Formation of face-centered cubic titanium in laser surface re-melted commercially pure titanium plate

Micron-scale face-centered cubic titanium phase(named as δ phase) were noticed in the re-melted zone of laser surface re-melted commercially pure titanium plate.The morphology,sub-structure,orientation and distribution of δ phase were investigated by scanning electron microscopy,electron back-scattered diffraction and transmission electron microscopy.Three kind formation processes of δ phase were put forward based on the investigation.The first one is α'→δ transformation which takes place in single α'grains and leads to the orientation relationship {001}δ//{0001}α' 110 δ// 1120 α'.The second one is β→α'+ δ transformation which takes place at α'/α'interfaces and leads to the orientation relationship{001}δ//110β110 δ//111β.The third one is another kind of β→α'+ δ transformation that takes place at α'/α'interfaces and leads to the orientation relationship111δ//110β 110 δ// 111 β.It is believed that the transformations of δ phase are stress assistant ones and in the present investigation,the phase transformation stress of β→α'transformation acts as the assistant driving force for the formation of δ phase.     

Diffusion behavior and reactions between Al and Ca in Mg alloys by diffusion couples

The diffusion behavior and reactions between Al and Ca in Mg alloys by diffusion couple method were investigated. Results demonstrate that Al_2Ca is the only phase existing in the diffusion reaction layers.The volume fraction of Al_2Ca in diffusion reaction layers increases linearly with temperature. The standard enthalpy of formation for intermetallic compounds was rationalized on the basis of the Miedema model. Al-Ca intermetallic compounds were preferable to form in the Mg-Al-Ca ternary system under the same conditions. Over the range of 350–400?C, the structure of Al_2Ca is more stable than that of Al_4Ca, Al_(14)Ca_(13) and Al_3Ca_8. The growth constants of the layer Ⅰ, layer Ⅱ and entire diffusion reaction layers were determined. The activation energies for the growth of the layer Ⅰ, layer Ⅱ and entire diffusion reaction layers were(80.74 ± 3.01) k J/mol,(93.45 ± 2.12) k J/mol and(83.52 ± 1.50) k J/mol, respectively.In layer Ⅰ and Ⅱ, Al has higher integrated interdiffusion coefficients D~(Int, layer)ithan Ca. The average effective interdiffusion coefficients D_(Al)~(eff) values are higher than D_(Ca)~(eff) in the layer Ⅰ and Ⅱ.     

Molecular dynamic modelling of fatigue crack growth in aluminium using LEFM boundary conditions

A molecular dynamic (MD) model of a crack in pure aluminium has been developed with isotropic Linear Elastic Fracture Mechanics (LEFMs) boundary displacements that simulates the fatigue crack growth process. The model consists of a cylindrical region filled with atoms around a crack tip and subject to boundary displacements that change due to cyclic loading. A sinusoidal load that produced a $K_{\max }=1.0\text{MPa}\sqrt{\text{m}}$ was applied to produce fatigue crack growth using three different atomic potentials for aluminium at T = 20 K, and a range of different K_min. Each run consisted of the application of fifteen or more loading cycles. In some cases, the crack tip was seen to advance in each cycle typical of fatigue, however, growth was smooth and continuous during the entire cycle with contraction occurring during the unloading phase of the cycle. The model contained 3 × 10⁶ atoms and had a diameter and width of 20 nm. This width was just large enough for fragments of sessile dislocations to form and couple with the glissile dislocations emitted from the crack tip, resulting in work hardening about the crack tip. The model was oriented for cracking on the {1 1 0} plane in the 〈1 0 0〉 direction. Crack advance was observed to be due to a combination of dislocation emission and atomic separation.