α´+α heterostructure design in titanium alloy: A potent strategy to achieve high strength and work-hardening synergy

Abstract

Although the work-hardening ability of metals is extremely important, it has always been a weakness for titanium (Ti) alloys. Conventional approaches which utilize TRIP or TWIP effects to enhance the work-hardening ability are only limited to metastable β-Ti alloys with relatively high molybdenum equivalent, and come at the expense of low yield strength in most cases. With regard to the near α and α+β Ti alloys with small amounts of β phases, how to improve strength, work-hardening capacity and ductility synergistically is still a challenge. Regarding this issue, a simple nano-scale α´ martensite combined with micron-scale α phase heterostructure with features spanning across length scales was designed in a Ti15Zr5.5Cu (wt%) model alloy. This simple heterostructure design strategy not only offers a high strength-ductility balance, but also achieves unprecedented desirable work-hardening capacity, which is rarely attained in Ti alloys. The hard α´ martensite matrix serves as the foundation to enhance the strength via finer grain strengthening effect, while the soft lamellar α phases act as the sources of great ductility through the activation of slip systems with various types. Most importantly, the generated stronger and higher efficiency back stress strengthening effect originated from the abundant activated slip systems in α phases and the huge mechanical contrast between α´ martensite and α phase, which results in noticeable improvement of strength and work-hardening capacity. This simple α´ +α heterostructure design engineering provides a new avenue to realize the desirable strength-work hardening-ductility combination for near α and α+β types Ti alloys.