Study on hot deformation behavior and microstructure evolution of cast-extruded AZ31B magnesium alloy and nanocomposite using processing map

The hot deformation behavior and microstructural evolution of cast-extruded AZ31B magnesium alloy and nanocomposite have been studied using processing-maps. Compression tests were conducted in the temperature range of 250–400 °C and strain rate range of 0.01–1.0 s⁻¹. The three-dimensional (3D) processing maps developed in this work, describe the variations of the efficiency of power dissipation and flow instability domains in the strain rate ($\dot{\epsilon }$) and temperature (T) space. The deformation mechanisms namely dynamic recrystallization (DRX), dynamic recovery (DRY) and instability regions were identified using processing maps. The deformation mechanisms were also correlated with transmission electron microscopy (TEM) and optical microscopy (OM). The optimal region for hot working has been observed at a strain rate ($\dot{\epsilon }$) of 0.01 s⁻¹ and the temperature (T) of 400 °C for both magnesium alloy and nanocomposite. Few instability regimes have been identified in this study at higher strain rate ($\dot{\epsilon }$) and temperature (T). The stability domains have been identified in the lower strain rate regimes.     

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.     

Effect of crystal orientation on corrosion behavior of directionally solidified Mg-4 wt% Zn alloy

Microstructure and crystallographic orientation of directionally solidified Mg-4 wt% Zn alloy were characterized by X-ray computed tomography (XCT) and electron backscatter diffraction (EBSD) in this study. Results reveal that Mg-4 wt% Zn alloy with dendritic microstructure exhibits typical $\left\{0002\right\}$ basal texture along growth direction. Based on this, the effect of grain orientation on corrosion behavior of directionally solidified Mg-4 wt% Zn alloy in 0.9 wt% NaCl solution was investigated. Result shows that $\left\{0002\right\}$ oriented planes have better corrosion resistance than $\left\{11{\displaystyle \overline{2}}0\right\}$ and $\left\{10{\displaystyle \overline{1}}0\right\}$ ones, which is attributed to a synergistic effect of surface energy, atomic packing density and the stability of oxidation film.     

3D atomistic simulation of fatigue behaviour of cracked single crystal of bcc iron loaded in mode III

We present new results of 3D molecular dynamic (MD) simulations of a mechanical behaviour of an edge crack $(\overline{1}10)$ [1 1 0] (crack plane/crack front) in a single crystal of bcc iron cyclically loaded in mode III at room temperature. Our previous 3D molecular dynamic simulations showed that this crack under monotonic uniaxial tension loading in mode I emitted blunting dislocations in the inclined $〈1\overline{1}1〉\{\overline{1}12\}$ slip systems and it was stable up to very high loads, in qualitative agreement with experiments and continuum predictions. Our question is how the cyclic loading in mode III changes the crack behaviour and its stability. The paper also brings information on how the distribution of external forces in mode III influences the fatigue crack behaviour and how the defects generated by the crack can produce experimentally observed advances of the crack front in local modes I, II, I + II and II + III.     

Visible photoluminescense of the (112¯0), (101¯1) and (0001) surfaces of ZnO nanofilms

Thin ZnO nanofilms 25 nm thick with (0001), $\left(11\overline{2}0\right)$ and $\left(10\overline{1}1\right)$ as surfaces were grown epitaxially on the NaCl (111) and (001) surfaces. The room temperature photoluminescence (PL) spectrum from the $\left(11\overline{2}0\right)$ nanofilm has a sharp UV emission but negligible green emission, indicating that it has good quality and low defect density. However, the PL spectra from the $\left(10\overline{1}1\right)$ and (0001) surface nanofilms have a broad green emission, and that of the $\left(10\overline{1}1\right)$ surface is stronger than the (0001) surface. The result supports that the surface oxygen vacancies are the probable origins of the green emission.     

Effect of aging treatment on the microstructures and mechanical properties evolution of 25Cr-20Ni austenitic stainless steel weldments with different Nb contents

The microstructure evolutions and the mechanical properties of the 25Cr-20Ni austenitic stainless steel weld metals with different Nb contents were investigated during the long term aging treatment at 700~?C.M_(23)C_6,Nb(C,N),α-Cr phase and Nb-nitride phase(Z phase)were observed in the microstructures of the aged weld metals.The results showed that theα-Cr phase precipitated in the interdendritic regions of the weld metals after being exposed to~ 700?C for 500 h and the element Nb accelerated the precipitation of theα-Cr phase significantly.The density of theα-Cr phase decreased with the increase of the distance away from the primary Nb(C,N).Additionally,theα-Cr phase showed a crystallographic relationship with the austenitic matrix,■.It was observed that the Z phase precipitated in the periphery of the Nb(C,N)and may replace the Nb(C,N)after long term exposure to high temperature.The transformation of the Nb(C,N)into Z phase suggested that the Z phase had a higher stability than the Nb(C,N)par~ticles at 700?C for long term aging.The tensile strength of the Nb-bearing weld metal showed a continuous decrease at the initial stage of the aging treatment and then went up slightly with the prolonged aging time.However,the elongations and the impact energies of the weld metals decreased monotonously with the increase of the aging time.