Search

Two Fe-Al-based intermetallic aluminide coatings were fabricated on 430-SS (Fe-Cr) and 304-SS (Fe-Cr-Ni) substrates by pressure-assisted solid diffusion bonding with coating on pure Fe as control. The microstructure and intermetallic phases of the coatings were characterized by SEM, EDS and EBSD. A network of Cr2Al13 with matrix of Fe4Al13 was formed by inter-diffusing of Al with the substrates. The corrosion behavior of intermetallic coatings was investigated in 0.5 mol/L HCl solution by mass-loss, OCP, Tafel plot and EIS. It was found that corrosion resistance was greatly enhanced by dozens of times after the addition of Cr and Ni compared with that on pure Fe. The presence of cracks in the coating on 430-SS provided a pathway for corrosion media to penetrate to the substrate and accelerated the corrosion rate. Moreover, the corrosion product was analyzed by XRD, demonstrating that the addition of Cr and Ni facilitated the formation of more corrosion resistant phases, and therefore improved corrosion resistance.

A novel Al–Ti–C master alloy containing Al4C3 and TiC particle clusters, which exhibits great refining potential for Mg, was prepared. With the addition of 2% Al–Ti–C master alloy, the grains transform to equiaxed crystal with a diameter of (110±17) μm. The results indicate that Al4C3 and TiC particle cluster, rather than a single particle, plays an important role in the refining process. Compared with the simplex smooth nucleating substrate, concave regions on the particle cluster provide easier route for the transformation from liquid Mg atoms to stable nucleus. Nucleus with a small size can also reach the critical nucleation radius when they attach on the concave regions of the substrate. Al4C3 and TiC particle clusters thus become more favorable nucleating substrate for α-Mg grains.

Microstructures and element distributions of the as-cast, hot-rolled and cold-rolled Cu–Mg–Te–Y alloys were studied. Effects of rolling process and annealing temperature on the properties of the Cu–Mg–Te–Y alloys were correspondingly investigated. The results indicate that the Mg element is homogeneously distributed in the matrix and the fragmentized Cu2Te phase is dispersed in the matrix after hot rolling. Then, the Cu2Te phase is further stretched to strip shape after the cold rolling process. The microstructures of the cold-rolled alloy keep unchanged for the sample annealed below 390 °C for 1 h. However, after annealing at 550 °C for 1 h, the copper alloy with fibrous microstructures formed during the cold rolling process recrystallizes, leading to an obvious drop of hardening effect and an increase of electrical conductivity. The Cu–Mg–Te–Y alloy with better comprehensive properties is obtained by annealing at 360–390 °C.

New copper alloys with high mechanical properties and high electrical conductivity were prepared, and the effects of addition of minor Mg and Y elements on microstructures and properties were studied. The high tensile strength of above 510 MPa, high elongation of 11% and high electrical conductivity of over 63% IACS can be simultaneously obtained in Cu–0.47Mg–0.20Te–0.04Y alloy after deforming and annealing treatment. Effects of purification together with the grain refining by Y and solid-solution strengthening by Mg are appropriate for enhancing mechanical properties and electrical conductivity of the copper alloys.

Al-5%C master alloy was prepared by powder in situ synthesis process, and its effects on grain refinement of AZ31 alloy and refining mechanism were investigated. The results indicate that the Al-5C master alloy consists of α(Al) and Al4C3 phases, and the size distribution of Al4C3 particles is controlled by sintering time. The Al-5C master alloy can remarkably reduce the grain size of AZ31 alloy, which decreases with the increasing addition amount of Al-5C master alloy when the addition amount is below 2%. The refining mechanism is attributed to the formation of new compounds of Al-C-Mn particles by Al4C3 and Mn, which might act as nucleating substrates for α-Mg grain.

In melt foaming method, the thermal stability and foaming speed of blowing agent significantly affect the pore structure, pore size and porosity of metal foams. To retard the foaming speed and increase thermal stability, Na2O·nSiO2 and dilute hydrochloric acid were used to coat SiO2 passive layer on the surface of CaCO3 which is the blowing agent of Mg foams. Thermal stability and microstructure of the SiO2 passive layer were studied by thermo gravimetric analyzer-differential thermal analysis (TGA-DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the thermal stability of coated CaCO3 is improved. The SiO2 layer is amorphous with reticular structure. Contrasting experiments reveal that the coated CaCO3 can foam placidly. Moreover, large size and homogeneous Mg foams were prepared without protective gas with the ignition- proof by alloying. And the porosity of the Mg foams is 60%-70%.

