The origin of different morphology of internal oxide precipitates in ferritic and austenitic steels

The internal oxide precipitates were supposed to be spherical in Wagner’s original theory,while the fol-lowing research demonstrated that this assumption is an exception rather than the truth,which caused deviations in the application of this theory.In this study,the internal oxide precipitates have a needle-like and a near-spherical morphology in a Fe-9Cr ferritic and a Fe-17Cr-9Ni austenitic steels after expo-sure to 600℃ deaerated steam for 600 h,respectively.The nano-to-atomic scale characterization shows that the morphology of the internal oxide precipitates is controlled by the structure of the interfaces be-tween the metal matrix and the internal oxide,while the interface structure is mainly affected by the crystallographic structure of the two phases and their orientation relationship.In addition,the internal oxide precipitation-induced volume expansion and the outward Fe diffusion-induced volume shrink oc-cur simultaneously during the oxidation process.The stress status in the internal oxidation zone(IOZ)is the competing result of the two factors,which could dynamically affect the high-temperature oxidation.The results obtained in this study suggest that there is potential to control the distribution,morphology,and interface structure of the internal oxide precipitates by selecting appropriate base metal and internal oxide-forming element,in order to obtain better high-temperature oxidation-resistant materials.

Structural and compositional analysis of(InGa)(AsSb)/GaAs/GaP Stranski-Krastanov quantum dots

We investigated metal-organic vapor phase epitaxy grown(InGa)(AsSb)/GaAs/GaP Stranski–Krastanov quantum dots(QDs)with potential applications in QD-Flash memories by cross-sectional scanning tunneling microscopy(X-STM)and atom probe tomography(APT).The combination of X-STM and APT is a very powerful approach to study semiconductor heterostructures with atomic resolution,which provides detailed structural and compositional information on the system.The rather small QDs are found to be of truncated pyramid shape with a very small top facet and occur in our sample with a very high density of∼4×10^(11) cm^(−2).APT experiments revealed that the QDs are GaAs rich with smaller amounts of In and Sb.Finite element(FE)simulations are performed using structural data from X-STM to calculate the lattice constant and the outward relaxation of the cleaved surface.The composition of the QDs is estimated by combining the results from X-STM and the FE simulations,yielding∼In_(x)Ga_(1−x)As_(1−y)Sb_(y),where x=0.25–0.30 and y=0.10–0.15.Noticeably,the reported composition is in good agreement with the experimental results obtained by APT,previous optical,electrical,and theoretical analysis carried out on this material system.This confirms that the InGaSb and GaAs layers involved in the QD formation have strongly intermixed.A detailed analysis of the QD capping layer shows the segregation of Sb and In from the QD layer,where both APT and X-STM show that the Sb mainly resides outside the QDs proving that Sb has mainly acted as a surfactant during the dot formation.Our structural and compositional analysis provides a valuable insight into this novel QD system and a path for further growth optimization to improve the storage time of the QD-Flash memory devices.