arXiv subjects

Graeme J Ackland

Publications and source records attributed to Graeme J Ackland.

22 records · Page 2Linked to original sources

arXiv2015

Theory of high pressure hydrogen, made simple

Phase I of hydrogen has several peculiarities. Despite having a close-packed crystal structure, it is less dense than either the low temperature Phase II or the liquid phase. At high pressure, it transforms into either phase III or IV, depending on the temperature. Moreover, spectroscopy suggests that the quantum rotor behaviour disappears with pressurisation, without any apparent phase transition. Here we present a simple thermodynamic model for this behaviour based on packing atoms and molecules and discuss the thermodynamics of the phase boundaries. We also report first principles molecular dynamics calculations for a more detailed look at the same phase transitions.

physics.comp-ph↗

arXiv2015

Shear melting and high temperature embrittlement: theory and application to machining titanium

We describe a dynamical phase transition occurring within a shear band at high temperature and under extremely high shear rates. With increasing temperature, dislocation deformation and grain boundary sliding is supplanted by amorphization in a highly localized nanoscale band, which allows massive strain and fracture. The mechanism is similar to shear melting and leads to liquid metal embrittlement at high temperature. From simulation, we find that the necessary conditions are, lack of dislocation slip systems, low thermal conduction and temperature near the melting point. The first two are exhibited by bcc titanium alloys, and we show that the final one can be achieved experimentally by adding low-melting point elements: specifically we use insoluble rare earth metals (REMs). Under high shear, the REM becomes mixed with the titanium, lowering the melting point within the shear band and triggering the shear-melting transition. This in turn generates heat which remains localized in the shear band due to poor heat conduction. The material fractures along the shear band. We show how to utilize this transition in the creation of new titanium-based alloys with improved machinability.

cond-mat.mtrl-sci↗

arXiv2013

High Temperature Raman analysis of Hydrogen Phase IV from Molecular Dynamics

We extend our previous studies on phase IV of solid hydrogen by employing larger cells and k-sampling. We show that uncorrelated hexagonal rotations in the weakly bounded G"-layers are needed to account for the experimentally measured Raman spectrum. In large simulations we find no evidence of proton diffusion or layer fluctuations, which we believe are the result of finite size effects. In calculations at higher pressures (above 400 GPa) we identify a new possible candidate for phase IV. We, finally, proposed a revised phase diagram based on our previous and present results.

cond-mat.mtrl-sci↗

arXiv2009

Income and Poverty in a Developing Economy

We present a stochastic agent-based model for the distribution of personal incomes in a developing economy. We start with the assumption that incomes are determined both by individual labour and by stochastic effects of trading and investment. The income from personal effort alone is distributed about a mean, while the income from trade, which may be positive or negative, is proportional to the trader's income. These assumptions lead to a Langevin model with multiplicative noise, from which we derive a Fokker-Planck (FP) equation for the income probability density function (IPDF) and its variation in time. We find that high earners have a power-law income distribution while the low income groups have a Levy IPDF. Comparing our analysis with the Indian survey data (obtained from the world bank website) taken over many years we obtain a near-perfect data collapse onto our model's equilibrium IPDF. The theory quantifies the economic notion of "given other things". Using survey data to relate the IPDF to actual food consumption we define a poverty index, which is consistent with traditional indices, but independent of an arbitrarily chosen "poverty line" and therefore less susceptible to manipulation.

q-fin.GN↗