Superplasticity in Nanocrystalline Ni3Al and Ti Alloys

2000 ◽  
Vol 634 ◽  
Author(s):  
Sam X. Mcfadden ◽  
Alla V. Sergueeva ◽  
Tomas Kruml ◽  
Jean-Luc Martin ◽  
Amiya K. Mukherjee
Keyword(s):  
Grain Size ◽  
Strain Rate ◽  
Strain Hardening ◽  
Nanocrystalline Materials ◽  
Tensile Elongation ◽  
Constant Strain Rate ◽  
Grain Boundary Sliding ◽  
Simple Extension ◽  
Size Dependent ◽  
Enhanced Properties

ABSTRACTThe advent of nanocrystalline materials has provided new opportunities to explore grain size dependent phenomenon. Superplasticity is such a grain size dependent phenomenon defined by the ability to attain tensile elongation of 200% or more. Superplasticity in microcrystalline materials has been well characterized. The constitutive equations that describe microcrystalline superplasticity predict enhanced properties for nanocrystalline materials. Enhanced properties in such nanocrystalline material include lower superplastic temperature at constant strain rate, higher superplastic strain rate at constant temperature, and lower flow stresses. Investigations with nanocrystalline Ni3Al and ultra-fine grained Ti-6Al-4V alloy have shown a reduction in the superplastic temperature. However, the flow stresses in these materials are significantly higher than expected. The high flow stresses are accompanied by strong strain hardening.Transmission electron microscopy in situ straining of nanocrystalline Ni3Al has shown that grain boundary sliding and grain rotation occurred during straining. The sliding and rotation decreased with strain. Dislocation activity was observed but was not extensive. There was no observable dislocation storage. The parameters of the generalized constitutive equation for superplasticity for nanocrystalline Ni3Al and Ti-6Al-4V are in reasonable agreement with the parameters for microcrystalline material. The rate parameters suggest that nanocrystalline superplasticity shares common features with microcrystalline superplasticity. In contrast, the observed flow stresses and strong strain hardening indicate that nanocrystalline superplasticity is not a simple extension of microcrystalline behavior scaled to finer grain size.

scholarly journals The Hall–Petch and inverse Hall–Petch relations and the hardness of nanocrystalline metals

2019 ◽  
Vol 55 (7) ◽  
pp. 2661-2681 ◽  
Author(s):  
Sneha N. Naik ◽  
Stephen M. Walley
Keyword(s):  
Grain Size ◽  
Yield Strength ◽  
Nanocrystalline Materials ◽  
Grain Boundary Sliding ◽  
Polycrystalline Materials ◽  
Size Dependent ◽  
Physical Mechanisms ◽  
Grain Sizes ◽  
Boundary Sliding ◽  
Fundamental Physical

AbstractWe review some of the factors that influence the hardness of polycrystalline materials with grain sizes less than 1 µm. The fundamental physical mechanisms that govern the hardness of nanocrystalline materials are discussed. The recently proposed dislocation curvature model for grain size-dependent strengthening and the 60-year-old Hall–Petch relationship are compared. For grains less than 30 nm in size, there is evidence for a transition from dislocation-based plasticity to grain boundary sliding, rotation, or diffusion as the main mechanism responsible for hardness. The evidence surrounding the inverse Hall–Petch phenomenon is found to be inconclusive due to processing artefacts, grain growth effects, and errors associated with the conversion of hardness to yield strength in nanocrystalline materials.

On the grain-size-dependent elastic modulus of nanocrystalline materials with and without grain-boundary sliding

2003 ◽  
Vol 18 (8) ◽  
pp. 1823-1826 ◽  
Author(s):  
P. Sharma ◽  
S. Ganti
Keyword(s):  
Grain Size ◽  
Elastic Modulus ◽  
Grain Boundary ◽  
Closed Form ◽  
Low Temperatures ◽  
Nanocrystalline Materials ◽  
Grain Boundary Sliding ◽  
Size Dependent ◽  
Form Model ◽  
Boundary Sliding

A closed-form model was proposed to evaluate the elastic properties of nanocrystalline materials as a function of grain size. Grain-boundary sliding, present in nanocrystalline materials even at relatively low temperatures, was included in the formulation. The proposed analytical model agrees reasonably well with the experimental results for nanocrystalline copper and palladium.

