*3.1. Experimental Materials*

Laboratory heats of mass 45 Kg were vacuum melted and solidified in cast iron molds. The chemical compositions of the lab heats are listed in Table 2. These were hot rolled to 25 mm "slabs" in five passes with finishing temperature of 890 ◦C and air cooled to room temperature. The Nb-bearing heats were designated by L4, L8, H4 and H8, where the letter indicates the carbon level (i.e., L = 0.03 wt.% and H = 0.06 wt.%) and the number indicates Nb level (0 = Nb-free, 4 = 0.04 wt.% and 8 = 0.08 wt.%).


**Table 2.** Chemical composition of the experimental alloys in wt.%.

From these slabs, 12 mm blanks were cut for producing cylinders for hot compression testing. Selected samples were machined to right cylindrical shapes of dimensions 12 mm (diameter) × 19 mm (height), as shown in Figure 5. A thermocouple hole 1.6 mm in diameter and approximately 5 mm deep was drilled into the cylindrical samples to control and monitor the temperature.

**Figure 5.** Schematic of the cylinders used in this study for hot compression testing.
