10 research outputs found
Lanczos exact diagonalization study of field-induced phase transition for Ising and Heisenberg antiferromagnets
Using an exact diagonalization treatment of Ising and Heisenberg model
Hamiltonians, we study field-induced phase transition for two-dimensional
antiferromagnets. For the system of Ising antiferromagnet the predicted
field-induced phase transition is of first order, while for the system of
Heisenberg antiferromagnet it is the second-order transition. We find from the
exact diagonalization calculations that the second-order phase transition
(metamagnetism) occurs through a spin-flop process as an intermediate step.Comment: 4 pages, 4 figure
Metamagnetism in the 2D Hubbard Model with easy axis
Although the Hubbard model is widely investigated, there are surprisingly few
attempts to study the behavior of such a model in an external magnetic field.
Using the Projector Quantum Monte Carlo technique, we show that the Hubbard
model with an easy axis exhibits metamagnetic behavior if an external field is
turned on. For the case of intermediate correlations strength , we observe a
smooth transition from an antiferromagnetic regime to a paramagnetic phase.
While the staggered magnetization will decrease linearly up to a critical field
, uniform magnetization develops only for fields higher than .Comment: RevTeX 5 pages + 2 postscript figures (included), accepted for PRB
Rapid Communication
A model of linear chain submonolayer structures. Application to Li/W(112) and Li/Mo(112)
We propose a lattice gas model to account for linear chain structures
adsorbed on (112) faces of W and Mo. The model includes a dipole-dipole
interaction as well as a long-ranged indirect interaction. We have explicitly
demonstrated that the periodic ground states depend on a competition between
dipole-dipole and indirect interaction. The effect of temperature is studied
within the molecular-field approximation. The numerical results show that for
dipole-dipole interaction only, all long periodic linear chain phases are
suppressed to low temperatures. However, when the long-range indirect
interaction becomes important, the long-periodic linear chain phases start to
fill up the phase diagram and develop a high thermal stability. Model
parameters are chosen to reconstruct a sequence of long-periodic phases as
observed experimentally for Li/Mo(112) and Li/W(112).Comment: RevTeX 9 pages + 5 Postscript figures (included), uses newdoc.sty
(included), to be published in Surface Scienc
Ground states of a one-dimensional lattice-gas model with an infinite range nonconvex interaction. A numerical study
We consider a lattice-gas model with an infinite range pairwise noncovex
interaction. It might be relevant, for example, for adsorption of alkaline
elements on W(112) and Mo(112). We study a competition between the effective
dipole-dipole and indirect interactions. The resulting ground state phase
diagrams are analysed (numerically) in detail. We have found that for some
model parameters the phase diagrams contain a region dominated by several
phases only with periods up to nine lattice constants. The remaining phase
diagrams reveal a complex structure of usually long periodic phases. We also
discuss a possible role of surace states in phase transitions.Comment: 16 pages, 5 Postscript figures; Physical Review B15 (15 August 1996),
in pres
High cycle fatigue and fatigue crack growth rate in additive manufactured titanium alloys
The Wire + Arc Additive Manufacture (WAAM) process can produce large metal parts in the metre scale, at much higher deposition rate and more efficient material usage compared to the powder bed fusion additive manufacturing (AM) processes. WAAM process also offers lead time reduction and much lower buy-to-fly ratio compared to traditional process methods, e.g. forgings. Research is much needed in the areas of fatigue and fracture performance for qualification and certification of additive manufactured aircraft components.
In this study, specimens made of WAAM Ti-6Al-4V alloy were tested and analysed focusing on two key areas of structural integrity and durability:
(1) High cycle fatigue and effect of defects: crack initiation at porosity defects was investigated via fatigue and interrupted fatigue-tomography testing performed on specimens with porosity defects purposely embedded in the specimen gauge section. Key findings are as follows. Presence of porosity did not affect the tensile strengths, however both ductility and fatigue strength were significantly reduced. Fatigue life could not be correlated by the applied stress, e.g. in terms of the S-N curves, owing to the different pore sizes. Using the fracture mechanics approach and Murakami’s stress intensity factor equation for pores, good correlation was found between the fatigue life and stress intensity factor range of the crack initiating defects. Predictive methods for fatigue strength reduction were developed taking account of the defect size, location, and distribution.
(2) Fatigue crack growth rate: effect of heterogeneous microstructure was investigated via two different material deposition methods and testing two crack orientations. Fatigue crack growth rates were measured for damage tolerance design considerations. Unique microstructure features and their effect on the property anisotropy are discussed