Mechanical Properties of Novel Nanocrystalline Materials

2001 ◽  
Author(s):  
J. Narayan ◽  
H. Wang ◽  
A. Kvit
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Nanocrystalline Materials ◽  
High Surface ◽  
Grain Boundary Sliding ◽  
Boundary Shear ◽  
Functionally Gradient ◽  
Boundary Sliding ◽  
First Time ◽  
Critical Grain Size

Abstract We have synthesized nanocrystalline thin films of Cu, Zn, TiN, and WC having uniform grain size in the range of 5 to 100 nm. This was accomplished by introducing a couple of manolayers of materials with high surface and have a weak interaction with the substrate. The hardness measurements of these well-characterized specimens with controlled microstructures show that hardness initially increases with decreasing grain size following the well-known Hall-Petch relationship (H∝d−½). However, there is a critical grain size below which the hardness decreases with decreasing grain size. The experimental evidence for this softening of nanocrystalline materials at very small grain sizes (referred as reverse Hall-Petch effect) is presented for the first time. Most of the plastic deformation in our model is envisioned to be due to a large number of small “sliding events” associated with grain boundary shear or grain boundary sliding. This grain-size dependence of hardness can be used to create functionally gradient materials for improved adhesion and wear among other improved properties.

scholarly journals Statistical characteristics for the type and length of deformation-induced cracks in columnar-grain ice

1997 ◽  
Vol 43 (144) ◽  
pp. 311-320 ◽  
Author(s):  
Lorne W. Gold
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Strain Rate ◽  
Relative Proportion ◽  
Constant Strain Rate ◽  
Statistical Characteristics ◽  
Columnar Grain ◽  
Log Normal ◽  
Almost All ◽  
Log Normal Distribution

AbstractObservations are reported on cracks formed during compressive, unidirectional, constant-strain-rate deformation of columnar-grain ice. The axis of hexagonal crystallographic symmetry of each grain tended to be in the plane perpendicular to the long direction of the grains and to have a random orientation in that plane. For stress applied perpendicular to the long direction of the grains, the deformation was practically two-dimensional. It was found that the relative proportion of grain-boundary cracks increased with increasing strain rate, decreasing temperature and, for strain rate greater than 7 × 10−5 s−1, with decreasing grain-size. Almost all the grain-boundary cracks had at least one edge at a triple point. For each test, the grain-boundary and transcrystalline crack lengths tended to have a log-normal distribution. The logarithmic mean crack length (LMCL) decreased with increasing strain rate, decreasing grain-size and decreasing temperature and tended to a constant value of 0.75 mm at 10°C. For grain-size of 3 mm or greater, the LMCL had a maximum at a strain rate of 10−5 to 10−6 S−1 at −10°C. The LMCLs and the relative proportion of grain-boundary cracks tended to be normally distributed for given load conditions.

What is Behind the Inverse Hall-Petch Behavior in Nanocrystalline Materials?

2006 ◽  
Vol 976 ◽  
Author(s):  
Christopher Carlton ◽  
P. J. Ferreira
Keyword(s):  
Grain Size ◽  
Yield Strength ◽  
Strain Rate ◽  
Grain Boundaries ◽  
Nanocrystalline Materials ◽  
Critical Value ◽  
Petch Relationship ◽  
And Diffusion ◽  
Critical Grain Size

AbstractAn inverse Hall-Petch effect has been observed for nanocrystalline materials by a large number of researchers. This result implies that nanocrystalline materials get softer as grain size is reduced below a critical value. Postulated explanations for this behavior include dislocation based mechanisms and diffusion based mechanisms. In this paper, we report an explanation for the inverse Hall-Petch effect based on the statistical absorption of dislocations by grain boundaries, showing that the yield strength is both dependent on strain rate and temperature, and that it deviates from the Hall-Petch relationship at a critical grain size.

