Phase Changes in Silica and Their Impact on Mechanical Properties in 3D Printed Investment Casting Molds.

Abstract

The devitrification of amorphous silica to cristobalite and the subsequent beta-alpha transformation of cristobalite were studied as well the impact of both transformations on mechanical properties. The work was motivated by the need to transform 3-D printed green molds composed of acryaltes and 60 vol% silica particles into refractory molds for the casting of single crystal Ni-superalloy. The 3-D printed molds were produced by Large Area Maskless Photopolymerization (LAMP), a process that selectively cures layers of photo-suspension with patterned UV light to produce solid objects. Cristobalite content was required to achieve sufficient sintering and creep resistance in the in the sintered silica molds. However, cristobalite lowers room temperature strength due to cracking caused by the beta-alpha transition, requiring a compromise between creep resistance and strength. The development of crack-like flaws in LAMP molds prior to sintering was also addressed. The kinetics of the crystallization of amorphous silica to cristobalite was investigated and a time-temperature-transformation or TTT diagram was developed. The impact of seeding with cristobalite powder and use of silica with residual crystalline quartz were examined. The stress relaxation of partially crystalline sintered silica under fixed strain was studied as a function of cristobalite content. The results are used to infer the impact of cristobalite on the creep and sintering resistance of partially crystallized sintered silica. The impact of cristobalite on strength of sintered silica was explored. The strength of partially crystalline sintered silica samples were tested at 350°C as beta-cristobalite having never cooled through the beta-alpha transition, at 25°C as alpha-cristobalite having passed the beta-alpha transition, and at 350°C as beta cristobalite having been cooled through the beta-alpha transition and subsequently reheated. The stress and fracture induced by the beta-alpha transition was also examined in the context of a model system: bulk silica glass. The formation of cristobalite spherulites on the surface of the glass and the development of stress and fracture were observed. The beta-alpha transition of cristobalite was also explored by differential scanning calorimetry (DSC) of sintered cristobalite repeatedly cycled through the beta-alpha transformation and the impact of cycling on hysteresis and transformation enthalpy was observed.