Tuning the mechanical properties of metastable beta titanium alloys through the control of microstructure and deformation mechanisms

• Autores: Nana Chen Nana
• Directores de la Tesis: Jon Mikel Molina Aldareguia (dir. tes.)
• Lectura: En la Universidad Politécnica de Madrid ( España ) en 2022
• Idioma: español
• Tribunal Calificador de la Tesis: María Teresa Pérez-Prado (presid.), Paula Alvaredo Olmos (secret.), Elisa María Ruiz Navas (voc.), Carmen María Cepeda Jimenez (voc.), Ilchat Sabirov (voc.)
• Programa de doctorado: Programa de Doctorado en Ingeniería de Estructuras, Cimentaciones y Materiales por la Universidad Politécnica de Madrid
• Materias:
  • Ciencias tecnológicas
    • Tecnología de materiales
      • Ensayo de materiales
• Enlaces
  • Tesis en acceso abierto en: Archivo Digital UPM
• Resumen

  • Metastable B-Ti alloys have been widely used as structural components in aerospace applications, due to their merits of deep hardenability, ease of processability, excellent damage tolerance and strength - ductility combinations. Despite their advantages, the most challenging aspect of this type of alloys is that they are extremely sensitive to microstructure and thermomechanical processing history, especially with respect to the ductility at high strength levels. A lack of sufficient strain-hardening and a relatively low ductility impede the use of metastable B-Ti alloys in advanced structural applications, where superior combinations of strength and ductility are required. Therefore, the main aim of this thesis is to investigate the relationship amongst microstructure, mechanical properties, and underlying deformation mechanisms, all of which play a decisive role in designing ultrahigh strength B-Ti alloys. For this, firstly the bimodal microstructure of a metastable BTi-7Mo-3Nb-3Cr-3Al (Ti-7333) alloy was successfully tuned through various thermomechanical processing steps. The huge variation in mechanical properties results from the interplay between different deformation mechanisms controlled by the size, distribution, morphology, volume fraction and orientation relationship (oR) of different types of a phase, including primary a (aP), secondary a (aS) and grain boundary a (aGB). The strength of bimodal Ti-7333 is mainly governed by secondary aS precipitation through the introduction of a high density of a/Binterfaces that effectively block dislocation transmission. However, ductility is tuned through the control of the primary aP, rod-like ar and retained Bphases, and can be extremely deteriorated if the formation of a continuous grain boundary aCGB layer is not prevented. The plastic deformation is dominated by dislocation slip and tangling, whereas the microstructural sensitivity that accelerates at high strength levels is originated from the deformation incompatibility between the fine acicular aS and the parent Bphase. Secondly, the deformation mechanisms of the Bphase during straining were investigated. The predominant deformation mechanisms for the Ti-7333 alloy with a fully-Bmicrostructure are stress-induced a″ martensitic (SIM a″) phase transformations and SIM a″ twinning. As strain increases, the deformed microstructures evolve from a single band morphology to a complex morphology of intersecting deformation bands, due to the activation of (130)(130)a″ twins, (111)a″ type I twins and nanoscale secondary a″ twins. The selection of the lattice correspondent SIM a″ variant is determined by the one that provides the “maximum transformation strain” in the tensile direction. The prevalence of (130)(310)a″ compound twinning is associated with the comparatively small shear (0.1872) and the simple shuffle mechanism (q=2, AIa=0.3257 A) involved, when compared to other possible twinning systems. Moreover, the first direct evidence of reversion martensitic transformation of the (130)(310)a″ compound twins into (332)(113)Btwins was observed and rationalized, which undisputedly reveals the origin of this peculiar twinning mode that prevails in metastable B-Ti alloys. Finally, a high-strength and high-ductility strain-transformable microstructure was successfully designed by the introduction of -28% a precipitates in the same metastable B-Ti Ti-7333 alloy. The designed lamellar microstructure was predominantly deformed by SIM a" transformation, SIM a" twinning and dislocation slip of the parent Bgrains and a laths. The B-a" transformation followed the [113]B//[112]a"//[-310]a"//[1-21]a" orientation relationship, with the (133)Bhabit plane. A novel martensitic (211)a" type II twinning mode was also found, in addition to the (111)a" type I mode at the SIM a"/a impinging regions. The a precipitates not only play a role on precipitation strengthening, but they can also effectively block SIM a" propagation at the initial stages of deformation, whereas apparent SIM a" transmission across the a laths was observed at large strains. Moreover, (c+a) pyramidal slip and shear of the a laths also contribute to the accommodation of internal stresses. The origin of the enhanced tensile mechanical properties was attributed to the combined effects of a precipitation strengthening coupled with the TRIP softening effect. This work provides a comprehensive investigation on the interplay between microstructure, tensile properties and deformation mechanisms, which may shed light for the design of novel high-performance metastable B-Ti alloys.