Effects of Grain Size
The effects of grain size refer to the impact of the size of individual grains in a material on its properties. In materials science and engineering, smaller grain sizes generally lead to increased strength and hardness, as well as improved resistance to deformation. This is due to the increased grain boundary area, which impedes the movement of dislocations within the material.
3 Key excerpts on "Effects of Grain Size"
eBook - ePubNanomaterials and Nanocomposites
Synthesis, Properties, Characterization Techniques, and Applications
- Rajendra Kumar Goyal(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
...3 Effect of Particle Sizes on Properties of Nanomaterials As discussed in Chapter 1, the nanomaterials are materials which have at least one dimension of the order of 1–100 nm. Due to high-surface area to volume ratio, significant fraction of surface atoms, reduced grain size, and their significant volume fraction of grain boundaries and triple junctions, nanomaterials exhibit many unusual thermal, mechanical, electrical, chemical, and electrochemical properties compared to conventional polycrystalline or amorphous materials. In this chapter, the effect of particle or grain size on the thermal, mechanical, electrical, magnetic, optical, and catalytic properties of nanomaterials is discussed. 3.1 Thermal Properties 3.1.1 Melting Point Melting point is the temperature when atoms, ions, or molecules of a crystalline material change their periodic ordered state to the disordered state. A number of studies reveal that the melting point of metals such as In, Sn, Pb, Bi, Cd, Al, Ag, and Au decreases with decreasing their size particularly below 30 nm. The melting initiates from the surface of the materials and is characterized by the increased mobility of the atoms or molecules in the top surface layers. The diffusion coefficient of these atoms approaches liquid-like values at temperatures much lower than the melting point of the bulk material [ 1 – 6 ]. This is because of the high-surface area to volume ratio of the nanoparticles which in turn have high-surface energies; hence, the activation energy required for the melting of the surface atoms is lower than the bulk. An example of a decrease in melting point of aluminum (Al) as a function of Al clusters is shown in Figure 3.1. There is a decrease in the melting point as the cluster size decreases. A reduction of 140°C has been reported for the Al clusters with radii of ∼ 2 nm [ 5 ]. Figure 3.1 Melting point as a function of Al cluster size, where T m is the melting point and R is the radii of Al cluster...
eBook - ePubEngineering Physics of High-Temperature Materials
Metals, Ice, Rocks, and Ceramics
- Nirmal K. Sinha, Shoma Sinha(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
...Thus, AE techniques may be used for ascertaining tensile strengths from uniaxial compression tests of coarse‐grained materials at high temperatures for which it is difficult to perform direct tensile tests. Yet, information on the tensile strength of freshwater columnar‐grained ice in lakes and rivers in cold regions of the earth is essential for the estimation of the bearing capacity of floating ice covers used for making roads or airstrips for transportation purposes and recreation facilities, such as skating rinks (see Chapter 10). 7.2 Grain Size Effects on Strength 7.2.1 Popular Low‐Temperature Concept of Strength Many important engineering concepts and relationships for the characterization of fracture failures developed over many decades and these have been used extensively. It is well established that at ordinary normal temperatures, the finer the grain, the higher the strength of the solid. The most important engineering requirement for maintaining the strength of a solid is not to allow its constituent grains to grow. Grain growth is detrimental at regular or thermodynamically low homologous temperatures, <0.3 T m. The “tensile yield” stress (or yield strength) for most polycrystalline metals at everyday temperatures is related to the grain size by: (7.4) where σ i is assumed to be a friction stress opposing motion of a dislocation, K y is a measure of the extent to which dislocations are piled up at barriers, and l is the “length” of grain intercept. Here, the microstructural parameter, l, represents the mean value of intercepts measured by a linear grain count on two‐dimensional sections. Equation (7.4) is essentially built on the phenomenological observation that is commonly known as the Hall–Petch (HP) relationship established on the works of Hall (1951) and Petch (1953)...
eBook - ePubMechanical Alloying
For Fabrication of Advanced Engineering Materials
- M. Sherif El-Eskandarany(Author)
- 2001(Publication Date)
- William Andrew(Publisher)
...3.1), since the former has a greater total grain boundary area to impede dislocation motion. For many materials the yield strength, σ y, varies with grain size according to: Figure 3.1 The influence of grain size on the yield strength of Cu-30% Zn brass alloy. (After Suzuki.) [5] Eq. (3.1) In this expression, termed the Hall-Petch equation, d is the average grain diameter, and σ 0 and k y are constants for a particular material. The information for the average grain diameters can be primarily obtained from TEM and/or HRTEM observations, and x-ray line broadening. 3.3 FORMATION OF NANOCRYSTALLINE MATERIALS BY BALL MILLING TECHNIQUE Nanocrystalline materials can be successfully synthesized by several techniques, including inert gas condensation, [6] rapid solidification, [7] electrodeposition, [8] sputtering, [9] crystallization of amorphous phases, [10] and chemical processing. [11] (See Fig. 3.2.) Figure 3.2 Nanocrystalline materials are a unique class of the advanced engineering materials that can be prepared by several methods. It has been reported that composites of amorphous and nanocrystalline phases have unique properties when compared with those of amorphous phases. [12] Amongst the different options for preparations, the mechanical alloying method has been considered the most powerful tool for nanostructured materials [13] because of its simplicity, relatively inexpensive equipment, and the possibility of producing large quantities, that can be scaled up to several tons. [14] The formation of nanocrystalline materials during MA of ceramics or metallic powders is attributed to the intense cold working on the ball milled powders. This leads to a dramatic increase in the number of imperfections (e.g., point and lattice defects) which leads to decreasing the thermodynamic stability of the starting materials...
