5.3 Volumetric Physical Aging Experiments for Vitreloy 1
5.3.2 Specimen Preparation
The procedure of stainless steel (SS) tube casting has been described in Section 4.2.1. The casting here for preparing the TMA samples differs in two respects: (i) the geometry of the tube in the active (molding) section; (ii) the feeding of a thermocouple probe through the vacuum into the casting chamber.
A cross section of the mold region is shown in Figure 5.3(a). It comprises three rectangu- lar chambers machined from an AISI 316 SS block using a wire electric discharge machine.
The entire cross section is evacuated, and only the middle chamber is filled with Vit.1 melt when the assembly is heated to ∼900◦C. During the quench, the evacuated chambers on either side of the central melt inhibit heat transfer in the y direction. The cooling is thus dominated by the heat drawn form the top and bottom surfaces. The intent of this design is to render the heat transfer problem approximately 1-D, i.e., allow thermal gradients only in the x direction. Without insulating the sides, this would require casting a large plate, which is not possible with the current procedure (due to the fixed 50 mm diameter of the vertical furnace tube) and not desirable for it would require a very large amount of BMG alloy. The thickness, width and length (dimensions inx, y, zdirections in this order) of the
stainless steel
cast Vit 1 evacuated
chamber evacuated
chamber z
y x
slab TC probe
(a)
(b)
TC probe elements h
slab elements
Figure 5.3: (a) Cross section of the designed SS mold that has three rectangular chambers.
The middle chamber is filled with Vit.1 melt whereas the chambers on both sides remain empty. The thermocouple (TC) probe that is fed through the vacuum resides approximately in the corner of the cast BMG section. The thickness, width and length (dimensions in x, y, z directions, respectively) of this section are 12.7, 19, 127 mm, respectively. The slab that has been cut out for TMA measurements is shown with bold lines. (b) The 2-D geometry of the finite element heat transfer analysis of the quench. The convective heat transfer occurs from the free surfaces, shown in the figure with arrows and the heat transfer coefficient, h. The temperature history of the TC probe is obtained from FE analysis as the average response of the elements that reside in the cross section of the TC (TC probe elements). Similarly, the thermal history of the slab is averaged from the slab elements.
BMG beam cast in this way are 12.7, 19, 127 mm, respectively. The three-chamber mold is closed on one end whereas the other end is welded to a 600 mm long, 19 mm diameter tube of the same material. The open end of this assembly is connected to vacuum circuitry.
This connection is outside the furnace tube and remains under ∼ 200◦C during furnace operation. Therefore, we will call it the ‘cold zone’.
To increase the cooling rate of the BMG section, the SS thickness on the top and bottom surfaces is kept to a mere 0.6 mm. Also, to promote the intensity of convection, the quenching water is salted with 15 volume pct. NaCl (brine quench), iced and stirred.
The thermocouple probes are fed from the cold zone. A three-way connector is used to connect the tube to both the vacuum circuitry and a low-temperature thermocouple feed- through. The 120 mm long, fast response probes (0.5 mm diameter, type HKMTSS-020G with AISI 304 sheath from Omega Engineering, Stamford CT) are extended to the hot zone
0 5 10 15 20 0
200 400 600 800 1000
t(s) T ( ° C)
0 5 10 15 20
0 0.5 1 1.5 2 2.5 3 3.5x 104
h (W / m2
⋅
K)t(s)
(a) (b)
Figure 5.4: (a) Temperature vs. time data from the thermocouple. (b) Back-calculated time variation of the heat transfer coefficient, h.
from inside the tube. The grounded probe which envelopes the thermocouple wires and junction with a thin AISI 304 sheath could survive the hostile environment of the BMG melt and yield temperature data throughout the process. This temperature data is shown in Figure 5.4(a). At the end of the process, the probe is, naturally, cast in the BMG beam.
The exact location of the probe tip is found out by sectioning (see Figure 5.3(a)). The temperature data are then used to solve the inverse problem: what heat transfer coefficient as a function of time would yield this temperature history at the thermocouple location?
The analysis is implemented in ABAQUSTMand Figure 5.3(b) demonstrates the considered domain. Only a quarter of the casting section needs to be considered due to symmetry. The
*AMPLITUDE option is used to vary the magnitude ofhin time, by iteratively comparing the calculated thermocouple temperature with the experimental data. The thermocouple temperature from the FE analysis is taken as the mean temperature of the elements that lie in the thermcouple section. The heat transfer coefficient as a function of time obtained
in this manner is shown in Figure 5.4(b).
A 2.4 mm thick, 13 mm wide slab is cut out of the mid-plane of the beam as shown in Figure 5.3(a). Note that the thermal gradients in the x direction are smallest at the mid- plane where the slab is extracted. The temperature history of the slab is taken by averaging over the slab elements (see Figure 5.3(a)) upon the solution of the inverse problem.
Finally, TMA samples are cut off the slab in thexdirection yielding 13 mm long samples with a 2.4×2.4 mm cross section.