Effect of Changing Strain Rate during Warm Deformation on Static Recrystallisation of IF Steel

2007 ◽  
Vol 550 ◽  
pp. 3-12
Author(s):  
C. Prentice ◽  
C.M. Sellars
Keyword(s):  
Grain Size ◽  
Strain Rate ◽  
Subgrain Size ◽  
Constant Strain Rate ◽  
Salt Bath ◽  
Interstitial Free ◽  
Compression Tests ◽  
Constant Rate ◽  
Interstitial Free Steel ◽  
Kinetics Of

Plane strain compression tests have been carried out on Ti stabilised interstitial free steel at 700oC with constant and changing strain rates. Specimens were annealed in a salt bath at 750oC to determine the effects of changing strain rate on the kinetics of static recrystallisation and on the recrystallised grain size. After relatively slow changes in rate, the recrystallisation behaviour at the end of the change was the same as for tests at constant strain rate with the final value. For faster changes in rate, there were transients in recrystallisation rate and recrystallised grain size at the end of the change in strain rate at a strain of 1.0. These were removed by a further increment of 0.1 strain at constant rate. In all cases the recrystallised grain size correlated with the subgrain size present at the end of deformation.

scholarly journals Molecular Dynamics as a Means to Investigate Grain Size and Strain Rate Effect on Plastic Deformation of 316 L Nanocrystalline Stainless-Steel

Materials ◽  
2020 ◽  
Vol 13 (14) ◽  
pp. 3223 ◽  
Author(s):  
Abdelrahim Husain ◽  
Peiqing La ◽  
Yue Hongzheng ◽  
Sheng Jie
Keyword(s):  
Plastic Deformation ◽  
Grain Size ◽  
Stainless Steel ◽  
Grain Boundary ◽  
Strain Rate ◽  
Deformation Mechanism ◽  
Grain Boundary Sliding ◽  
Plastic Deformation Mechanism ◽  
Grain Sizes ◽  
Critical Grain Size

In the present study, molecular dynamics simulations were employed to investigate the effect of strain rate on the plastic deformation mechanism of nanocrystalline 316 L stainless-steel, wherein there was an average grain of 2.5–11.5 nm at room temperature. The results showed that the critical grain size was 7.7 nm. Below critical grain size, grain boundary activation was dominant (i.e., grain boundary sliding and grain rotation). Above critical grain size, dislocation activities were dominant. There was a slight effect that occurred during the plastic deformation mechanism transition from dislocation-based plasticity to grain boundaries, as a result of the stress rate on larger grain sizes. There was also a greater sensitive on the strain rate for smaller grain sizes than the larger grain sizes. We chose samples of 316 L nanocrystalline stainless-steel with mean grain sizes of 2.5, 4.1, and 9.9 nm. The values of strain rate sensitivity were 0.19, 0.22, and 0.14, respectively. These values indicated that small grain sizes in the plastic deformation mechanism, such as grain boundary sliding and grain boundary rotation, were sensitive to strain rates bigger than those of the larger grain sizes. We found that the stacking fault was formed by partial dislocation in all samples. These stacking faults were obstacles to partial dislocation emission in more sensitive stress rates. Additionally, the results showed that mechanical properties such as yield stress and flow stress increased by increasing the strain rate.

Coupled effect of grain boundary sliding and dislocation emission on fracture toughness of nanocrystalline materials

2016 ◽  
Vol 01 (02) ◽  
pp. 1650008 ◽  
Author(s):  
Q. H. Fang ◽  
L. C. Zhang
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Crack Growth ◽  
Nanocrystalline Materials ◽  
Crack Tip ◽  
Grain Boundary Sliding ◽  
Dislocation Emission ◽  
Disclination Dipole ◽  
Coupled Effect ◽  
Wedge Disclination

This paper establishes a theoretical model to explore the coupled effect of grain boundary (GB) sliding deformation and crack tip dislocation emission on the critical stress intensity factor (SIF) for crack growth in ultrafine-grained and nanocrystalline materials (NCMs). The model postulates that the stress concentration near a crack tip initiates GB sliding. It is found that GB sliding leads to the formation of wedge disclination dipole at the triple junctions of grain boundaries. Under the external load and stress fields produced by wedge disclinations, dislocations are emitted from crack tips but will stop at the opposite GBs. The influence of the wedge disclination dipole and the dislocation emitted from crack tip on the critical SIF for crack growth is investigated. The model prediction shows that the critical SIF varies with the decrement of grain size, and that there is a critical grain size corresponding to a minimum value of SIF. Compared with the pure brittle fracture in NCMs at the grain sizes of tens of nanometers, the combined deformation mechanisms can bring an increase of the critical SIF for crack growth.

Sign in / Sign up

Export Citation Format

Share Document