Various Rejuvenation Behaviors of Zr-Based Metallic Glass by Cryogenic Cycling Treatment with Different Casting Temperatures

Abstract The rejuvenation behavior of an Zr50Cu40Al10 (at.%) metallic glass upon cryogenic cycling treatment has been investigated. At a high casting temperature, the microstructure of the glass is quite homogenous and thus, internal stress cannot be generated during cycling. Therefore, the glass cannot be rejuvenated by cryogenic cycling treatment. In the contrary, by lowering the casting temperature, nano-sized heterogeneity can be induced and subsequently generates the internal stress and rejuvenates the glass. Once the glass is rejuvenated, the more induced free volume can plasticize the glass with a higher plastic strain. These findings point out that the synthesis conditions can tailor the heterogeneity of the glass and subsequently affect the following rejuvenation behavior upon thermal treatment. It can also help understand the mechanisms of rejuvenation of metallic glass upon cryogenic cycling treatment

Research progress on microstructure evolution of semi-solid aluminum alloys in ultrasonic field and their rheocasting

The effects of ultrasonic vibration (UV) treatment on microstructure of semi-solid aluminum alloys and the application of UV in rheocasting process are reviewed. Good semi-solid slurry can be produced by high-intensity UV process for aluminum alloys. The microstructures of Al-Si, Al-Mg and Al-Cu alloys produced by rheocasting assisted with UV are compact and with fine grains. The mechanical properties of the UV treated alloys are increased by about 20%-30%. Grain refinement of the alloys is generally considered because of cavitation and acoustic streaming caused by UV. Apart from these mechanisms, a hypothesis of the fuse of dendrite root caused by capillary infiltration in the ultrasonic field, as well as a mechanism of crystallites falling off from the mould-wall and crystal multiplication by mechanical vibration effect in indirect ultrasonic vibration are proposed to explain the microstructure evolution of the alloys

Porous NiTi Particle Dispersed Mg-Zn-Ca Bulk Metallic Glass Matrix Composites

Even though the Mg-based bulk metallic glasses (BMGs) have shown superior anti-corrosion properties compared with their crystalline counterparts, the brittleness of them limits the widespread application of these materials. In the present study, we have firstly introduced porous NiTi shape memory alloy particles into an Mg-Zn-Ca BMG by the direct adding method. This composite showed both improved compressive strength and corrosion resistance in Hank’s solutions than its monolithic glassy counterpart. The NiTi dispersoids among the matrix were likely to hinder the main shear band propagation, and also acted as the corrosion barriers. Furthermore, the porous nature of present added particle could further increase the interface areas, which should enhance the reinforcing effects compared with solid ones. This low-cost, high-anticorrosive composite was a good candidate as an engineering material

Effects of Low Nickel Content on Microstructure and High-Temperature Mechanical Properties of Al-7Si-1.5Cu-0.4Mg Aluminum Alloy

In this paper, the effects of Ni content on the room and elevated temperature (250 °C) tensile strength of Al-7Si-1.5Cu-0.4Mg-0.3Mn-0.1RE-xNi (x = 0, 0.3, 0.6, 0.9 wt.%) alloys were investigated, along with microstructure characterization and tensile testing. In the as-cast state, the dominant Ni-rich phases were primarily the γ-Al7Cu4Ni and δ-Al3CuNi phases. Following the solution heat treatment, a significant reduction in the γ-Al7Cu4Ni phase was noted, accompanied by the emergence of numerous small ε-Al3Ni phases. Both room temperature strength and high temperature strength at 250 °C exhibited a consistent increase with rising Ni content, reaching 405 MPa and 261 MPa, respectively, at 0.9 Ni content, which were increased by 6.4% and 16.8%, respectively, compared with 0 Ni content. The elongation exhibited an oscillating increase within the Ni content range of 0 to 0.6, reaching peak values of 2.6% in room temperature and 4.3% in high temperature at 0.6 Ni, followed by a rapid decline. At 0.6 Ni content, the alloy demonstrated a well-balanced combination of mechanical properties, featuring commendable strength and plasticity

Effects of Excessive Zr Content and Ultrasonic Treatment on Microstructure and Mechanical Properties of Al-Zn-Mg-Cu Alloy

