Study Of Microstructures And Properties Of Zr And ZrTi-based Alloy Processed By Severe Plastic Deformation

High strength and good ductility are essential for their wide applications of structuralmaterials. However, the strength and ductility of most materials are often mutuallyexclusive. For example, high strength has already been observed in many nanocrystallinemetals and alloys, but in most cases these materials exhibit very low ductility, whichgreatly limits their practical applications. Therefore, it is significant to make structuralmaterials possess both the high strength and good ductility for developing and applyingadvanced structural materials. Furthermore, the structural thermal stabilities of materialsare also important for their services and applications. In the present study, using a Zr metaland a metastable β-ZrTiAlV alloy as model systems, the subjects about the microstructures,mechanical properties and structural thermal stabilities of the Zr metal and ZrTiAlV alloyprocessed by severe plastic deformation and subsequent thermal treatments have beenstudied by employing an X-ray diffraction (XRD), an optical microscopy (OM), atransmission electron microscopy (TEM), a scanning electron microscopy (SEM) and amicro material test machine (Instron5948) as follows:Using severe cryorolling to tailor the dislocation morphology, an extraordinarytoughening behavior has been observed in cryorolled Zr, which shows a simultaneousenhancement of both the strength and ductility with strain. This unusual increase ofductility with strain results from the motion of preexisting high-density dislocations,which are induced in the metal by cryorolling, under high stress in tensile deformationprocesses. The deformation-induced high-density dislocations also contribute to theenhancement of strength in the cryorolled Zr. Tailoring the dislocation morphology can beas an alternative to improve the mechanical properties of deformed materials.Hierarchical-structured Zr metals composed of nano-grains (NG), ultrafine-grains (UFG)and micro-grains (CG) with various grain size distributions have been produced viacryorolling followed by thermal annealing. The strength of the hierarchical-structured Zrconsisting of UFG and CG follows approximately the rule-of-mixtures with the volumefraction of CG, while its ductility in the rolling direction deviates positively from therule-of-mixtures. The78%UFG+22%CG sample with a grain size distribution in the range of200nm-1.6μm exhibits a good combination of high tensile strength (σb~650MPa)and uniform elongation (εu~13.4%), much better than that of CG Zr, σb~350MPa andεu~13.8%. Tailoring an appropriate grain size distribution can optimize the strength andductility of hierarchical-structured materials.An hierarchical and multiphase nanolaminated structure consisting of microscaleprimary αplarge grains (~1.5μm), sub-microscale α plates (~200nm) and nanoscaleacicular isothermal (orthorhombic) α″martensites (~15nm) has been produced in theZrTiAlV with a fine (~12μm) β grain size by employing severe plastic deformation (SPD)combined with subsequent recrystallization annealing and aging treatments. This specificstructure results in excellent combinations of tensile properties, e.g., an ultimate tensilestrength σb~1545MPa and an elongation to failure εf~7.9%. The formation of thehierarchical and multiphase nanolaminated structure can be as an alternative route toobtain advanced titanium alloys with excellent combination of mechanical properties.Using SPD and subsequent recrystallization annealing and aging treatments, varioushierarchical nanolaminated structures has been produced in the ZrTiAlV, and the effect ofthese hierarchical nanolaminated structures on the thermal stabilities and mechanicalproperties has been studied. The large strain accumulation (93%) is suggested to favor theformation of a fine (~8μm) β microstructure and the hierarchical and multiphasenanolaminated structure consisting of microscale αplarge grains, sub-microscale α lathsand nanoscale acicular isothermal α″martensites. This hierarchical and multiphasenanolaminated structure leads to an excellent combination of enhanced thermal stability(Tβ~275℃), i.e., the onset temperature of phase transformation of βâ†'α″+β, andsuperior mechanical properties (σb~1550MPa and εf~8.0%), compared with those (e.g.,Tβ~105℃, σb~1490MPa and εf~5.9%) of its coarse-laminated counterpart without αplarge grains.