I ncreasing attention has been paid due to increased demand of heat resistant magnesium alloys by heatresistant parts, such as powertrain and transmission components [1] for lightweight vehicles. Mg-Al system is one focus for development of this kind of materials. In alloying elements, calcium makes a most attractive option since it provides acceptable levels of castability and increased corrosion and creep resistance at low cost in comparison with RE [2][3][4] . The addition of Ca to the MgAl system resulted in high pressure die casting MRI230D and MRI153 alloys, both of which present significantly better creep resistance than AE42 alloy [5,6] . Mg-Sn-based alloys have great potential for creep resistance because of the formation of thermally stable Mg 2 Sn phase in the as-cast condition, and many Ca-containing Mg-Sn alloys exhibit good creep resistance [4,[7][8][9][10][11] . Therefore, the consideration for the development of Mg-Al-Sn-Ca alloy becomes necessary. In our previous studies, Mg-5Al-5Ca-2Sn (AXT552) presents good thermal stability [12] due to the formation of connected hard skeleton [13] composed of Abstract: In the present study, creep properties of as-cast Mg-5Al-5Ca-2Sn (AXT552) alloy were investigated by means of a GWT304 creep testing machine at temperatures of 175 °C and 200 °C in the stress range of 35-90 MPa. Results show that creep rates increase with applied stress at an identical temperature. Creep strain at 100 hours is 0.0518% and 0.083% at creep conditions of 175°C/75MPa and 200°C/60MPa, respectively, which is comparable to MRI230D and much lower than most of AX series alloys. By the observation and analysis for samples before and after creep tests using a Shimadzu XRD-7000 type X-ray diffractometer (XRD) and a Hitachi S-3400N type scanning electron microscope (SEM), it was found that Al 2 Ca (C15) phase precipitated out of C36 phase or matrix. The cavity formation and connection at the interface of soft matrix and hard intermetallics caused the propagation of cracking along the eutectic phase during creep process and dislocation accommodated grain/ phase boundary sliding is expected to be the dominant creep mechanism.Key words: as-cast AXT552 alloy; creep properties; creep rate; creep strain; microstructure Al 2 Ca and (Mg,Al) 2 Ca compounds. It was found that the skeleton is able to effectively shield load from the softer α-Mg matrix [14] and increase the creep resistance of the alloys [15,16] . Therefore, the tensile creep property of ascast AXT552 alloy at temperatures of 175 °C and 200 °C was checked and compared with other heat-resistant Mg alloys in this study. Experimental procedureMg-5Al-5Ca-2Sn (wt.%), or AXT552, alloy was prepared using commercial Mg, Al, Sn (with purity of 99.9%) metals and Mg-20Ca master alloy. The alloy was melted in a mild steel crucible under the protection of a mixed gas atmosphere of 1% SF 6 and 99% Ar. After the master alloy added was melted, the melt was held at 750 °C for 10 min, and then poured into a cylindrical mold made of cast iron with a diam...
The microstructure, phase structure, mechanical properties and damping capacity of Al x Fe2CrNiCu (x = 0.0, 0.5, 0.75, 1.0, 1.5) (x in molar ratio) HEAs were investigated. The results show that with the increase of x value, the volume fractions of BCC phase increase from 0 for x = 0.0 to 100% x = 1.5 for the as-cast Al x Fe2CrNiCu HEAs. Among them, a novel dual phase microstructure consisting of 51 vol.% FCC and 49 vol.% BCC with large interface area was obtained in Al0.5FeCrNiCu HEA. Especially the compressive yield strength of the alloy with BCC structure is more than 5 times larger than that of the alloy with FCC structure, indicating that a typical damping alloy with a soft second phase distributed on the hard matrix was successfully fabricated. Compared with other HEAs, the damping capacity (Q−1) of Al0.5 HEA is 0.1, which is the largest one obtained up to now. The foundational contribution of this paper is to show that the damping capacity of the HEAs can be adjusted by tuning the volume fraction of BCC and FCC phases.
Multilayer dielectric grating (MDG) is one of the key optical elements of high-power laser systems. To meet the need of MDGs for high-power laser systems, experimental investigation on MDG with a top layer of HfO2 has been carried out using Kaufman-type ion beam etcher. The optimal ion source conditions have been obtained by etching of HfO2 in pure Ar and Ar/CHF3 mixture plasmas. Compared with pure Ar plasma etching, better selectivity was achieved with Ar/CHF3. The redeposition of sidewalls effects are quite obvious during etching, which results in the increase in duty cycle of etched grating. As there is a distribution of etch rate along the direction normal to the scan movement, a special-shaped mask was made to be used as a substrate holder, which increases uniformity of the etched profile. In order to process repeatability, the ion source should be cleaned up, the cathode and neutralizer filament should be changed after etching process to full completion. Based on the above techniques, a number of MDGs have been achieved, each of which has a mean diffraction efficiency greater than 95%, a line density 1480 lines/mm, and on aperture up to 80 mm×150 mm. Experimental results agree fairly well with the designed, which provides a good reference for the large aperture MDGs ion beam etching.
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