Titanium, which is referred as a “wonder metal” has been in use for structural application for more than 50 years both in the form of commercially pure titanium and alloys. The wide range of mechanical properties exhibited by titanium led to the development of various alloys tailored for specific application in areas spanning aerospace to sports. The innovatively engineered titanium alloy ATI 425TM is an emerging high performance, high strength alloy and a viable replacement to the work horse and most commercially popular titanium alloy Ti-6Al-4V. This newly emerged alloy offers the inherent advantage of being receptive to mechanical deformation by cold working. Initially this alloy was developed and put forth for use as armor plate for ballistic protection. This alloy also shows promise for use in aerospace-related applications. In this thesis report is presented and discussed the final fracture behavior of the alloy deformed under both quasi static and cyclic fatigue loading conditions, highlighting the role of product form in governing the mechanical deformation and fracture behavior. Samples of the alloy were prepared from both rod stock and sheet stock, and deformed with stress axis both parallel and perpendicular to the longitudinal direction for the sheet stock and along the longitudinal axis for the rod stock.
The intrinsic influence of processing on microstructure of the rod revealed alpha grains of varying size and shape being well distributed through the transformed beta matrix. The hardness measurements were consistent and the macrohardness was found to be about half the value of the microhardness. Tensile properties of this alloy are comparable with the commercial alloy Ti-6Al-4V, within the limits of experimental scatter. The tensile deformed fracture surface was macroscopically rough and microscopically, reminiscent of locally ductile and brittle failure mechanisms. The influence of intrinsic microstructural features of the alloy product and nature of loading on final fracture behavior is discussed. The high cycle fatigue resistance of the chosen titanium alloy revealed a trend shown by most non- ferrous metallic materials. The final fracture behavior of the alloy under cyclic loading conditions showed differences in the nature and volume fraction of the features with maximum stress at a given load ratio.
The processing on the sheet stock of both orientations resulted in alpha plus beta microstructure. The microhardness and macrohardness data reveals the alloy to be harder in the transverse orientation than in the longitudinal orientation. The tensile properties of the sheet stock with transverse orientation, when compared to the commercial alloy Ti-6Al-4V were observed to be better than sheet stock with longitudinal orientation. The tensile fracture surface of the alloy sheet along the longitudinal orientation revealed at the macroscopic level a fairly rough transgranular region and at microscopic level a healthy population of microscopic voids and shallow dimples of varying size and shape. For the transverse test specimen, the tensile fracture surface was macroscopically rough and globally at an inclination to the far field stress axis and microscopically it was rough and covered with a healthy population of voids of varying and dimples of varying size and shape. The high cycle fatigue resistance of the chosen titanium alloy revealed that the transverse oriented specimens showed more fatigue resistance compared to the longitudinal ones. Cyclic fatigue fracture surfaces revealed differences in the nature and volume fraction of the features with maximum stress at a given load ratio. The region of crack initiation and early crack growth and stable crack growth was essentially flat and transgranular.