The Zr element is one of the important grain refiners for 7xxx series Al-Zn-Mg-Cu alloys, but the effect of Zr content more than 0.15 wt.% needs to be deeply investigated under the action of ultrasonic vibration. In this study, the effects of Zr contents (0.1 to 0.25 wt.%) on microstructure and mechanical properties of Al-Zn-Mg-Cu alloy were studied. The results showed that Zr element could refine grains, but when the Zr content was greater than 0.15 wt.%, the grain size was not uniform, the number of second phase particles increased, and the segregation of components became more serious. It was found that after ultrasonic treatment, the grain-size inhomogeneity was greatly improved, and the Zr content could be added up to 0.2 wt.%. When the Zr content is equal or lower than 0.2 wt.%, ultrasonic treatment can effectively improve the mechanical properties of materials by refining grains and weakening segregation. However, when the Zr content is up to 0.25 wt.%, the effect is getting worse

Effects of Si Content and Ca Addition on Thermal Conductivity of As-Cast Mg–Si Alloys

The Mg⁻Si alloys have low CTEs (coefficients of thermal expansion) and other merits, which contribute to their application potential in the electronic industry. However, the details of their thermal properties are still unclarified, and need more research. In this study, the thermal conductivities (TC) of Mg⁻xSi (x = 1 wt.%, 1.38 wt.%, 2 wt.%, 3 wt.%, and 4 wt.%) binary alloys and Mg⁻4Si⁻yCa (y = 0.2 wt.%, 0.4 wt.%, 0.6 wt.%, 0.8 wt.%, and 1.0 wt.%) alloys over the temperature range of 25⁻300 °C were systematically studied. The results show that the TC of Mg⁻xSi binary alloys decreases with the increase of Si content, while it increases slightly near the eutectic composition. The addition of a Ca element to Mg⁻4Si alloys has an obvious modification effect on the Mg2Si phase. When the Ca content increases constantly, the TC of the alloys decreases at first; then, there is a significant increase at the content of 0.8 wt.% Ca, and after that, it continues to decline. The mechanism is mainly related to the precipitation of the CaMgSi phase at 0.8 wt.% Ca content

Effects of Al-Ti-C Refiner and Forming Processes on the Microstructure and Properties of Al-Zn-Mg-Cu Alloys

In this paper, the refinement effect of Al-5Ti-0.2C refiner on Al-Zn-Mg-Cu alloys was first investigated, and then the effects of three forming processes, i.e., Gravity Casting (GC), Squeeze Casting (SC), and Squeeze Casting after Ultrasonic Treatment (UT-SC), on microstructure and properties of Al-Zn-Mg-Cu alloys were studied. The results show that the refining effect of Al-5Ti-0.2C refiner is obvious; first, the average grain size of the alloy decreases and then increases with the increase in Ti content from 0.15 wt.% to 0.3 wt.%. The optimal amount of added Al-5Ti-0.2C is 0.2 wt.% Ti content. The good refining effect is attributed to the formation of TiC particles and Al3Ti compounds by the refiner, which can all be the nucleus of solidification. The poor refining effect when the Ti content was more than 0.2 wt.% is due to the formation of coarse Al3Ti particles. The results of three forming processes that cast Al-Zn-Mg-Cu alloys under the addition of Al-5Ti-0.2C with 0.2 wt.% Ti content show that the mechanical properties under the UT-SC process are the best; the tensile strength in the as-cast state reaches 367 MPa, and the elongation is 3.84%. The effect of tiny TiC particles in the refiner on the microstructure and properties of Al-Zn-Mg-Cu alloys is also discussed

Phase evolution and strengthening mechanisms in a cast magnesium-zinc-yttrium-zirconium alloy following different heat treatments

The phase transformation sequences of a quasicrystal-reinforced magnesium-zinc-yttrium-zirconium (Mg–Zn–Y–Zr, labeled as ZWK3) alloy after annealing, T4, and T6 heat treatments, respectively, were determined by using electron microscopes. Nanoscale Mg7Zn3 phase and I–Mg3YZn6 strengthening particles are the two primary precipitates in the annealed ZWK3 alloy. Substantial nanoscale I-phase particles formed during solidification remained stable during the T4 treatment indicating their high stability at elevated temperatures, while a large number of nanoscale MgZn2 precipitates appeared during the following T6 treatment. There is a clear orientation relationship between the precipitates and the matrix; consequently, these precipitates are supposed to improve the strength of the heat-treated ZWK3 alloys significantly. T6 heat-treated ZWK3 alloy appears to contain thin and slender-like twins and long period stacking ordered (LPSO) structure. T4 and T6 heat treatment enhance the tensile strength of ZWK3 alloy, though with a minor reduction in elongation compared to the as-cast one