Preparation, oxidation and ablation resistance of IrAl intermetallic coating

Iridium (Ir) has been selected as the protective coating on the rhenium thruster chamber of the liquid rocket engine, due to its high melting point, excellent corrosion resistance and quite low oxygen permeability. However, Ir forms gaseous oxides rather than a protective oxide barrier above 1100°C under oxidizing environments, leading to a limited lifetime at high temperatures. To improve the oxidation resistance, in the present work pure Ir was modified by pack cementation to produce a single phase IrAl intermetallic coating. The bond strength of the coating was examined by coating-pull-off test. The oxidation and ablation resistance was assessed by cyclic oxidation test at 1800°C and high frequency plasma wind tunnel test (heat flux: 2.03MW/m2 and enthalpy: 19MJ/kg), respectively. It was found that the IrAl coating is well boned to the substrate with a bond strength above 30MPa. The oxidation and ablation resistance of the Ir was significantly enhanced after the pack cementation treatment (see Figure 1). The improvement in oxidation and ablation resistance can be ascribed to the excellent comprehensive properties of the in-situ formed Al2O3 barrier and outstanding physical and chemical compatibility among the phases in the multilayer coating system. Please click Additional Files below to see the full abstract

Feasibility research of gaining “refractory high entropy carbides” through in situ carburization of refractory high entropy alloys

Abstract: High entropy alloys (HEAs) refer to solid solution alloys which contain five or more principal elements in equal or near equal atomic percent. Due to their unique structures, HEAs have superior comprehensive properties compared with the conventional alloys based on only one element. The property improvement based on the effect of high entropy may works on the refractory carbides used in ultra-high temperature ceramics. Therefore, a solid carburization method was employed on a refractory high entropy alloy of HfZrTiTa to prepare in situ the possible “Refractory High Entropy Carbides”. The microstructure, micro-hardness and oxidation resistance of the carburized layer obtained at 900 ℃ for 10 h were investigated. It can be concluded that the carburized layer, ~120 μm thick in total, had a double layer structure. The Ti-depleted outer layer had an average hardness of ~1200 HV, while the inner layer, with evenly distributed and equimolar elements, had a maximum hardness of ~1590 HV. Although the final phase identification by TEM is under way, we believe that the carburized layer is composed of uniformly distributed carbides based on the results of hardness and element distribution. The substrate closely adjacent to the carburized layer had a higher hardness of ~725 HV compared with the HfZrTiTa alloy (~500 HV), due to the formation of Zr and Hf-rich Needle-like phases. The cyclic oxidation test showed that the carburization treatment on HfZrTiTa can improve its oxidation resistance to some extent. Please click Additional Files below to see the full abstract

Microstructure，mechanical property and oxidation behavior of HfZrTiTaBx HEAs

The unique structural and thermal features of high-entropy alloys (HEAs) conduce to their excellent stability and mechanical properties. Recent researches have suggested that the high-entropy alloys composed of refractory metals exhibit competitive phase-stability and strength at elevated temperatures, which made them the promising candidate materials for high-temperature structural applications at even higher temperatures compared with the Ni-based superalloys. However, the alloys barely consisting of refractory metal elements are usually oxidized easily in oxidizing environment at high temperatures. This work aims to prepare a refractory HEA with both excellent mechanical properties and outstanding oxidation resistance by alloying of B element. In this study, an equimolar quaternary HfZrTiTa alloy and three kinds of HfZrTiTaBx(x=1.1, 2.3, 4.7) alloys with different amounts of B-addition were produced by vacuum arc melting technique in argon atmosphere. The structures of the prepared alloys were characterized via X-Ray diffraction and TEM. The oxidation behaviors of these alloys were investigated by differential scanning calorimeter (DSC)from 25℃ to 1300℃ in air. Their mechanical properties at room temperature and phase-stability at different annealing temperatures from 800℃ to 1600℃ were also examined. The results show that the HfZrTiTa alloy consists of a fully disordered body-centered cubic (BCC) solid solution phase due to the high mixing entropy, while the alloys with B addition have some nano particles uniformly distributed in the BCC solid solution matrix. The lattice parameters and Vicker hardness of the B-containing alloys increase with increasing B content due to the interstitial solid solution strengthening of B element and nanoprecipitation strengthening. The BCC structure of all alloy samples remains stable up to 1200℃. The quaternary HfZrTiTa alloy has a flexural strength of 2.3GPa with a typical dimple fracture morphology, indicating that the alloy shows ductile to some extent. The oxidation rates of the HfZrTiTaBx (x=1.1, 2.3, 4.7) alloys at 1300℃ were about 0.13~0.15g•mm-2•h-1, obviously lower than that of the HfZrTiTa alloy (0.454g•mm-2•h-1)

The association between stress hyperglycemia and unfavorable outcomes in patients with anterior circulation stroke after mechanical thrombectomy

