The Analysis of Thermal-Induced Phase Transformation and Microstructural Evolution in Ni-Ti Based Shape Memory Alloys By Molecular Dynamics

Shape memory alloys has been widely applied on actuators and medical devices.The transformation temperature and microstructural evolution play two crucial factors and dominate the behavior of shape memory alloys.In order to understand the influence of the composition of the Ni-Ti alloys on the two factors,molecular dynamics was adopted to simulate the temperature-induced phase transformation.The results were post-processed by the martensite variant identification method.The method allows to reveal the detailed microstructural evolution of variants/phases in each case of the composition of Ni-Ti.Many features were found and having good agreement with those reported in the literature,such as the well-known Rank-2 herringbone structures;the X-interface;Ni-rich alloys have lower transformation temperature than Ti-rich alloys.In addition,some new features were also discovered.For example,the Ti-rich alloys enabled an easier martensitic transformation;the nucleated martensite pattern determined the microstructural evolution path,which also changed the atomic volume and temperature curves.The results generated in the current study are expected to provide the design guidelines for the applications of shape memory alloys.

CFD simulation of wind and thermal-induced ventilation flow of a roof cavity

The hygrothermal performance of a ventilated roof cavity is greatly affected by the airflow passing through it.This ventilation flow is mainly driven by the wind pressure difference between openings and the thermal-induced buoyancy.However,the wind effect is not well understood as it is often neglected in previous studies.The present study investigates the properties of such airflows,including the flow pattern,flow regime,and flow rate,using a CFD method.The target building is a large-span commercial building with a low-pitched roof.To study the wind-induced airflows,the onset atmospheric boundary layer wind flow was modelled,and the results were compared with the site-measured data recorded in the literature.To study the thermal-induced buoyancy effects,a roof cavity model found in the literature with experimental data was adopted.The findings show that the flow pattern in the roof cavity varied with the airflow driven factors.The flow separation at the windward eave inlet of the thermally induced flows are more pronounced compared with those of the wind-induced flows.Furthermore,the wind-induced airflows can generate around two times more ventilation flow rate through the roof cavity compared to the thermal-induced airflow.The findings indicate that wind-induced ventilation flows are the dominant factor of the roof cavity ventilation in a large-span,low-pitched building.

Preparation and Characterization of Low Density Poly(Imino Imino Ketone)Foam

Poly（Imino Ketone） （PIK） is a group of novel high performance polymer material with excellent thermal properties and dissolubility. Aiming at the requirements of inertial confined fusion （ICF） studies on low density polymer foams, we firstly synthesized poly（imino imino ketone） （PIIK） by palladium catalyzed C-N cross-coupling reaction, and successfully prepared the PIIK foam material with a density of 80-300 mg/cm3 by using PIIK as the raw material with thermal-induced phase separation and lyophilization technique. Mercury injection method was used to determine the structure of PIIK foams, and the results indicated that the mean pore diameter was lower than 5 ~tm and it had relatively high voidage and specific surface area.