Shuxian He

Grain refinement of Al–Si alloy (A356) by melt thermal treatment

Abstract In order to increase the casting quality and material plasticity of hypoeutectic Al–Si alloys, the effects of melt thermal treatment on the solidification structure of the A356 alloy were analyzed. Unlike the conventional melt addition treatment, no refinements are required during this novel processing. The structural characteristics and mechanical properties (plasticity and strength) were demonstrated. Results of quantitative metallorgraphic analysis showed that the primary dendrite size obviously reduces and the dendrite grains change into equiaxed ones. However, the secondary dendrite arm spacing changes little. Experimental results show that there is an optimal cooling rate range for thermal treatment, in which a maximum microstructure refining effect can be obtained. By the optimized parameters obtained with orthogonal experiments, the elongation ratio of the castings increases by 46.2% and the tensile strength by 8.6%. These results are explained by means of microheterogeneous melt theory, which is a result of the multiplication of the nuclei in the melt thermal treating procedure.

Effects of melt thermal treatment on hypoeutectic Al-Si alloys

Abstract A new refining method—melt thermal treatment without additive to melt is studied in this paper. Microstructure analysis and property evaluation of hypoeutectic Al–Si alloys treated with this method show that the solidification microstructure can be refined significantly with a considerable increase in elongation ratio and strength. Effects such as cooling rate, holding time and alloy composition on the solidification microstructure and mechanical properties have been evaluated. It is shown that the strengthening and toughening effects on the treated samples vary with alloy composition. The property increment of the alloy rich of iron is relatively more remarkable than those rich of Cu or Mg element. Specifically, the structure of the low temperature melt is identified as a primary factor, on which the solidification structure of the treated melt is dependent.

Evaluation of the optimum solute concentration for good glass forming ability in multicomponent metallic glasses

Abstract Previous work suggests that the size factor may be the most crucial one on the glass forming ability (GFA). But the alloys in a system have significant deference for GFA. A criterion λ n is defined in the present work to comprehensively evaluate concentration dependence of GFA of alloys in a system. It is found that the values of λ n of bulk glass formers with greatest GFA in Zr-, Pd-, Mg-, Nd- and Fe-based systems are nearly constant of 0.18. Based on the mathematical description of regular polytopes and the dense random packing of hard sphere (DRPHS) model, a cluster model is suggested. This model provides the physical meaningful origin of the λ n from the view of free volume. The glass structure of bulk glass formers with λ n ≈0.18 has optimum defect concentration within the framework of the cluster model. In the term of λ n criterion, an alloy with λ n of about 0.18 will be one of bulk glass former candidates, which is verified by the experimental result of ZrAlNi system.

The relation between formation of compounds and glass forming ability for Zr–Al–Ni alloys

Abstract This paper reports the formation of compounds during the solidification process of liquid Zr 85− x Al 15 Ni x ( x =15, 20, 25, 30, 35) alloys studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The results indicate that compound AlNi 4 Zr 5 forms when liquid alloys containing 20, 25 and 30 at.% nickel solidify. It is suggested in this work that the large glass forming ability (GFA) of the Zr–Al–Ni system is attributed to the difficulty of formation of compound AlNi 4 Zr 5 as a primary phase during solidification process of liquid alloys. In comparing the phases appearing in the solidification structure of the liquid Zr 60 Al 15 Ni 25 alloy with the products formed during crystallization process of this amorphous alloy, it is assumed that Al 2 NiZr 6 is a metastable phase, which decomposes into equilibrium phases during the subsequent cooling process.

Effect of repeated melting of the ingots on the glass-forming ability of Zr-based alloys

Abstract This letter reports the effect of repeated melting of the ingots on the glass-forming ability (GFA) of Zr 60 Al 15 Ni 25 and Zr 65 Al 7.5 Ni 10 Cu 17.5 alloys. The results indicate that the glass transition temperature T g and the onset temperature of crystallization T x of Zr 60 Al 15 Ni 25 alloy increase as melting times increase. However, the repeated melting of the ingot brings only a little change to Zr 65 Al 7.5 Ni 10 Cu 17.5 glassy alloy. Within the framework of structure heredity and thermodynamics, the reason for the improvement of the GFA of alloys is discussed.

