| Fe-based bulk metallic glasses (BMGs) are characterized by ultrahigh strength, large elastic strain and relatively low material cost, the most significant disadvantage of the BMGs is the lack of macroscopic plasticity (usually less than 0.2% compressive plastic strain) at room temperature. For industrial applications, it is of great interest to develop new Fe-based bulk metallic glasses with good plasticity in addition to the high strength. This issue has been actually the subject of intense research in recent years.In this dissertation, the formation mechanism of Fe-based metallic glasses that developed in past years was overviewed systematically. According to analyses of the compositional characteristics of Fe-based metallic glasses developed so far, the metallic glasses can be roughly divided into two families:Fe(B, Si)-and Fe(P, C)-system, between which the latter one exhibits better plasticity and toughness. Based on the Fe(P, C)-system, a series of novel Fe-based bulk metallic glasses and their composites reinforced by a-Fe dendrite phase were developed through compositional and structure design. The structure, thermal stability and mechanical behavior of these alloys were then systematically investigated by X-ray diffraction (XRD), optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), differential scanning calorimetry (DSC) and mechanical properties testing. According to these detailed investigations, the following important results have been obtained:(1) By substituting Fe with an appropriate amount of Ni, FeNiMoPCB bulk metallic glasses have been synthesized by arc-melting and water-cooled copper mould casting. It was found that the substitution of Ni for Fe can enhance the compressive plasticity of the Fe-based BMG up to 5%, which is one of the largest plastic strains of Fe-based bulk metallic glasses developed up to date. The enhancement of the plasticity was explained in terms of the decrease in glass transition temperature and supercooled liquid region due to the substitution.(2) Based on the phase selection principle, a Fe-based bulk metallic glass matrix composite (with the composition of Fe77Mo5P9C7.5B1.5) reinforced with in-situ formed ductile a-Fe dendrite phase was developed. The composite exhibits a compressive plastic strain more than 30% at room temperature. The considerable enhancement in plasticity is due to the intensive interaction between shear bands and a-Fe dendrites, which block the rapid propagation of main shear band and promote the generation of multiple shear bands. (3) The Fe75Mo5P10C8.3B1.7 bulk metallic glass was chosen as a based alloy, a series of metallic glass matrix composites were developed through the compositional modification along the horizontal right (along the Mo-constant line) and right corner (along the Mo/PCB-ratio-constant line) directions in the simplified triangle composition diagram Fe-Mo-(PCB) toward Fe-rich side, respectively. It is found that the a-Fe dendrites are more uniformly dispersed in the composite developed along the horizontal right than along the right corner direction.(4) The distribution and volume fraction of a-Fe dendrites strongly affect compressive plasticity of the Fe-based BMG composites. For the composite with low-volume fraction of a-Fe crystalline phase (about 5%), the a-Fe dendrite cannot effectively block the rapid propagation of dominant shear band, leading to poor plasticity; For the composites with critical volume fraction of 30-50%, the a-Fe dendrite phase forms a continuous network structure, which can sustain severe plastic deformation through multiplication of shear bands and thus increase the plasticity to a large extent. However, when the volume fraction of second phase exceeds the percolation threshold, the plasticity decreases significantly.(5) Three-point single edge notched bending test were performed on Fe7.5Mo5P10C8.3B1.7 metallic glass, Fe77Mo5P9C7.5B1.5 and Fe79Mo5P8C6.7B1.3 metallic glass matrix composites to measure notch fracture toughness, yielding the value of 27.2±2.1 MPa-m1/2,36.7±4.3 MPa-m1/2 and 25.5±4.0 MPa-m1/2, respectively. The low glass transition temperature and low shear modulus might account for the relatively high toughness of the Fe-Mo-(P, C, B) metallic glass.(6) The Fe77Mo5P9C7.5B1.5 BMG composite contains single a-Fe dendrite phase, which tends to prevent the rapid unstable operation of the shear band and promote the crack bifurcation, resulting in a better notch toughness than the monotonic BMG. Nonetheless, the fracture mode of Fe79Mo5P8C6.7B1.3 alloy transformed into intergranular fracture mode due to the additional precipitation of Fe2MoP12 hard brittle phase apart from a-Fe phase, which detrimentally spawned the descending of the fracture toughness.
|
PDF Full Text Request