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The ideal processing region is defined by the lowest processing pressure where effects such as turbulent flow, wetting, and gravitational influences can be neglected on the time scale of the experiment. This region can be accessed by plastics and also by some of the recently developed highly processable BMGs12-15, but not by conventional metals or even by SPF alloys. Compared to plastics, such BMGs exhibit a room temperature strength which is two orders of magnitude higher. Thus, BMGs can be considered high strength metals that can be processed like plastics. of handling low viscosity liquids. However, these improvements are marginal when compared to plastics and can only be realized in a limited family of alloys.Bulk metallic glassRecent decades have witnessed the development of a new class of metallic alloys that solidify into an amorphous structure even at moderate (<100 K/s) cooling rates4. The amorphous structure of these bulk metallic glass (BMG) forming alloys results in strength and elasticity values that typically surpass those of conventional structural metals4. In order to harness their full range of attractive properties, BMGs should be used in geometries where at least one dimension is below approximately9 1 mm. This length scale depends on the critical crack length and varies among BMGs and is due to size effect on the BMGs’ mechanical properties5-8. For example, despite the fact that BMGs lack a strain hardening mechanism, which results in shear instability and has been considered the Achilles’ heel of BMGs4, significant bending ductility has been observed when one dimension is reduced to approximately5 1 mm, Thermoplastic based processing of bulk metallic glassThe main challenge associated with net shape processing of BMGs is to avoid crystallization. The stability against crystallization exhibited by BMG formers allows for two principally different processing methods10. One is direct casting, where the liquid BMG former must fill a mold and simultaneously be cooled sufficiently fast to avoid crystallization. Only a careful balance of processing parameters can satisfy these contradictory requirements and, as a consequence, this only allows the casting of some limited geometries10. Geometries with thin sections are particularly difficult to cast, a fact which has hampered the widespread use of BMGs. Alternatively and unique among metals, BMG formers can be thermoplastically formed (TPF)10. During TPF, the pre-shaped BMG is reheated into its supercooled liquid region and formed into its final shape. The supercooled liquid region is the temperature region where the BMG former first relaxes into a supercooled liquid before it eventually crystallizes. Despite the early recognition and utilization of TPF11, its potential as a net shape process to enable a wider array of forming methods was not explored10 until the recent development of BMGs with high formability12-15.Blow molding of bulk metallic glassEven though fast cooling and forming are decoupled during TPF of BMGs, thin sections with a high aspect ratio remain challenging to create when using techniques where the BMG is in physical contact with the mold. This is due to stick conditions between the BMG and the mold and the resulting parabolic flow patterns16. In order to eliminate such stick conditions, physical contact between the BMG and the mold must be avoided, at least while significant tangential strain isgenerated.
We will show that this can be achieved by TPF-based blow molding. The required minimum pressure for TPF-based blow molding is defined by the flow stress of the BMG. From a processing point of view, ideal conditions are those under which strain exceeds 100 % at strain rates of ≤10-1 sec-1 using a forming pressure of 1 atm. We have concluded from theoretical considerations17 that recently developed BMG formers with high formability12-15 fulfill these requirements. Fig. 2a shows a BMG disc which was deformed by approximately 400 % using blow molding as experimental verification. However, deforming beyond 500 % caused rupture, terminating the forming operation and thereby limiting experimentally achievable overall strains (Fig. 2b). This is due to geometrical thinning: a non-uniform decrease in thickness upon deformation18. For pre-shapes other than perfect spheres, the stress distribution is non-uniform. This leads to non-uniform strains, and consequently the thickness decreases in a non-uniform manner throughout the pre-shape. The magnitude of the thinning is controlled by the strain rate sensitivity exponent, m = dd— σε⋅ —, which reflects the material’s thinning resistance, i.e., its strain rate dependent resistance to deformation. By extrapolating data on the effect of the strain rate on viscosity19 we can conclude that under processing conditions suited for blow molding of BMG formers, m = 1, indicating Newtonian behavior (σ = η⋅3ε ⋅ ). This assumption is supported by finite element modeling (FEM) (Fig. 2c), which predicts rupture at the same latitude as experimentally observed (Fig. 2b) at processing conditions under which Zr44Ti11Cu10Ni10Be25 BMG exhibits Newtonian behavior. The condition m = 1 represents the highest possible thinning resistance for a metal. In comparison to BMGs, SPF metals are significantly more prone to thinning, reflected in their typical m values ranging from 0.4 to 0.718. As a result, the overall strain that can be achieved in the SPF process is limited.To achieve larger overall strains the thickness distribution in the initial pre-shape must be modified such that locations which will undergo large strains have an increased thickness. This is demonstrated in Fig. 2d, where BMG discs with varying thickness at the edge, dE, and center, dC, were used. The overall strain increases from 470 % for dC/dE = 1 to 686 % for dC/dE = 3. This strategy is limited because for dC/dE > 3 the geometry becomes unstable, as evidenced by the fact that it ruptures at a different latitude than predicted by FEM (Fig. 2c). In order to increase the range of complexity that can be net shaped, the pre-shape has to more closely resemble the final shape.