Improvement for creep strength of a second-generation single crystal superalloy by design of heat treatments

Design of heat treatments is related to the key technology for development of nickel-based single crystal superalloys(Ni-SXs). Based on the full understanding of the solidification characteristics, this work applies optimization design of heat treatments for a second-generation Ni-SX. Microstructure evolution and creep properties are compared in the material under conventional/standard(Std.) and optimized(Opt.) treatments. For the Std. sample,strong dendritic segregations determine inconsistent microstructure evolution in the dendritic(D) and interdendritic region(ID), while the latter serves as weak area to have the prior microcrack initiation, damaging overall performance of the alloy. The Opt. treatment applies higher homogenization temperature, leading to overall reduced segregations, while not inducing incipient melting. A lower temperature of first-step ageing is used to lower the size ofγ'particles. These help to form the more uniform microstructure in dendritic and interdendritic region and relieve the inconsistent microstructure evolution. The balanced local strength makes ID no longer as the weak area,thus restricting microcrack initiation. Great improvement of high temperature and low stress property is obtained by this progress, leading to the pronounced increase of creep rupture life under 1100 °C/140 MPa.

WAPN:a distributed wormhole attack detection approach for wireless sensor networks

As the applications of wireless sensor networks(WSNs) diversify,providing secure communication is emerging as a critical requirement. In this paper,we investigate the detection of wormhole attack,a serious security issue for WSNs. Wormhole attack is difficult to detect and prevent,as it can work without compromising sensor nodes or breaching the encryption key. We present a wormhole attack detection approach based on the probability distribution of the neighboring-node-number,WAPN,which helps the sensor nodes to judge distributively whether a wormhole attack is taking place and whether they are in the in-fluencing area of the attack. WAPN can be easily implemented in resource-constrained WSNs without any additional requirements,such as node localization,tight synchronization,or directional antennas. WAPN uses the neighboring-node-number as the judging criterion,since a wormhole usually results in a significant increase of the neighboring-node-number due to the extra attacking link. Firstly,we model the distribution of the neighboring-node-number in the form of a Bernoulli distribution. Then the model is simplified to meet the sensor nodes' constraints in computing and memory capacity. Finally,we propose a simple method to obtain the threshold number,which is used to detect the existence of a wormhole. Simulation results show that WAPN is effective under the conditions of different network topologies and wormhole parameters.