The billets of Al-Zn-Mg-Cu-Zr and Al-Zn-Mg-Cu-Zr-0.5Er alloys were prepared by semi-continuous direct chill casting (DCC). The effects of trace Er on microstructure of Al-Zn-Mg-Cu-Zr alloy under as-cast and homogenization conditions were studied. The results show that the grain morphology is large dendritic structure and the grain size increases obviously by the addition of 0.5% Er. Moreover, most of Er element in the alloy segregates at grain boundary during solidification, resulting in ternary Al8Cu4Er phase. After homogenization, most of the MgZn2 phase at grain boundary has dissolved back to Al matrix in the two alloys. In the Er-containing alloy, the dissolution temperature of Al8Cu4Er phase is about 575 °C. Therefore, the homogenization treatment cannot eliminate Al8Cu4Er phase validity.

Ultrasonic vibration was introduced into the Mg-8Li-3Al alloy melt during its solidification process. The microstructure, corrosion resistance and mechanical properties of the Mg-8Li-3Al alloy under ultrasonic vibration were investigated. The experiment results show that the morphology of a phase is modified from coarse rosette-like structure to fine globular one with the application of ultrasonic vibration. The fine globular structure is obtained especially when the power is 170 W, and the refining effect also gets better with prolonging the ultrasonic treatment time. The corrosion resistance of the alloy with 170 W of ultrasonic vibration for 90 s is improved apparently compared with the alloy without ultrasonic vibration. The mechanical properties of alloys with ultrasonic vibration are also both improved apparently. The tensile strength and elongation of alloy improve by 9.5% and 45.7%, respectively, with 170 W of ultrasonic treatment for 90 s.

Effects of electromagnetic stirring on the microstructure and mechanical properties of the magnesium-lithium-aluminum alloy were studied. The results reveal that, the morphology of the α phase changes from the long block to globular structure and β phase distributes more widely in the periphery of α phase when the electromagnetic stirring voltage is higher than 110 V. The mechanical properties are increased significantly with the increasing electromagnetic stirring. The tensile strength is improved from 172 to 195 MPa, and the elongation is increased from 10.65% to 25.75%.

The effects of yttrium addition on microstructure and mechanical properties of as-cast Mg-6Zn-3Cu-0.6Zr- x Y ( x =0, 0.5, 1.0, 1.5 and 2.0, mass fraction, %) (ZCK630+ x Y for short in this study) alloys were investigated by means of OM, XRD and SEM. The results show that the average grain size of Mg-Zn-Cu-Zr magnesium alloy is effectively reduced (from 57 μm to 39 μm) by Y addition. The analysis of XRD indicates the existence of I-phase (Mg 3 Zn 6 Y) and W-phase (Mg 3 Zn 3 Y 2 ) in ZCK630 alloys with Y addition. The ultimate tensile strength of ZCK630 alloys is significantly deteriorated with increasing Y addition, which is possibly related to the continuous networks of intergranular phases and the increase of W-phase.

The effects of external fields such as electromagnetic field on the structure and heat treatment behavior of Mg-Li-Al alloys were studied. Mg-8Li-3Al alloys cast with and without electromagnetic stirring were used for solution treatment and aging treatment. Experimental results show that the dendritic arms are broken and a large quantity of equiaxed grains appear in the microstructure of specimens with electromagnetic stirring (EMS). With the increase of the quenching temperature (150-350℃), the solution of Mg and Al in solid β phase increases, and the Brinell hardness of the alloy increase as well. Aging peak and over aging happen because θ(MgLi2Al) which precipitates in the β matrix and strengthens the alloy is unstable and transforms to stable AlLi phase. Aging curves of EMS specimens change in a smaller amplitude.

CaC2 particulates reinforced magnesium materials were synthesized using a new technique-electromagnetic suspension casting (ESC). The microstructure and composition of magnesium alloys were investigated and the mechanical properties of alloys were tested. The tensile fracture surfaces were also studied by scanning electron microscopy. And the grain refinement effect of alloys was also examined. The results indicate that proper additions of CaC2 particulates are found to be efficient in refining the microstructure of magnesium materials. The tensile properties of alloys are consistent with the grain sizes. 0.26% (mass fraction) of CaC2 addition reduces the mean grain size of pure magnesium from more than 2mm to 125μm, and the ultimate tensile strength of pure magnesium at room temperature decreases from 77 to 148MPa.