• K T Kashyap

Articles written in Bulletin of Materials Science

• Role of diffusional coherency strain theory in the discontinuous precipitation in Mg-Al alloy

Discontinuous precipitation (DP) occurs in many alloy systems under certain conditions. It is called discontinuous precipitation because precipitation occurs on prior matrix grain boundaries followed by grain boundary movement. The DP nodule consists of alternate lamellae of the precipitate and the matrix respectively. The chemical driving force for DP is one of solute supersaturation. Although solute supersaturation is responsible for precipitation, it has to be coupled with another driving force to explain grain boundary migration. This coupling driving force has been identified to be diffusional coherency strain which has been verified to be active in diffusion induced grain boundary migration and liquid film migration.

To test diffusional coherency strain theory for discontinuous precipitation Mg-7Al and Mg-7Al-1Pb alloys were studied. While the fraction transformed was high at 6% in Mg-7Al alloy, dit was significantly low at 2% in Mg-7Al-1Pb alloy. The velocity of DP nodules decreased by half in alloy with Pb as compared to the alloy without Pb. Theoretical calculations also predict that the misfit parameter δth decreases with the addition of Pb. These observations are an evidence to the fact that diffusional coherency strain is the most active driving force for the movement of the grain boundaries of the DP nodules during discontinuous precipitation in Mg-Al alloy.

• Role of work hardening characteristics of matrix alloys in the strengthening of metal matrix composites

The strengthening of particulate reinforced metal-matrix composites is associated with a high dislocation density in the matrix due to the difference in coefficient of thermal expansion between the reinforcement and the matrix. While this is valid, the role of work hardening characteristics of the matrix alloys in strengthening of these composites is addressed in the present paper. It is found that commercial purity aluminium which has the lowest work hardening rate exhibits the highest strength increment. This effect is due to increased prismatic punching of dislocations. This relationship of decreasing work hardening rate associated with increasing prismatic punching of dislocations in the order 7075, 2014, 7010, 2024, 6061 and commercial purity aluminium leading to increased strength increments is noted.

• A test for diffusional coherency strain hypothesis in the discontinuous precipitation in Mg-Al alloy

Discontinuous precipitation (DP) occurs in many alloy systems under certain conditions. Although solute supersaturation is the chemical driving force for DP, this has to be coupled with another driving force for grain boundary migration. This was identified to be diffusional coherency strain ahead of the moving boundary in the case of diffusion induced grain boundary migration (DIGM) and liquid film migration (LFM). In the present work, the validity of diffusional coherency strain hypothesis is verified in Mg-Al alloy, which exhibits discontinuous precipitation. Samples were tested with an applied stress simultaneously with discontinuous precipitation and it was found that the velocity of the boundaries both parallel and transverse to the stress axis obeys the model for diffusional coherency strain. This work can be used as a conclusive evidence for diffusional coherency strain hypothesis for the occurrence of discontinuous precipitation in Mg-Al alloys.

• A model for two-step ageing

In commercial practice, two-step ageing is commonly used in Al-Zn-Mg alloys to produce a fine dispersion of η′ precipitates to accentuate the mechanical properties and resistance to stress corrosion cracking. While this is true in Al-Zn-Mg alloys, two-step ageing leads to inferior properties in Al-Mg-Si alloys. This controversial behaviour in different alloys can be explained by Pashley’s Kinetic model. Pashley’s model addresses the stability of clusters after two-step ageing. In the development of the model, the surface energy term between cluster and matrix is taken into account while the coherency strains between the cluster and matrix are not considered. In the present work, a model is developed which takes into account the coherency strains between cluster and matrix and defines a new stability criterion, inclusive of strain energy term. Experiments were done on AA 7010 aluminium alloy by carrying out a two-step ageing treatment and the results fit the new stability criterion. Thus it is found that the new model for two-step ageing is verified in the case of Al-Zn-Mg alloy.

