Ultra Fine Grain of Cu-Zn Alloy Evolved by Multi Directional Forging and Its Thermal and Mechanical Properties

Abstract

A Cu 30 massZn alloy was multi directionally forged (MDFed) at 300 K and 77 K. Dislocation walls, subboundaries and grain boundaries were gradually developed with increasing strain to form ultra fine grains (UFGs). Furthermore, the evolution of UFGs was drastically stimulated by mechanical twinning. Therefore, it was revealed that the grain refinement in a Cu Zn alloy during MDF was induced by both mechanisms of mechanical twinning and continuous dynamic recrystallizatrion. When MDFed to a cumulative strain of ∑De＝6, the grain size obtained was 18 nm at 77 K and 22 nm at 300 K. The MDFed alloy exhibited almost constant ductility irrespective of cumulative strain and MDF temperature, while the tensile strength increased monotonically with increasing cumulative strain. Thermal stability of the UFGed Cu Zn alloy decreased with increasing cumulative strain and decreasing MDF temperature.

Nano-grain evolution in austenitic stainless steel during multi-directional forging

“…12 shows the summarized relationships between 0.2% proof stress, 0.2 , and grain size d. The results from previous studies are also shown for comparison [23,33,34]. For the SUS 316 stainless steel and the Cu-Zn alloy [18], the packet size d p is plotted instead of the actual grain size. This is because the actual grain sizes in these materials could not be accurately measured because of the high dislocation density and/or the magnetic field, as mentioned previously.…”

“…One explanation for these mechanical properties is the rather short mean free path of the dislocations. However, it has also been shown that a Cu-Zn alloy composed of 20 nm grains exhibits rather good ductility [18]. This implies that ductility may be recovered by the evolution of UFGs much smaller than 0.1 m and by control of the grain-boundary character distribution.…”

“…It is known that mechanical twins evolve easily at cryogenic temperatures in the metallic materials that possess low stacking-fault energies [13,14]. Because the twin boundary, as well as the general grain boundaries, contribute to strengthening [15][16][17], UFGed materials fragmented by mechanical twinning exhibit high strength [18].…”

Nano-Grained Structure Induced by Mechanical Twinning during Multi-Directional Forging and the Mechanical Properties

“…It is known that SPD materials possess quite high strength but poor ductility. The above nano-grained Cu-Zn alloy, however, exhibited superior balance of strength and ductility [16]; about 940 MPa ultimate strength and about 20 % fracture strain. This would be induced by extremely small grain size and specific nature of twin boundary.…”

Comparisons of Microstructures and Mechanical Properties of Heterogeneous Nano-Structure Induced by Heavy Cold Rolling and Ultrafine-Grained Structure by Multi-Directional Forging of Cu–Al Alloy

“…Because no clear evidence of evolution of dislocation substructures in the grain interior could be confirmed, a possibility of recrystallization at room temperature due to thermal instability after SPD was considered. 20) It was revealed by high resolution TEM observation and lattice-line analysis (Fig. 4(b)), however, that the areas with white contrast were composed of equi-axed extra UFGs with a diameter of 5 nm having large misorientation angles among the neighboring grains.…”