The Study of A New Symmetrical Rod Phase in Mg-Zn-Gd Alloys

Quasicrystal alloys have a wide application prospect because of excellent performances and characteristics; meanwhile, magnesium alloys are known as green engineering materials because of their high specific strength and light weight. Therefore, the study of Mg-Zn-Gd quasicrystal alloys is of great significance for the development of new materials. In this paper, Mg(70-x)Zn30Gdx(x=3,4,5) alloys were prepared by a conventional casting method and the morphologies and properties of these alloys were studied. There was a new symmetrical rod phase found in the Mg66Zn30Gd4 alloy and the symmetrical rod phase was identified as a ternary phase by mapping scanning and energy dispersive spectroscopy (EDS) analysis. The Zn/Gd ratio of the symmetrical rod phase was found to be 4.8 and the TEM images obtained were different from the typical diffraction spots patterns of quasicrystalline, which means it is unlikely to be quasicrystalline. With different melt holding time, the symmetrical rod phase evolved gradually over time from a lamellar eutectic structure; differential scanning calorimetry (DSC), heat treatment, and microhardness tests showed that the melting temperature of the rod phase was 453 °C and that its thermal stability and microhardness are better than quasicrystalline. Hence, the symmetrical rod phase is a new kind of complex metallic alloy phase whose composition and properties are close to those of quasicrystals but is not quasicrystalline.

Effect of cooling rate on microstructure of B2-NiSc intermetallics

Ni-50at%Sc alloy was prepared by centrifugal casting method. Volume fraction, the actual content of B2-NiSc and second phase Ni2Sc in alloy were analyzed with an Image-Pro Plus software. The cooling rates for the solidified thin plate with thickness of 2.65mm, 1.2mm, 0.75mm and 0.35 mm are 1164, 2570, 4112 and 8811 K·s1, respectively. It is found that d=0.5 mm was an critical dimension which corresponds to an abrupt change in the solidification rate. It is also found that (Ni2Sc+NiSc)eutectic was dispersed at grain boundary or between dendritic arms due to the loss of Sc element during melting. While d＞0.5mm (corresponding to the thin plate with thickness of 0.75mm, 1.20mm and 2.65 mm), the solidification structure consists of primary phase B2-NiSc and (Ni2Sc+NiSc)eutectic. While d ＜ 0.5mm (corresponding to the thin plate with thickness of 0.35 mm), the solidification structure is composed of fine globular B2-NiSc and relatively small amounts of (Ni2Sc+NiSc)eutectic. Based on the phase volumetric analyzing of the microstructure with an Image-Pro Plus software, the loss of Sc element during melting was about 3.01~3.10 at%. The eutectic NiSc in the lamellar eutectic structure together with the primary phase B2-NiSc form a larger single phase NiSc, while Ni2Sc with the form of particles is distributed on the grain boundaries after (970 ℃, 72 h) homogenization heat treatment.

Eutectic Structures Competition in the Stripes Strengthening the (Zn) – Single Crystal

Some eutectic stripes have been generated in a hexagonal (Zn) - single crystal. The stripes are situated periodically with the constant interstripes spacing. The eutectic structure in the stripes consists of strengthening inter-metallic compound, Zn16Ti, and (Zn) - solid solution. The rod-like irregular eutectic structure (with branches) appears at low growth rates. The regular lamellar eutectic structure is observed at middle growth rates. The regular rod-like eutectic structure exists exclusively in the stripes at some elevated growth rates. A new thermodynamic criterion is recommended. It suggests that this eutectic regular structure is the winner in a morphological competition for which the minimum entropy production is lower. A competition between the regular rod-like and the regular lamellar eutectic growth is described by means of the proposed criterion. The formation of branches within irregular eutectic structure is referred to the state of marginal stability. A continuous transitions from the marginal stability to the stationary state are confirmed by the continuous transformations of the irregular eutectic structure into the regular one.

Lamellar and Rod-Like Eutectic Growth of TiB2-TiC-TiN Composites by Arc-Melting

TiB2-TiC, TiB2-TiN and TiB2-TiCxN1-x composites were prepared by arc-melting mixtures of TiB2, TiC and TiN powders. 28TiB2-72TiC (mol%) composite showed a lamellar eutectic structure, while 55TiB2-45TiN (mol%) composite with a lamellar structure consisted of TiB2, TiN and TiB. 36TiB2-44TiC-20TiN (mol%) was a quasi-binary eutectic composite having a rod-like structure.

Hot Compression Deformation Behaviors and Microstructure Evolution for Brittle Au-20Sn Eutectic Alloy

The hot compression test of Au-20Sn alloys with different solidification structures was carried out. The deformation behaviors and microstructure evolutions during hot compression were investigated. The results indicate that the fine fully lamellar eutectic structure without primary phase exhibited a very low yielding stress and a low stress platform, while the coarse lamellar with large primary Au5Sn phase showed rather high in that value. After compression, the lamellar eutectic tended to be equiaxed due to dynamic recrystallization, the large primary phase became separate dendrites or split into small dendrites, and the small roselike primary phase partly dissolved in the eutectic matrix. The rate of recrystallization also increased as the primary phase eliminated and lamellae was refined. It is suggested that refining eutectic lamellae and eliminating the primary Au5Sn phase in the as-cast microstructure by solidification optimization are an effective way to improve the workability greatly for brittle Au-20Sn eutectic alloy.