinson Pressure (Tension) Bar (SHPB and SHTB). The mechanical properties were found to vary significantly with grain size, and strain rate. The relationship between flow stress and grain size can be expressed by a Hall - Petch relation with the different slope for both compressive tests and tensile tests. The influence of strain rate on the slope of the Hall - Petch relation is such that in compression, the slope does not change much, but in tension, there is an increase in the slope value. The strain hardening rate was seen to increase with increasing strain rate. The strain rate dependence of flow stress is obvious, and is seen to be more significant for the smallest grain size specimens. The 3D fractographs illustrated that the numbers of the dimples decrease with the increase of the grain size.

662

Influence of Annealing on the Microstructure and Mechanical Properties of Electrodeposited Nanocrystalline Nickel

Authors: B. Yang

Abstract: The evolution of the microstructure and mechanical properties of electrodeposited nanocrystalline Ni with different annealing procedures was studied systematically. For the annealed specimens hardness decreases with increasing average grain size but the dependence changes at different grain size ranges. The specimens annealed at a low temperature show higher hardness compared to the as-deposited nanocrystalline Ni, despite an increased measured average grain size. In association with this hardening an increase in elastic modulus and a decrease in microstrain was observed after annealing. With increasing annealing temperature both the tensile strength and the fracture strain were observed to decrease, this is companied with a transition from ductile to brittle in the fracture surfaces. These results indicated that the mechanical behaviour of nanocrystalline Ni depends not only on the average grain size but also on the grain boundary structure. A change in the grain boundary state arising from annealing may be responsible for the observed increase in hardness and elastic modulus as well as the deterioration of tensile properties.

103

Inverse Hall-Petch Effect of Hardness in Nanocrystalline Ta Films

Authors: Zhen Hua Cao, Xiang Kang Meng

Abstract: Hardness and creep property of nanocrystalline Ta films were studied by nanoindentation tests. Experimental results suggested that hardness decreases with the decrement of grain size, which exhibits an inverse Hall-Petch effect. A remarkable room temperature creep behavior of nanocrystalline Ta films was revealed during indentation response. Creep stress exponent decreases with the decrement of feature scale, such as grain size and indent displacement. Grain boundary (GB) mediated process involving atomic diffusion and the emission of dislocation at GB is believed to be the dominant deformation mechanism.

575

Effects of Strain Rate and Grain Size on Behavior of Nano Crystalline Materials

Authors: Reza Jafari Nedoushan, Mahmoud Farzin, Mohammad Mashayekhi

Abstract: Recent Experiments on Nano-Crystalline Materials Show an Increase of Strain-Rate Sensitivity in Contrast to the Conventional Coarse-Grained Materials. these Materials Also Show a Different Grain Size Dependency as Compared to Coarse-Grained Materials. to Explain these Issues, a Constitutive Equation Is Proposed which Considers Dominant Deformation Mechanisms Including Grain Interior Plasticity, Grain Boundary Diffusion and Grain Boundary Sliding. the Stresses Obtained from these Constitutive Equations Match Well with the Experimental Data for Nanocrystalline Copper at Different Strains and Strain Rates. the Model Also Well Predicts Variation of Strain Rate Sensitivity Parameter. this Variation Can Be Explained with Regard to the above Mentioned Effective Deformation Mechanisms. Deviation from the Hall-Petch Law and Inverse Hall-Petch Effect Are Also Well Illustrated by the Model.

Modeling of the Plastic Deformation of Polycrystalline Materials in Micro and Nano Level Using Finite Element Method

Authors: Mohammad Jafari, Saeed Ziaei-Rad, Nima Nouri

Abstract: Recent experiments on polycrystalline materials show that nanocrystalline materials have a strong dependency to the strain rate and grain size in contrast to the microcrystalline materials. In this study, mechanical properties of polycrystalline materials in micro and nanolevel were studied and a unified notation for them was presented. To completely understand the rate-dependent stress-strain behavior and size-dependency of polycrystalline materials, a dislocation density based model was presented that can predict the experimentally observed stress-strain relations for these materials. In nanocrystalline materials, crystalline and grain-boundary were considered as two separate phases. The mechanical properties of the crystalline phase were modeled using viscoplastic constitutive equations, which take dislocation density evolution and diffusion creep into account, while an elasto-viscoplastic model based on diffusion mechanism was used for the grain boundary phase. For microcrystalline materials, the surface-to-volume ratio of the grain boundaries is low enough to ignore its contribution to the plastic deformation. Therefore, the grain boundary phase was not considered in microcrystalline materials and the mechanical properties of the crystalline phase were modeled using an appropriate dislocation density based constitutive equation. Finally, the constitutive equations for polycrystalline materials were implemented into a finite-element code and the results obtained from the proposed constitutive equations were compared with the experimental data for polycrystalline copper and good agreement was observed.