Optimizing materials for the purposes of toughening performance to achieve high-strength and allow for higher working loads to be applied, has been a long-term research target for many scholars. One of the proven techniques of achieving this in metallic materials is by decreasing the grain size, which in turn leads to the emergence of ultrafine-grained and nanocrystalline materials. Previous studies have revealed that when the grain size is reduced to nearly zero, material microstructure becomes disordered, which leads to the formation of amorphous alloys and the material strength is actually enhanced. However, this technique has a limit in that when the material size is increased, structural relaxation or/and brittle phase precipitation will occur, which extensively deteriorates the mechanical performance. Recently, martensitic transformation has become a hot research topic prevailing in (Zirconium, Copper)-based metallic alloy, which has an important effect on the mechanical performance of the large-size materials. Unfortunately, ZrCu is a metastable phase at high temperature, which tends to decompose into brittle compounds. Therefore, there is a need to try and resolve this drawback.
Recently, Dr. Jitang Fan and Mr. Yongming Yan from the State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing, China proposed a study whose main objective was to precipitate amorphous and ZrCu (B2) austenitic phases by employing the component of Zr48Cu48Al4 in atom ratio and a fast cooling rate technology. Additionally, they also aimed at utilizing minor Al element so as to improve the glass forming ability and to yield a positive influence on the phase transformation. Their work is currently published in the research journal, Materials & Design.
Briefly, the research technique commenced the preparation of alloy ingots made up of various elements in this study: copper, zirconium and Al in a predetermined ratio. Next, the researchers determined the gradient microstructure and phases distribution at different locations of the as-cast cylindrical metallic alloy rod. Eventually, quasi-static uniaxial compression tests were carried out on the specimens, and the deformation and fracture behavior as well as the toughening mechanisms were analyzed.
The authors observed that the hardness in the cross section of the metallic alloy rod showed a decreasing tendency from the top surface to center. Moreover, the compressive mechanical tests revealed an initial linear elastic deformation to yielding, followed by a fast strain hardening to the final fracture. The mechanical properties showed a unique characteristic of low modulus and high fracture strength, with a relation of strength = modulus/20, which is close to the theoretical strength (about E/10) of materials. So, this developed metallic alloy is soft but strong. Furthermore, shearing fracture model with shear banding, nano wrinkling and micro cracking was revealed, which indicated the mixed mechanical behavior of ductile and brittle models.
Jitang Fan et al’s study presented the architecture of a Zr48Cu48Al4 alloy rod using fast cooling rate technology. They investigated the gradient microstructure, stress-strain characteristics as well as the deformation behavior and mechanisms. The main results directed that the various characteristics investigated were collectively responsible for the desired mechanical performance. Altogether, the designing concept in their work is a stepping-stone and guide to the development of high-performance metallic materials.
Jitang Fan, Yongming Yan. Gradient microstructure with martensitic transformation for developing a large-size metallic alloy with enhanced mechanical properties. Materials & Design, volume 143 (2018) page 20-26.Go To Materials & Design