Processing effects on the ductility of chromium-vanadium alloys

Graduation date: 2011The next generation of high temperature structural alloys must exhibit exceptional resistance to fracture, creep, oxidation, and fatigue at high temperatures. Chromium is being considered due to its high melting point (>1800°C), relatively low density (~7.2 g/cc), high temperature strength, and oxidation resistance. Limiting the use of chromium is its high ductile-to-brittle transition temperature. Previous efforts to reduce the ductile-to-brittle transition temperature have considered elimination of embrittling interstitals, increasing the grain size, pre-deformation and alloying. Recent computational studies have renewed interest in using alloying additions to ductilize chromium. A series of first principles calculations recently revealed elements (Ti, Zr, Hf, Nb, V) which have potential to reduce the ductile-to-brittle transition temperature when alloyed with chromium. To experimentally measure the effects of alloying and processing on the ductile-to-brittle transition temperature, chromium-vanadium alloy samples of 3 different compositions were prepared and compared to nominally pure chromium.\ud Chromium vanadium samples were prepared from powders, hot isostatically pressed, heat treated at 1300°C and extruded at 1200°C. Elevated temperature Vickers hardness, elevated temperature tensile and elevated 3-point bend tests were performed. A diffusion study was performed using microprobe analysis to assist in the development of homogenization procedures for the alloys. Fracture surface\ud analysis of bend and tensile specimens was used to determine the mode of fracture in bend and tensile specimens. Elevated temperature tensile tests determined a relationship between temperature and ductility in 100%Cr and 25%Cr-75%V alloys.\ud Results from elevated temperature Vickers hardness suggest alloying with vanadium increases the strength by a factor of two. Premature failure in elevated temperature 3-point bend specimens and elevated temperature tensile tests made it difficult to quantify the effect of vanadium in affecting the ductility of chromium. Electron microscopy observations indicate that mechanical damage, likely caused by extrusion, is responsible for initiating the premature failures. Despite this complication, evidence of micro- and macro-scale ductile fracture mechanisms on the fracture surfaces of the Cr-V alloys indicate that the addition of vanadium indeed improves the ductility of chromium