Model Of Bulk Metallic Glasses In Thermoplastic Forming Based On Aspect Ratio And Crystallization Control

Patterns on mold can be transferred onto the surface of bulk metallic glassesthrough thermoplastic forming （TPF） which enables fabrication of MEMS devices.Candidates of mold material include silicon and metal. The former is brittle, andrequires expensive fabrication procedures and long fabrication cycles, althoughpatterns on silicon mold are fabricated with high dimensional accuracy. Metalsexhibit high strength, but their wider commercial application in MEMS is limitedsince the fabricated patterns on metal cannot smaller than their grain size. However,bulk metallic glasses（BMGs） exhibit superior mechanical properties including highstrength, large elastic strain limit, high corrosion resistance, abrasive resistance, andcan be thermoplastic formed without grain size limitation in its supercooled liquidregion. However, few model could serve as a quantitative and reliable guideline forimprinting parameters selection, thus people could only choose imprint parametersempirically according to different microstructures fabricated on BMGs. Moreover,few crystallization theories have been introduced to describe TPF process and thusto predict or control imprint results.Accordingly, the contents of this paper are listed as follows:（1） La₆₂Al₁₄Cu₁₂Ni₁₂BMG was prepared by copper mold suction casting, andwas then wire-electrode cut into smaller samples in order to reduce contact areabetween mold and BMG samples, which provides large pressure to imprint higheraspect ratio microstructures. Thermal properties of La₆₂Al₁₄Cu₁₂Ni₁₂BMG weredetermined by differential scanning calorimetry （DSC）, and then was used to fitsupercooled viscosity, crystal nucleation rate, crystal growth rate, and the functionbetween crystallized volume fraction and time. Three-point bending was introducedto measure the supercooled viscosity of La₆₂Al₁₄Cu₁₂Ni₁₂BMG, which fitted toVogel-Fulcher-Tamman equation. Best fits of constant0, Vogel-FulcherTemperature T0, and fragility parameter D*were obtained to develop the model forparameter selection.（2） Isothermal DSC studies were carried out at assigned temperature todetermine the time-temperature-transformation（TTT） diagram forLa₆₂Al₁₄Cu₁₂Ni₁₂BMG.. TTT curves were fitted to classical crystal nucleation rateand crystal growth rate, and best fits were obtained. Accordingly, when temperatureremains constant, the function between crystallized volume fraction and time wasdeveloped. La₆₂Al₁₄Cu₁₂Ni₁₂BMG with same crystallized volume fraction can beimprinted under different combination of temperature, pressure and time, whose set can be described by a surface defined as iso-crystallized volume fraction surfaces.These iso-crystallized volume fraction surfaces could help to develop the model forparameter selection, and predict or control the crystallized volume fraction orimprinted samples.（3） A model based on thermoplastic forming was developed for parameterselection according to the aspect ratio of expected imprinted patterns on BMGs,taking La₆₂Al₁₄Cu₁₂Ni₁₂BMG for example. The chosen parameters promiseexpected imprinting patterns, together with avoiding crystallization to maximumthermoplastic formability. When selecting parameters according to the aspect ratioof expected imprinted patterns, onset crystallization surface（5%crystallized volumefraction surface） was introduced to avoid the occurrence of isothermalcrystallization. When predicting and controlling the crystallized volume fraction ofimprinted BMG samples, several iso-crystallized volume fraction surfaces wereintroduced.（4） Several methods were proposed to predict and control the crystallizedvolume fraction of imprinted BMG samples by describing temperature-time path incertain pressure cross-section of banding model and iso-crystallized volume fractionsurfaces. Parameter selection criteria were present for imprinted BMGs withdifferent degree of crystallization. Temperature-time path and pressure can serve asnew imprint parameters for BMGs. Higher crystallized imprinted BMG sample canbe used as mold to pattern lower crystallized or uncrystallized imprinted BMGsample. Although being imprinted, lower crystallized BMG sample allows to berepeated imprinted before it is totally crystallized.