The effect of initial grain size on the strength property of copper with gradient microstructure

Abstract

Gradient structure (GS) is beneficial for metal materials to obtain excellent comprehensive properties of strength and toughness, since the geometrically necessary dislocations induced by GS constraint can inhibit the strain localization during the deform process. Although the investigation on the GS of metal materials has gained more and more attention, there are still problems that need to be solved concerning the controllable preparation of GS and optimization of its mechanical properties, and the systematically and quantitatively research on the strengthening mechanism of GS. Herein, ultrasonic rolling process (USRP) combined with rolling was used to prepare GS of pure copper with different initial grain sizes. The GS samples (morphology, grain size and volume fraction, etc.) were characterized by scanning electron microscope (SEM) with electron backscattering diffraction (EBSD) and transmission electron microscope (TEM), and the strength and toughness were tested by universal testing machine. The results indicate that GS layer of pure copper consists of nanocrystalline layer (NL), deformed layer (DL), and center layer (CL). The thickness of GS layer first increases to the maximum and then decreases with the decrease of the initial grain size (21.2 μm–9.6 μm). The sample with the thickest GS layer (initial grain size of 11.4 μm) can obtain the highest yield strength (YS) (239 MPa) and ultimate tensile strength (UTS) (293 MPa). Compared with the raw sample, YS is improved significantly and good plasticity is simultaneously retained. In addition, the greatest contribution of strengthening effect comes from the fine grain strengthening.

Keywords

• Pure copper
• Gradient structure
• Initial grain size
• Strengthening mechanism