Novel Nanocrystalline Composites

2002 ◽  
Author(s):  
J. Narayan
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Tungsten Carbide ◽  
Nickel Aluminide ◽  
Processing Technique ◽  
High Temperature Strength ◽  
Softening Behavior ◽  
Boundary Shear ◽  
Nanocrystalline Composites

We have developed a novel processing technique to fabricate tungsten carbide (WC) nanocomposites with uniform grain size. In this method, pulsed laser deposition of WC in conjunction with a few monolayers of nickel aluminide (NiAl) is used to control the grain size of nanocrystalline composites. The grain size of WC was controlled by the thickness of tungsten carbide and the substrate temperature. The role of NiAl is to ensure the nucleation of tungsten carbide islands, and it is relatively insoluble in WC. Using this approach, we have fabricated nanocomposites of grain sizes ranging from 6 nm to 35 nm. The hardness of the composite increases with the decrease in grain size, following approximately Hall-Petch relationship. Below a critical value, we observed a softening behavior which has been modeled to be related to intragrain deformation or grain boundary shear. The role of NiAl in grain boundary deformation is of particular interest in strengthening and stabilizing against the grain growth of nanocrystalline composites. The new WC-NiAl composite is expected to have superior high-temperature strength compared to conventional microcrystalline WC-Co composites.

Novel Tungsten Carbide Nanocrystalline Composites by Pulsed Laser Deposition

2000 ◽  
Vol 634 ◽  
Author(s):  
Ravi K. Venkatesan ◽  
A. Kvit ◽  
Q. Wei ◽  
J. Narayan
Keyword(s):  
Grain Size ◽  
Tungsten Carbide ◽  
Pulsed Laser Deposition ◽  
Nickel Aluminide ◽  
Pulsed Laser ◽  
Processing Technique ◽  
Laser Deposition ◽  
Grain Sizes ◽  
Nanocrystalline Composites

ABSTRACTWe have developed a novel processing technique to fabricate “artifact free” tungsten carbide (WC) nanocomposites. In this method, pulsed laser deposition of WC in conjunction with a few monolayers of nickel aluminide (NiAl) is used to control the grain size of nanocrystalline composites. The grain size of WC was controlled by the thickness of tungsten carbide and the substrate temperature. The role of NiAl is to ensure the nucleation of tungsten carbide islands, and it is also insoluble in WC. Using this approach, we have fabricated nanocomposites of grain sizes ranging from 6 nm to 35 nm. The hardness of the composite increases with the decrease in grain size, following approximately Hall-Petch relationship. The role of NiAl in grain boundary deformation is of particular interest in strengthening the nanocrystalline composites. The potential of this technique to go to even lower grain sizes is discussed.

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.

Is There a Critical Size in Nano Grained Metals for Ductile to Brittle Transition?

2004 ◽  
Author(s):  
Aman Haque ◽  
Taher Saif
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Critical Size ◽  
Volume Ratio ◽  
Metal Films ◽  
Metal Structures ◽  
Brittle Transition ◽  
Ductile To Brittle Transition ◽  
Metal Properties

Nanoscale metal films and electrodes are extensively used in today’s micro and nano electronics as well as nano mechanical systems. These metal structures are usually polycrystalline in nature with nano scale grains connected to each other by grain boundaries. The small size offers large grain boundary to volume ratio that is likely to affect the metal properties significantly. Here, we discuss the role of grain size and boundaries in determining the mechanical behavior of metals, such as elasticity and yielding.

A Re-Appraisal of Cavity Growth Processes in Superplasticity

1990 ◽  
Vol 196 ◽  
Author(s):  
Yan Ma ◽  
Terence G. Langdon
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Diffusion Process ◽  
Superplastic Deformation ◽  
Cavity Growth ◽  
Normal Model ◽  
Experimental Conditions ◽  
Diffusion Growth ◽  
New Model

ABSTRACTIt is well known that cavities are nucleated and grow during the superplastic deformation of many materials. The various theories for cavity growth are examined with special emphasis on the role of growth by diffusion. It is demonstrated that the normal model for the diffusion growth of cavities is inadequate for superplastic materials when the grain boundary lengths are very small. By developing a new model for the growth of an isolated cavity to sizes exceeding the grain size, it is shown that the diffusion process may play a major role in cavity growth under a range of experimental conditions.

A new design of tungsten carbide tools with diamond coatings

1996 ◽  
Vol 11 (9) ◽  
pp. 2220-2230 ◽  
Author(s):  
Volker Weihnacht ◽  
W.D. Fan ◽  
K. Jagannadham ◽  
J. Narayan ◽  
C-T. Liu
Keyword(s):  
Wear Resistance ◽  
Tungsten Carbide ◽  
Nickel Aluminide ◽  
Diamond Films ◽  
Diamond Growth ◽  
Nucleation Density ◽  
High Temperature Strength ◽  
Binder Phase ◽  
Chemical Vapor Deposition Diamond ◽  
Diamond Coatings

We have designed tungsten carbide tools with a new binder, which makes them suitable for advanced diamond tool coatings. The new tool substrates, made of tungsten carbide and nickel aluminide as binder phase, are produced by sintering and hot isostatic pressing, and also by combustion synthesis. The high temperature strength of nickel aluminide is key to superior tool performance at elevated temperatures. More importantly, nickel aluminides reduce the formation of graphite and promote diamond growth during chemical vapor deposition. Diamond films are deposited on the new tool substrates to investigate the nucleation density, adhesion, and wear resistance. The diamond coatings are characterized by scanning electron microscopy and Raman spectroscopy. The graphitizing tendency due to cobalt in the tungsten carbide tools was found to be a limitation to improve adhesion of diamond films. The new tool substrates with nickel aluminide binder have been found to exhibit good adhesion and wear resistance. The implications of these results in advanced cutting tools are discussed.