Effect of Solidification Behavior on Microstructures and Mechanical Properties of Ni-Cr-Fe Superalloy Investment Casting

The effect of solidification behavior on the microstructures and mechanical properties of Ni-Cr-Fe superalloy investment casting is given. Metallographic and image analysis have been used to quantitatively examine the microstructures’ evolution. For the parts with the thickness of 3 mm and 24 mm, the volume fraction and maximum equivalent radius of the Laves phase increases from 0.3% to 1.2%, from 11.7 μm to 23.4 μm, respectively. Meanwhile, the volume fraction and maximum equivalent radius of carbides increase from 0.3% to 0.5%, from 8.1 μm to 9.9 μm, respectively. In addition, the volume fraction of microporosity increases from 0.3% to 2.7%. As a result, the ultimate tensile strength is reduced from 1125.5 MPa to 820.9 MPa, the elongation from 13.3% to 7.7%, and the quality index from 1294.2 MPa to 954.0 MPa, respectively. A typical brittle fracture is observed on the tensile fracture. As the cooling rate decreases, the microstructures become coarser.

Relation between the mother ingot microstructure and the glass forming ability of a bulk glass forming alloy

Amorphous alloys are formed when the liquid atoms are frozen into a non-crystalline arrangement. In general, rapid quenching from the melt at cooling rates about 106 K/s is required to obtain glass formation in conventional metallic alloys. However, recent development have shown that glasses can be produced at cooling rates as low as about 102 K/s in some multicomponent systems [1–6], the so-called bulk glassy alloys. The low critical cooling rates for the bulk glassy alloys indicate that they have a very high glass forming ability (GFA). The reason for the high GFA of bulk glassy alloys has been discussed, and has resulted in the three empirical rules [7]: (1) multicomponent systems consisting of no less than three elements, (2) significant difference in atomic size ratios above about 12% among the main three constituent elements, and (3) large negative heats of mixing among the main three constituent elements. It is known that there exist short-range order in liquid alloys, and the short-range order play an important role in forming a glassy phase. So, the liquid state will determine the size of the short-ranged orders in the glassy phase. A liquid structure is inherited from the mother ingot. Hence, it is assumed that there exists a relation between the mother ingot microstructure and the GFA of an alloy. This work attempts to uncover this relation. The original ingots with nominal composition Zr60Al15Ni25 were prepared by arc melting a mixture of pure Zr (99.9 wt%), A1 (99.99 wt%) and Ni (99.9 wt%) metals in a water-cooled copper crucible under titanium-gettered argon atmosphere. To prevent segregation, the original ingots were melted 4 times at 1300 K, and were marked A0. Then, two A0 ingots were repeatedly melted 12 times at 1300 K and 1580 K, respectively, and marked AL12 and AH12. The time of one melting operation was 60 s. The melting temperatures were measured by thermocouple (type B). The solidification structures of the ingots were observed using optical microscopy. Samples with a cross section of 1 × 10 mm2 were produced by suction casting in a cop∗Author to whom all correspondence should be addressed. per mold. The amorphous nature of the as-cast samples and the phases in the ingot microstructures were identified by X-ray diffraction (XRD) using Cu Kα radiation (Rigaku Dmax-rc). Thermal analysis was performed in a differential scanning calorimeter (Netzch DSC 404) at a heating rate 10 K/min. The XRD patterns of the ingots A0, AL12 and AH12 are shown in Fig. 1. The phases in the ingots remain unchanged after repeated melting. All the ingots contain Al2Zr3, Ni42Zr58 and AlNi4Zr5 phases. Fig. 2 shows the ingot microstructures. After the repeated melting of the ingots, the microstructure becomes finer, and AH12 is finer than AL12. Fig. 3 shows the XRD patterns of the as-cast samples of A0, AL12 and AH12. All the suction cast samples are verified to be single glassy phase. The DSC curves of Zr60Al15Ni25 glasses are shown in Fig. 4. All the samples show an endothermic event, which is characteristic of glass transition, followed by an exothermic event corresponding to crystallization process. The detailed resuls of DSC are summarized in Table I. After the repeated melting of the ingots at 1300 K and 1580 K, the glass transition temperature Tg increases from 686.4 K to 694.1 and 697.7 K and the onset temperature of crystallization Tx from 758.0 K to 760.2 and 763.7 K. Meanwhile, the fusion enthalpy of the alloy decreases from 110.9 J/g to 82.55 and 80.35 J/g. The results show that the repeated melting of the mother ingot, especially the repeated melting at higher temperature, can improve the GFA of Zr60Al15Ni25 glassy alloy. The critical cooling rates Rc, above which a liquid can be frozen into glass, have a strong correlation with reduced glass transition temperatures Trg [8]. The higher the Trg, the lower the Rc is. In this work, the experimental results show that the Trg increases from 0.571 to 0.577 and 0.580 after the repeated melting 12 times at 1300 K and 1580 K, indicating that the repeated melting of an ingot improves the GFA of the alloy. The heat of mixing of Zr Al, Zr Ni and Al Ni atomic pairs is −44, −49 and −22 kJ/mol respectively [9]. Thus, the atoms in this system show a strong