• On the origin of recrystallization textures

The development of recrystallization textures has been debated for the past 50 years. Essentially the rival theories of evolution of recrystallization textures i.e. oriented nucleation (ON) and oriented growth (OG) has been under dispute. In the ON model, it has been argued that a higher frequency of the special orientation (grains) than random occur, thus accounting for the texture. In the OG model, it has been argued that the specially oriented grains have a high mobility boundary and thus can migrate faster and grow to a larger size as compared to random orientations thus contributing to the final recrystallization texture.

In FCC metals and alloys like aluminium, cube orientation [(001) $\langle$100$\rangle$] is the recrystallization texture component. In the classic OG model, cube orientation is supposed to be misoriented from 𝑆-orientation [(123) $\langle$63$\bar{4}\rangle$] which is a deformation texture component by a 40° about $\langle$111$\rangle$ relationship which is supposed to be a high mobility boundary leading to faster growth of cube grains. Stereographic calculations and analytical calculations are presented in this paper to the effect that the 𝑆-orientation (123) $\langle$63$\bar{4}\rangle$ is not misoriented from cube (100) $\langle$001$\rangle$ by 40° (111) whereas another deformation texture component (123) $\langle$41$\bar{2}\rangle$ which is termed the 𝑅-component is misoriented from cube component by 40°$\ \langle$111$\rangle$ . 𝑅-component is also seen in deformation textures of aluminium and hence the classic OG model remains valid with respect to the 𝑅-component.

• Effects and mechanisms of grain refinement in aluminium alloys

Grain refinement plays a crucial role in improving characteristics and properties of cast and wrought aluminium alloys. Generally Al–Ti and Al–Ti–B master alloys are added to the aluminium alloys to grain refine the solidified product. The mechanism of grain refinement is of considerable controversy in the scientific literature. The nucleant effects i.e. which particle and its characteristics nucleate 𝛼-Al, has been the subject of intensive research. Lately the solute effect i.e. the effect of dissolved titanium on grain refinement, has come into forefront of grain refinement research. The present paper attempts to review the literature on the nucleant effects and solute effects on grain refinement and addresses the importance of dissolved titanium in promoting nucleation of 𝛼-Al on nucleant particles.

• Effect of zirconium addition on the recrystallization behaviour of a commercial Al–Cu–Mg alloy

It is well known that the second phase particles have an effect on recrystallization and grain growth behaviour of an alloy. Particularly the bimodal distribution of second phase particles has an effect which is opposite in sense where coarse second phase particles (&gt; 1 𝜇m) stimulate nucleation while fine particles exhibit Zener drag.

In the literature, the effect of zirconium addition to aluminium alloys has been well documented in order to produce superplasticity by giving ultra fine grain size to the alloy. Addition of zirconium produces Al3Zr particles which pin the grain boundaries during recrystallization and grain growth.

In the present work, zirconium was added to a commercial Al–Cu–Mg alloy and by heat treatment Al3Zr particles were precipitated and after forging, the grain size was an order of magnitude lower than the alloy without zirconium.

Transmission electron microscopy was employed to characterize the second phase particles, i.e. Al3Zr particles and found to be rod shaped and identified to be cubic ordered 𝐿12 phase with a lattice parameter of 0.408 nm. Further, it was observed that fine (100 nm) Al3Zr particles promote only continuous recrystallization which is polygonization of subgrains and subgrain growth.

It was found that the fine dispersion of Al3Zr particles inhibits both recrystallization and grain growth in the commercial Al–Cu–Mg alloy.

• Mechanism of cube grain nucleation during recrystallization of deformed commercial purity aluminium

Cube texture is a sharp recrystallization texture component in fcc metals like aluminium, copper, etc. It is described by an ideal orientation i.e. (100) $\langle 100 \rangle$. The subject of cube texture nucleation i.e. cube grain nucleation, from the deformed state of aluminium and copper is of scientific curiosity with concurrent technological implications. There are essentially two models currently in dispute over the mechanism of cube grain nucleation i.e. the differential stored energy model founded on the hypothesis proposed by Ridha and Hutchinson and the micro-growth selection model of Duggan et al. In this paper, calculations are made on the proposal of Ridha and Hutchinson model and the results are obtained in favour of the differential stored energy model. It is also shown that there is no need for the micro-growth model.

