| Bulk metallic glass (BMG) possesses many favorable properties including high strength, high hardness and high wear resistance, and has great potential to be utilized as structural materials. However, catastrophic failure in the form of sudden fracture usually takes place due to the highly localized shear behavior of BMG, with no obvious macroscopic plastic deformation occurs at room temperature. This severely restricts the large-scale application of BMG as the advanced structure materials in engineering. Consequently, to develop BMG and its composites with good room temperature plasticity and toughness becomes a focus in the research field of the metallic glass. In this dissertation, Zr-Cu-Ni-Al BMG with excellent plasticity at room temperature has been fabricated. The influence of the cooling rate on the mechanical properties and structure of Zr-based BMG have been deeply researched. BMG matrix composites have been synthesized by rapid cooling casting process, adding metal fiber, and crystallizing BMG, and the mechanical properties such as fracture toughness, impact toughness and compression have been studied. The following important results have been obtained:(1) By adjusting Al content, Zr63.36Cu14.52Ni10.12Al12BMG with excellent plasticity was successfully fabricated by copper mold suction casting method, and the compressive plastic strain is up to20.6%at room temperature. The stress-strain curve reflects the work hardening properties, and the yield strength, maximum strength and fracture strength are1740.6,2030.7and1510.5MPa, respectively. Compressive properties of the stepped Zr63.36Cu14.52Ni10.12Al12BMG with diameters of3-4-6mm were studied. The results show that the compressive plastic strain of Φ3mm specimen is20.6%, and the compressive plastic strain of Φ4and6mm specimen is only2.6%and0.2%, which show plastic strain decreases with the increase of the size of BMG. Compressive properties of the stepped Zr55Al10Ni5Cu30BMG with diameters of3-4-6mm were also studied, and the plastic strain of Φ3,4and6mm specimen are2.2%,1%and0.1%, respectively, which also show that plastic strain decreases with the increase of the size of BMG. This change is due to the different content of free volume in the alloy. The smaller specimen formed at a higher cooling rate contained more excess free volume, which contributed to the better plastic deformability.(2) Wedge-shaped Zr55Al10Ni5Cu30BMG specimen was fabricated by copper mold suction casting method. With the increasing of the section width of the wedge-shaped Zr55Al10Ni5Cu3o specimen from the tip to the root, the microstructure evolution law is amorphous region→transition region with metallic glass and crystal→crystal region. The change law of microhardness is first increases then decreases, that is, the hardness is lower in the amorphous region and crystal region, while is higher in the transition region, and the HV value is710. Fracture morphology evolution law is full veins→transition region with veins and diverged pattern→full diverged pattern. The difference of the microstructure, microhardness and the fracture morphology of the wedge-shaped Zr55Al10Ni5Cu30specimen are also due to the difference of the cooling rate. The change of the section size of BMG determines the change of the cooling rate of the melt in the process of solidification, which affect the formation of the microstructure, and ultimately affect the macroscopic mechanical properties and fracture behavior of BMG.(3)(Zr0.55Al0.1Ni0.05Cuo0.30)100-xTix(x=0,2,4,6,8) alloys were prepared by rapid cooling casting process. The structure of the alloys are composed of metallic glass and crystal when the Ti content is x=0,2,4and the structure of the alloys are crystal when the Ti content is x=6,8, which indicate that Zr-based BMG matrix composites reinforced with in-situ formed phase can be fabricated by rapid cooling casting process. The fracture toughness of composites which Ti content x=0,2,4were measured by three-point single edge notched bending test, and the KIC value are10.53,5.14and3.45MPa-m1/2, respectively, which show that the fracture toughness KIC value decreases with the increasing of Ti content.(4) Zr-based BMG matrix composites can be obtained by adding metal fiber. Charpy impact tests were conducted at room temperature, and the results show that the impact toughness of V-notch specimens for Zr64.8Cu14.85Ni10.35Al10sheet(1#), Zr63.36Cu14.52Ni10.12Al12sheet(2#), copper metal fiber reinforced Zr63.36Cu14.52Ni10.12Al12sheet(3#) and stainless steel reinforced Zr63.36Cu14.52Ni10.12Al12sheet(4#) are14.75,11.43,10.83and9.14J·cm-2. The impact toughness of unnotched specimens for1#,2#,3#and 4#sheet specimens are11.23,14.70,5.94and14.12J·cm-2, respectively. Comparing the impact toughness of2#,3#and4#specimens with the same matrix composition Zr63.36Cu14.52Ni10.12Al12, we can find that the impact toughness of V-notch specimens and unnotched specimens for metal fiber reinforced Zr63.36Cu14.52Ni10.12Al12sheet(3#,4#) are lower than pure sheet (2#). It can be analysed that metal fiber is not forming a metallurgical bond with matrix, which bring about that the area near metal fiber is easy to produce stress concentration, and then contribute to cracks leading to fracture. For this reason, the impact toughness is lower. As can be seen from the fracture surface morphology, stainless steel metal fiber are easier to distribution in the center of the matrix, and copper metal fiber are easily swept to the copper walls by the melt, which show that stainless steel metal fiber can composite with matrix more easily.(5) For Cu5oZr42Al8BMG, annealing temperature127and327℃(T<Tg),470and480℃(Tg<T<Tx),600and700℃(T>Tx) were selected to heat treatment for30minutes and3hours. The results indicate that the structure of the specimens annealed at127,327,470and480℃are amorphous structure, the specimen annealed at600℃is BMG matrix composite, and the specimen annealed at700℃is crystal material. All specimens annealed at127,327,470and480℃for3hours are BMG matrix composites. Compressive curves of the samples annealed after30minutes and3hours exhibit linear elasticity and no inealstic yielding obviously. Compressive properties of Cu-based BMG are best after annealing at470℃for30minutes and compressive strength reaches2138MPa.(6) For Cu50Zr42Al8BMG, all specimens annealed at600,700and800℃(Txe<T<Tm),875℃(Tm),888℃(Tmp),900℃(T1),925and950℃(T>T1) for5minutes are BMG matrix composites, and precipitation phase has a tendency to increase with the increasing of annealing temperature. With the increase of annealing temperature, the fracture strength first decreases then increases. The fracture strength of specimen annealed at875℃(Tm) is lowest, and the value is only234.06MPa, while the fracture strength of specimen annealed at950℃is highest in all annealed specimens and the value reaches1442.36MPa. Serrate characteristic can be observed from the stress-strain curve for the specimen annealed at925℃for5minutes, which exhibits obvious yielding occurs before fracture, and yielding strength, fracture strength and plastic strain are819.78MPa,898.77MPa and0.7%. It indicate that Cu50Zr42Al8BMG matrix composites with room temperature compressive plasticity are likely obtained by high temperature and rapid annealing heat treatment. |
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