Solidification Behavior And Glass Formation Of Ni-P Alloys

Among numerous transition metal-metalloid binary glass-forming alloys,the Ni-P system has relatively large glass forming ability (GFA), and can bequenched into glass within a wide composition range, due to which it is anideal object to study glass formation. However, the compositionaldependence of the GFA of Ni-P alloys has not been established, and thecorresponding interpretation to the GFA from the point view of solidificationtheory is still absent.In this dissertation, bulk Ni80.4P19.6eutectic alloy and several Ni100P100-x(x=17.5~21) off-eutectic alloys were undercooled to different degrees belowthe liquidus temperature by employing the glass fluxing technique incombination with cyclical superheating, and the temperature history duringsolidification was monitored by a high-speed infrared pyrometer. Thesolidification microstructure was examined by an optical microscope and ascanning electron microscope. The orientation of crystals in the anomalouseutectics was identified using the electron backscattering diffractiontechnique. Ribbon samples and wedge-shaped samples were prepared bysingle-roller melt-spinning and suction casting, respectively. The microstructure, thermal stability and microhardness of the formed glasseswere investigated systematically, using an X-ray diffractometer, a differentialscanning calorimeter and a microhardness tester, respectively. Based on theexperimental results, the crystal growth mode and microstructure evolutionduring the solidification of the Ni-P melts were analyzed, and the relationshipbetween glass forming ability and crystal growth mode was discussed. Themain findings are as follows:During the solidification of undercooled Ni-P alloy melts, doublerecalescence events occur when the primary phase is one of the eutecticphases, but only one recalescence event occurs when regular eutectics formas the primary phase. Anomalous eutectics form as undercooling exceeds acritical value, and their volume fraction in the solidification structureincreases with increasing the undercooling. The anomalous eutecticformation results from the recalescence-induced partial remelting of theprimary phase and the subsequent ripening. The size and orientation of-Niparticulates in the anomalous eutectic change with their precursor. Thoseoriginated from-Ni dendrites are generally coarse and randomly distributed,while those from regular rod eutectics are relatively fine and well oriented.The Ni80.4P19.6eutectic alloy has the best GFA and thermal stability amongthe Ni-P alloys. As the alloys deviate from the eutectic composition, the GFA and thermal stability become deteriorated. A strong correlation betweenmicrohardness and GFA exhibits in the Ni-P amorphous alloys: the better theGFA, the higher the microhardness in the as-cast state, and the less themicrohardness enhancement upon annealing. This phenomenon is inaccordance to the fact that the alloy with high GFA generally lead to highatomic packing density in the glass.There is none of the studied Ni-P alloys whose melt can solidifyconstantly through coupled eutectic growth in the whole temperature rangefrom the liquidus to the glass transition temperature. Instead, coupled eutecticgrowth only takes place within a pear-shaped zone below the eutectictemperature line and another zone above the glass transition temperature. TheGFA of the Ni-P alloy is found to correlate with the crystal growth mode: thealloy prone to adopt decoupled growth mode generally has low GFA.Moreover, hypoeutectic alloys where-Ni solid solutions are preferred as theprimary phase have worse GFA than the hypereutectic ones.