Plastic flow and microstructure evolution are investigated at elevated temperatures in two unalloyed niobium sheet materials, Type 1 and Type 2 as designated in ASTM B393-18. Tensile tests are conducted at temperatures from 1473 to 1823 K (1200 to 1550°C) at constant true strain rates of 10⁻³ and 10⁻⁴ s⁻¹. Deformation microstructures are characterized using backscatter electron (BSE) imaging, electron backscatter diffraction (EBSD), and high-resolution EBSD (HR-EBSD). The mechanical behaviors of the Type 1 and Type 2 niobium materials are compared to relevant data from the literature. Elevated temperature deformation in both niobium materials is dominated by the five-power creep mechanism and the associated development of subgrains. The higher impurity content of the Type 2 niobium led to: 1. greater strength, 2. delayed recrystallization, 3. slower grain growth, 4. inhomogeneous microstructures, and 5. slower recovery which resulted in finer and less distinct subgrains compared to the Type 1 niobium. The smaller subgrain size of the Type 2 niobium produces, through the five-power creep mechanism, a higher strength at elevated temperature compared to the Type 1 niobium. This is the first mechanistic explanation supported by direct microstructural data for how impurity content affects strength in refractory metals. HR-EBSD analysis is performed on the deformed Type 2 niobium material by developing new techniques to: 1. utilize data from a new EBSD instrument, 2. expand the capabilities of the OpenXY open-source cross-correlation software, and 3. enable cross correlation calculations spanning the breadth of heavily deformed grains. This is the first successful implementation of these techniques.