• Modelling of soft impingement during solidification

It has been well established that spheroidal grain morphology in the microstructure forms during stir casting (rheocasting) and grain refinement of magnesium alloys by zirconium addition. This curious microstructure has been of interest both commercially from enhanced mechanical properties and also scientific interest in explaining the mechanism of spheroidal grain formation. Vogel and Doherty proposed a model describing the fracturing of dendrite arms during stir casting to produce a high density of nuclei which they presume to give rise to spheroidal grains. They proposed that there is soft impingement of diffusion fields of neighbouring nuclei, which reduces the concentration gradient ahead of the planar solid and liquid interface, which in turn negates shape instability. In this paper, the Vogel and Doherty model is pursued by quantitative modeling of soft impingement problem and related to shape instability by constitutional supercooling theory. This analysis correctly predicts the spheroidal grain formation during stir casting or rheocasting. This model can also be used to explain the grain refinement of magnesium alloys by zirconium addition wherein spheroidal grains are formed.

• On Young’s modulus of multi-walled carbon nanotubes

Carbon nanotubes (CNTs) were discovered by Iijima in 1991 as the fourth form of carbon. Carbon nanotubes are the ultimate carbon fibres because of their high Young’s modulus of ≈ 1 TPa which is very useful for load transfer in nanocomposites. In the present work, CNT/Al nanocomposites were fabricated by the powder metallurgy technique and after extrusion of the nanocomposites bright field transmission electron microscopic (TEM) studies were carried out. From the TEM images so obtained, a novel method of ascertaining the Young’s modulus of multi-walled carbon nanotubes is worked out in the present paper which turns out to be 0.9 TPa which is consistent with the experimental results.

• Discontinuous precipitation in copper base alloys

Discontinuous precipitation (DP) is associated with grain boundary migration in the wake of which alternate plates of the precipitate and the depleted matrix form. Some copper base alloys show DP while others do not. In this paper the misfit strain parameter, 𝜂, has been calculated and predicted that if 100 𝜂 &gt; ± 0.1, DP is observed. This criterion points to diffusional coherency strain theory to be the operative mechanism for DP.

• Small-angle scattering from GP zones in Al–Cu alloy

It is well known that Guinier Preston (GP) zones form in Al–Cu alloys upon solutionizing and artificial aging, which are extensively used in commercial practice. It is well established that GP zones are discshaped precipitates, i.e. disks of clusters of copper atoms in the FCC aluminium matrix. These disks have coherency strain fields in aluminium that give the alloy its high yield strength. The formation of GP zones in the supersaturated aluminium matrix is thought to be heterogeneous nucleation and growth. Some authors have believed that the formation of GP zones is by spinodal decomposition of the supersaturated Al–Cu solid solution. The main objective of the present work is to test whether spinodal decomposition is responsible for the formation of GP zones in Al–Cu alloy. The experimental alloy AA2219 was selected for its high copper content (Al–6%Cu–0.2%Zr). After solutionizing and artificial aging, the aging curve was plotted and smallangle scattering experiments were carried on the powdered samples as a function of time during artificial aging. Small-angle scattering data were analysed, and evidence has been obtained for the occurrence of spinodal decomposition as the mechanism responsible in the early stages of formation of GP zones.

• On grain growth kinetics in two-phase polycrystalline materials through Monte Carlo simulation

Monte Carlo Potts model simulation was carried out on a 2D square lattice for various surface fractions of second phase particles for over 50,000 iterations. The observations are in good agreement with known theoretical and experimental results with respect to both growth kinetics as well as grain size distribution. Further, the average grain size and the largest grain size were computed for various surface fractions which have indicated normal grain growth and microstructure homogeneity. The surface fraction of the second phase particles interacting with the grain boundaries (𝛷), hitherto not computed through the simulation route, is shown to vary inversely as the average grain size due to Zener pinning.

• # Bulletin of Materials Science

Current Issue
Volume 42 | Issue 6
December 2019

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019