Background and purposeStress hyperglycemia is common in critical and severe diseases. However, few studies have examined the association between stress hyperglycemia and the functional outcomes of patients with anterior circulation stroke, after mechanical thrombectomy (MT), in different diabetes status. This study therefore aimed to determine the relationship between stress hyperglycemia and the risk of adverse neurological functional outcomes in anterior circulation stroke patients with and without diabetes after MT.MethodsData of 408 patients with acute anterior circulation stroke treated with MT through the green-channel treatment system for emergency stroke at the First Affiliated Hospital of Jinan University between January 2016 and December 2020 were reviewed retrospectively. The stress hyperglycemia ratio (SHR) was calculated as fasting plasma glucose (mmol/L) divided by glycosylated hemoglobin (%). The patients were stratified into four groups by quartiles of SHR (Q1-Q4). The primary outcome was an excellent (nondisabled) functional outcome at 3 months after admission (modified Rankin Scale score of 0–1). The relationship between stress hyperglycemia and neurological outcome after stroke was assessed using multivariate logistic regression.ResultsAfter adjusting for potential confounders, compared with patients in Q1, those in Q4 were less likely to have an excellent outcome at 3 months (odds ratio [OR], 0.32, 95% confidence interval [CI], 0.14–0.66, p = 0.003), a good outcome at 3 months (OR, 0.41, 95% CI, 0.20–0.84, p = 0.020), and major neurological improvement (OR, 0.38, 95% CI, 0.19–0.73, p = 0.004). Severe stress hyperglycemia increased risks of 3-months all-cause mortality (OR, 2.82, 95% CI, 1.09–8.29, p = 0.041) and ICH (OR, 2.54, 95% CI, 1.21–5.50, p = 0.015).ConclusionStress hyperglycemia was associated with a reduced rate of excellent neurological outcomes, and increased mortality and ICH risks in patients with anterior circulation stroke after MT regardless of diabetes status

Effect of Ti on the Structure and Mechanical Properties of TixZr2.5-xTa Alloys

To determine the effects of Ti and mixing entropy (ΔSmix) on the structure and mechanical proper-ties of Zr-Ta alloys and then find a new potential energetic structural material with good me-chanical properties and more reactive elements, TixZr2.5−xTa (x = 0, 0.5, 1.0, 1.5, 2.0) alloys were investigated. The XRD experimental results showed that the phase transformation of TixZr2.5−xTa nonequal-ratio ternary alloys depended not on the value of ΔSmix, but on the amount of Ti atoms. With the addition of Ti, the content of the HCP phase decreased gradually. SEM analyses revealed that dendrite morphology and component segregation increasingly developed and then weakened gradually. When x increases to 2.0, TixZr2.5−xTa with the best mechanical properties can be ob-tained. The yield strength, compressive strength and fracture strain of Ti2.0Zr0.5Ta reached 883 MPa, 1568 MPa and 34.58%, respectively. The dependence of the phase transformation and me-chanical properties confirms that improving the properties of Zr-Ta alloys by doping Ti is feasible

Dl-3-n-Butylphthalide Treatment Enhances Hemodynamics and Ameliorates Memory Deficits in Rats with Chronic Cerebral Hypoperfusion

Our previous study has revealed that chronic cerebral hypoperfusion (CCH) activates a compensatory vascular mechanism attempting to maintain an optimal cerebral blood flow (CBF). However, this compensation fails to prevent neuronal death and cognitive impairment because neurons die prior to the restoration of normal CBF. Therefore, pharmacological invention may be critical to enhance the CBF for reducing neurodegeneration and memory deficit. Dl-3-n-butylphthalide (NBP) is a compound isolated from the seeds of Chinese celery and has been proven to be able to prevent neuronal loss, reduce inflammation and ameliorate memory deficits in acute ischemic animal models and stroke patients. In the present study, we used magnetic resonance imaging (MRI) techniques, immunohistochemistry and Morris water maze (MWM) to investigate whether NBP can accelerate CBF recovery, reduce neuronal death and improve cognitive deficits in CCH rats after permanent bilateral common carotid artery occlusion (BCCAO). Rats were intravenously injected with NBP (5 mg/kg) daily for 14 days beginning the first day after BCCAO. The results showed that NBP shortened recovery time of CBF to pre-occlusion levels at 2 weeks following BCCAO, compared to 4 weeks in the vehicle group, and enhanced hemodynamic compensation through dilation of the vertebral arteries (VAs) and increase in angiogenesis. NBP treatment also markedly reduced reactive astrogliosis and cell apoptosis and protected hippocampal neurons against ischemic injury. The escape latency of CCH rats in the MWM was also reduced in response to NBP treatment. These findings demonstrate that NBP can accelerate the recovery of CBF and improve cognitive function in a rat model of CCH, suggesting that NBP is a promising therapy for CCH patients or vascular dementia