Precipitation in a Ni–Al–Mo Directionally Solidified Eutectic Composite
The most recently developed high temperature materials are known as in-situ composites or directionally solidified eutectics. Since the composites are formed during solidification, they have excellent morphological stabilities up to near their melting temperatures. The Ni-Al-Mo system has received the greatest attention for recent several years(1)~(3) since the alloy is expected to form a structure consisting of ductile α-Mo fibers aligned in aductile γ+γ' matrix†. However variations in microstructures and the resultant strength by thermal treatments have not been investigated in earlier papers(1)~(3). In a separate paper(4) the present authors have reported the detailed microstructural analysis of as-grown composite and the microstructural and strength variations of the composite due to quenching from high annealing temperatures. The present paper describes the microstructural changes occurring during ageing of a Ni-12.7at%Al-21.6at%Mo alloy composite after quenching from 1533K studied by transmission electron microscopy
Microstructural characterization of rapidly solidified nickel-base superalloys
Several Ni-Al-Mo-based eutectic superalloys were rapidly solidified using a chilled block melt spinning process. The effects of rapid solidification on the microstructure were studied using optical microscopy, transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM). Results showed, except for the alloy containing chromium, that the microstructure varied as a function of ribbon thickness from segregationless solidification at the wheel side of the ribbon to dendritic solidification at the free side. In addition, alloys with the same solidification rate showed a large variation in microstructure depending upon the solid state cooling rate. The rapidly solidified eutectic Ni-Al-Mo alloy with a small amount of rhenium and vanadium did not show any improvement on delaying or prohibiting the formation of the embrittlingδ-NiMo phase on ageing at 1000‡ C. This was determined from microstructural as well as chemical analysis using STEM. Differential thermal analysis was used to obtain melting temperature,γ′-Ni3Al solvus, and heat of formation for the alloys.
Recent progress in engineering near-infrared persistent luminescence nanoprobes for time-resolved biosensing/bioimaging
Persistent luminescence nanoprobes (PLNPs) can remain luminescent after ceasing excitation. Due to the ultra-long decay time of persistent luminescence (PersL), autofluorescence interference can be efficiently eliminated by collecting PersL signal after autofluorescence decays completely, thus the imaging contrast and sensing sensitivity can be significantly improved. Since near-infrared (NIR) light shows reduced scattering and absorption coefficient in penetrating biological organs or tissues, near-infrared persistent luminescence nanoprobes (NIR PLNPs) possess deep tissue penetration and offer a bright prospect in the areas of in vivo biosensing/bioimaging. In this review, we firstly summarize the design of different types of NIR PLNPs for biosensing/bioimaging, such as transition metal ions-doped NIR PLNPs, lanthanide ions-doped NIR PLNPs, organic molecules-based NIR PLNPs, and semiconducting polymer self-assembled NIR PLNPs. Notably, organic molecules-based NIR PLNPs and semiconductor self-assembled NIR PLNPs, for the first time, were introduced to the review of PLNPs. Secondly, the effects of different types of charge carriers on NIR PersL and luminescence decay of NIR PLNPs are significantly emphasized so as to build up an in-depth understanding of their luminescence mechanism. It includes the regulation of valence band and conduction band of different host materials, alteration of defect types, depth and concentration changes caused by ion doping, effective radiation transitions and energy transfer generated by different luminescence centers. Given the design and potential of NIR PLNPs as long-lived luminescent materials, the current challenges and future perspective in this rapidly growing field are also discussed.