The Role of Zirconium Additions in Recrystallization of Aluminum Alloys

2007 ◽  
Vol 558-559 ◽  
pp. 383-387 ◽  
Author(s):  
Hasso Weiland ◽  
Soon Wuk Cheong
Keyword(s):  
Grain Size ◽  
Aluminum Alloys ◽  
Grain Boundary ◽  
Solute Drag ◽  
Heat Treatments ◽  
Boundary Mobility ◽  
Grain Boundary Mobility ◽  
Structural Applications ◽  
Zn Alloys

Control of grain size during recrystallization of aluminum alloys is critical when tailoring material properties for structural applications. Most commonly the grain size is controlled by adding alloying elements which form second phases during homogenization heat treatments small enough to impose a Zener drag on the grain boundary mobility. These phases are known as dispersoids and are in the 10 to 200 nm in diameter range. In Al-Zn alloys, zirconium has been successfully used in controlling the degree of recrystallization after solution heat treatments. It is commonly understood that the Al3Zr dispersoids of about 20 nm in diameter present in the microstructure are the key features affecting grain boundary mobility. With the success of controlling recrystallization in Al- Zn alloys, zirconium has been added to other alloy systems, such as Al-Cu-Mn, and a similar retarding effect in recrystallization kinetics has been observed as seen in the Al-Zn systems. However, in Al-Cu-Mn alloys, zirconium bearing dispersoids are not observable in the microstructure. Consequently, additional microstructural effects such as solute drag need to be considered to explain the experimental observations. In this paper, the role of zirconium additions in aluminum alloys will be summarized.

The Role of Crystallization of an Intergranular Glassy Phase in Determining Grain Boundary Residual Stresses in Debased Aluminas

1989 ◽  
Vol 170 ◽  
Author(s):  
Nitin P. Padture ◽  
Helen M. Chan ◽  
Brian R. Lawn ◽  
Michael J. Readey
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Residual Stresses ◽  
Glassy Phase ◽  
Analytical Techniques ◽  
Ceramic Materials ◽  
Strength Test ◽  
Second Phase

AbstractThe influence of microstructure on the crack resistance (R-curve) behavior of a commercial debased alumina containing large amounts of glassy phase (28 vol %) has been studied using the Indentation-Strength test. The effect of two microstructural variables, viz. grain size and the nature of the intergranular second phase (glassy or crystalline) has been evaluated. Crystallization of the intergranular glass was carried out in order to generate residual stresses at the grain boundaries, which have been shown to enhance R-curve behavior in ceramic materials. Enhancement of the R-curve behavior was observed with the increase in grain size. However, no effect of the nature of the intergranular second phase on the R-curve behavior, in small and large grain materials, was observed. The results from characterization of these materials using various analytical techniques is presented, together with possible explanations for the observed effects.

Revealing the unfavorable role of superfluous CH3NH3PbI3 grain boundary traps in perovskite solar cells on carrier collection

RSC Advances ◽  
2016 ◽  
Vol 6 (86) ◽  
pp. 83264-83272 ◽  
Author(s):  
Yangyang Du ◽  
Hongkun Cai ◽  
Hongbin Wen ◽  
Yuxiang Wu ◽  
Zhenglong Li ◽  
...  
Keyword(s):  
Grain Size ◽  
Solar Cells ◽  
Grain Boundary ◽  
Simulation Analysis ◽  
Perovskite Solar Cells ◽  
Carrier Collection ◽  
Experiment And Simulation

In this work, the perovskite films with controllable grain size are obtained by a facile method. And the unfavorable role of perovskite grain boundary traps is unveiled by the combination of experiment and simulation analysis.

Performance and Durability Enhancement of Tungsten Carbide Micro-Drills by Ti/ta-C Coatings

2019 ◽  
Vol 806 ◽  
pp. 76-80
Author(s):  
Vadim Yu. Samardak ◽  
Alexander S. Samardak ◽  
Alexey V. Ognev ◽  
Sergey Goloviatinski ◽  
Sergey N. Mikhallov ◽  
...  
Keyword(s):  
Grain Boundary ◽  
Tungsten Carbide ◽  
Amorphous Carbon ◽  
Vacuum Arc ◽  
Drilling Operation ◽  
Tetrahedral Amorphous Carbon ◽  
Sintered Powder ◽  
Micro Drilling ◽  
Filtered Cathodic Vacuum Arc

This paper presents results of the application of Ti/ta-C films to micro drilling operation for machining. Tetrahedral amorphous carbon (ta-C) films were successfully deposited on WC-Co substrates by a filtered cathodic vacuum arc (FCVA) system. The mechanical and flexion properties of Ti/ta-C films were systematically investigated. The experimental results show that the Ti/ta-C coated micro drills have the excellent microhardness, adhesion and flexion properties and represent the optimal coatings for micro drilling applications. The role of the Ti –sublayer on a sintered powder tungsten carbide substrate is not only limited by the adhesion improvement, but it is mainly used to neutralise the grain boundary microcracks on a surface. The results of drilling tests carried out on PCB boards showed that the durability and drilling efficiency of tools coated by Ti/ta-C films are significantly higher than that of uncoated ones.

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