Microstructure and crystallization mechanism of Ti-based bulk metallic glass by electron beam welding

Abstract In this work, we report on the successful welding of the Ti-based bulk metallic glass (BMG) plates via electron beam welding route. Microstructure determination shows that crystalline phases exist both in weld zone (WZ) and heat affected zone (HAZ). The critical cooling rate for glass formation in WZ is depended on the solidification condition. The continuous heating transformation curve (CHT) of glass transition temperature (Tg) and crystallization temperature (Tx) during heating process, time-temperature-transformation diagram (C-curve) during cooling process, and the thermal cycle curves are obtained by Kissinger equation, nucleation theory, and temperature field simulation, respectively. The crystallization mechanism in HAZ was discussed in details during the heating and cooling processes. The intersection between cooling curve and C-curve denotes the crystallization of HAZ during the cooling process

Tensile creep characterization and prediction of Zr-based metallic glass at high temperatures

Abstract The high temperature creep behaviors of a Zr-based bulk metallic glass (BMG) are studied by uniaxial tensile creep experiments under applied stresses of 50–180 MPa at temperatures of 660–700 K. The microstructural observations of the BMG samples after creep tests show that crystalline phases can be detected under high temperature or high applied stress. Constitutive models for predicting the high temperature creep behaviors of the studied Zr-based BMG are established based on the Ɵ projection method. The creep activation energy and stress exponent are also calculated to establish the creep model. The parameters of the established models are found to be closely associated with the applied stress and temperature. The results show an excellent agreement between the measured and predicted results, confirming the validity of the established model to accurately estimate the high temperature creep curves for the Zr-based BMG. Moreover, based on the classical diffusion creep theory, a schematic model is proposed to describe the creep behaviors of BMGs from the framework of free volume theory

Evolution of stomatal and trichome density of the Quercus delavayi complex since the late Miocene

A fossil oak species, Quercus tenuipilosa Q. Hu et Z.K. Zhou, is reported from the upper Pliocene Ciying Formation in Kunming, Yunnan Province, southwestern China. The establishment of this species is based on detailed morphologic and cuticular investigations. The fossil leaves are elliptic, with serrate margins on the apical half. The primary venation is pinnate, and the major secondary venation is craspedodromous. The tertiary veins are opposite or alternate-opposite percurrent with two branches. The stomata are anomocytic, occurring only on the abaxial epidermis. The trichome bases are unicellular or multicellular. The new fossil species shows the closest affinity with the extant Q. delavayi and the late Miocene Q. praedelavayi Y.W. Xing et Z.K. Zhou from the Xiaolongtan Formation of the Yunnan Province. All three species share similar leaf morphology, but differ with respect to trichome base and stomatal densities. Q. tenuipilosa, Q. praedelavayi, and Q. delavayi can be considered to constitute the Q. delavayi complex. Since the late Miocene, a gradual reduction in trichome base density has occurred in this complex. This trend is the opposite of that of precipitation, indicating that increased trichome density is not an adaptation to dry environments. The stomatal density (SD) of the Q. delavayi complex was the highest during the late Miocene, declined in the late Pliocene, and then increased during the present epoch. These values show an inverse relationship with atmospheric CO2 concentrations, suggesting that the SD of the Q. delavayi complex may be a useful proxy for reconstruction of paleo-CO2 concentrations