Solidification microstructure in a supercooled binary alloy

The maximum undercooling that has been achieved for Ni-Cu alloy, by using molten glass purification and cyclic super-heating technology, is 270 K. With the help of high-speed photography, the solidification front images of Ni-Cu alloy at various typical undercooling were obtained. Two grain refinements occurred in the range of 60 K< ΔT < 100 K and ΔT > 170 K, the solidification front became smoother, and the solidification position appeared randomly. With the increase of undercooling, the transition from solute diffusion to thermal diffusion leads to the transition from coarse dendrite to directional fine dendrite. At large undercooling, considerable stress is accumulated and some dislocations exist in the microstructure. However, the proportion of high-angle grain boundaries is as high as 89%, with twin boundaries of 13.6% and most strain-free structures, and the microhardness decreases sharply. This indicates that the accumulated stress at large undercooling causes the plastic strains in the microstructure, and in the later stage of recalescence, part of the plastic strains is dissipated by the system and acts as the driving force to promote the recrystallization of the microstructure.

Effect of the Third Element Ni on the Solidification Microstructure of Undercooled Cu-40 wt.% Pb Monotectic Alloy Melt

In this paper, the evolution of solidification microstructure of Cu-40 wt.% Pb monotectic alloy of the third element Ni pair under deep undercooling conditions was studied. By comparing the phenomena of liquid phase separation during deep undercooling and rapid solidification of Cu-40 wt.% Pb monotectic alloy, the melt of the alloy increases with the undercooling, and the solidification structure appears uneven or even stratified. With the addition of the third element Ni, the liquid phase separation can be effectively inhibited by the change of interfacial energy. The solidified structure undergoes the transformation from coarse dendrite to the first kind of granular and refined dendrite in a wide undercooling range. When the undercooling reaches 143 K, the structure begins to show an inhomogeneous trend.

The Influence of Calcium on the Primary Mg2Si Phase in the Hypereutectic Mg-Si Alloys

The microstructure of Mg-5Si alloy consists of the primary coarse Mg2Si phase, α-Mg solid solution and eutectic α-Mg + Mg2Si, in which eutectic Mg2Si phase solidifies in the form of Chinese script particles. When 0.2 wt.% of Ca was added to the Mg-5Si alloy the size of primary Mg2Si phase remained unchanged. The modification effect of calcium on the primary Mg2Si phase was effective only in the Mg-5Si-0.5Ca alloy. The morphology of the primary Mg2Si phase is changed from the coarse dendrite shape to polyhedral shape and the size of primary crystals is significantly reduced. The addition of 0.6 wt.% Ca to Mg-7Si alloy did not cause the modification of primary Mg2Si phase.

Effect of Al-5Ti-1B Grain Refiner on Microstructure and Mechanical Properties of 7075 Aluminum Alloy

The microstructure of Al-5Ti-1B grain refiner was analyzed by XRD and SEM. The effect of the Al-5Ti-1B grain refiner on the microstructure and mechanical properties of 7075 aluminum alloy were studied. Results show that when the addition amount of Al-5Ti-1B grain refiner is 0.1%, the microstructure of 7075 aluminum alloy is refined from coarse dendrite to equiaxed grains with an average diameter of 55.1μm. The tensile strength and elongation of 7075 aluminum alloy are improved by 11.32% and 36.04% compared with that of 7075 aluminum alloy without adding Al-5Ti-1B grain refiner. With increasing the addition amount of Al-5Ti-1B grain refiner from 0.1% to 0.5%, both the tensile strength and elongation are improved. When the addition amount of Al-5Ti-1B grain refiner increases to 0.5%, the microstructure of 7075 aluminum alloy is refined to uniform equiaxed grains with an average diameter of 32.5μm. The tensile strength and elongation are improved by 18.11% and 48.76%, respectively.

Effects of Non-Equilibrium Solidification and Aging Treatment on Microstructure Characteristic of a Ni-Base Superalloy

Copper mould spray casting and aging treatment were performed to investigate the microstructure evolution of a Ni-base superalloy. With increasing the cooling rate during solidification, the morphology of primary γ phase changes from coarse dendrite to fine dendritic structure with radial-like feature, accompanied by the inhibition of γ′ phase due to the shortened period during the subsequent solid state transformation process. After aging treatment, both the size and volume fraction of γ′ phase are increased with prolonging the isothermal time, which generate the morphology transition of precipitates from irregular and spherical to ellipse and rectangle due to the competition between the interface energy and strain energy.

Strengthen of Mechanism of 30CrMnSiNi2A Steel Welded Joint with Ultrasonic Impact Treatment

To analyze the strengthening mechanism of 30CrMnSiNi2A steel welded joint with ultrasonic impact treatment (UIT), the welded joint specimens were full coverage strengthened by the technology. The microstructure of the surface layer in fusion zone of the welded joint with and without UIT was investigated by optical microscopy (OM). The hardness and residual stress distributions along the thickness direction were also measured by micro-hardness tester and X-ray diffraction method respectively. The results show that the microstructure in fusion zone of the untreated 30CrMnSiNi2A steel welded joint were coarse dendrite, and there were many welding defects in this zone. UIT has the ability to achieve more compact microstructure with only small welding defects. The average hardness value of the treated specimens reached 571 HV, increased 14.4% as compared with that of the untreated specimen (499 HV). A residual compressive stress layer with thickness of 850 μm was also obtained from by UIT, and the maximum residual compressive stress was-347 MPa. The grain refinement, work hardening and residual compressive stress in fusion zone introduced by UIT increased its anti-fatigue performance.

Tem Investigation of Phases Formed During Aluminium Wetting of MgO at [100], [110] and [111] Orientations

The interaction of liquid aluminium (5N) with single crystal MgO substrates of [100], [110] and [111] orientations (surface roughness <1 nm) were studied using sessile drop wettability test performed at 1000ºC for 1 hour in vacuum (5 x 10-6 mbar). The observations performed using scanning electron microscopy (SEM) showed that the interaction of liquid metal with MgO crystals in all cases resulted in the formation of reaction products region (RPR) of thickness varying from ∽40 up to ∽80 microns in depth. In each case the RPR consisted mainly of coarse dendrite-like crystallites of few microns thick surrounded by net of much thinner channels. Occasionally away from the RPR centre the areas built of much finner but also dendrite- or filament-like crystallites were noted. The thin foils for transmission electron microscopy (TEM) investigations were cut using focused ion beam system (FIB) both from drop/RPR as well as RPR/MgO interfacial regions. The electron diffractions proved that the dominating coarse dendrite-like crystallites are of the same α-Al2O3 type throughout the whole RPR for all substrates orientations. Similarly, the colonies of finer crystallites always showed diffraction patterns characteristic for MgAl2O4 spinel. Therefore, the performed investigation indicated, that both the reaction layer depth and the reaction path represented by the sequence and type of phases present in Al/MgO RPR remain roughly similar for all examined orientations, i.e. that the substrate orientation control neither reaction kinetics, nor affects final phase composition of RPR.

Microstructure and Mechanical Properties of Extruded Mg-Zn-Y Alloy

A high strength and toughness extruded Mg-Zn-Y alloy based on quasicrystal-strengthening has been studied. The effect of extrusion and heat treatment on the microstructures and mechanical properties of Mg-Zn-Y alloy were studied by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrum (EDS), X-ray diffraction (XRD) and tensile testing. The experimental results indicated that the coarse dendrite crystals were broken through the hot extrusion, and dynamic recrystallization appeared during the hot extrusion, which obviously refined the hot-extruded microstructure to the average grain size about 20μm. A large amount of strengthening phases such as Mg3Zn6Y(I-Phase), Mg12ZnY(X-Phase) and MgZn2, which were massive, grainy and clavate, dispersedly precipitated from the matrix along grain boundary during ageing treatment at 225 after extrusion, and made the sliding of grain boundaries restrained, which resulted in an enhancement for mechanical properties to a great extent. At the same time, the tensile strength and yield strength increased after ageing treatment. After ageing treatment of 225×24h, the highest tensile strength and yield strength of the extruded Mg-Zn-Y alloy were obtained: σb=506.7MPa, σ0.2=373.5MPa, which were increased by 104.8% and 120.4%, respectively, compared with the extruded Mg-Zn-Y alloy, however the elongation decreased to 16.52%.

Research on Wear Resistance of Mg2Si Reinforced Hypereutectic Al-Si Alloy Composite

Mg2Si particle reinforced hypereutectic Al-Si composite was prepared by casting, and the microstructure and wear resistance of the composite were researched. The results indicated that the morphology of Mg2Si obviously changed with Mg2Si contents increasing, in which the morphology of Mg2Si in the composite had changed from polygon block to characters like and finally became coarse dendrite. There were effects of Mg2Si content and morphology on the wear resistance in the composite, which had the higher hardness and better wear resistance at the suitable Mg2Si content.

Effect of Ti, Al and Cu Addition on Structural Evolution and Phase Constitution of FeCoNi System Equimolar Alloys

FeCoNi system equimolar alloys were fabricated by a vacuum arc melting. The phase constitution of FeCoNi system alloys was determined by XRD analysis and the microstructure was observed by OM. The comprehensive atomic radius δ, the mixing enthalpy ΔHmix and the mixing entropy ΔSmix of alloys were also calculated according to relevant equations. The results show that the addition of Ti, Al and Cu has an obvious influence on the microstructure and phase constitution of FeCoNi system equimolar alloys. Single Ti addition resulted in almost entire solid solution with a typical dendrite growth character and a little unknown phase. However, further addition of Al, Cu or Al+Cu into the FeCoNiTi equimolar alloys led to the occurrence of an entire solution phase with dendrite, coarse dendrite, and rosette dendrite respectively. Such a phenomena suggested that the mixing entropy caused by the increase of components number rather than the comprehensive atomic radius between the elements or the mixing enthalpy of the alloy systems might be responsible for the formation of almost entire solid solution in FeCoNi system equimolar alloys.