Microstructures, strengthening mechanisms and fracture behavior of Cu-Ag alloys processed by high-pressure torsion

Disks of a eutectic Cu-Ag alloy were processed by high-pressure torsion (HPT) up to 20 revolutions to reveal the microstructural evolution and mechanical properties. Both the Cu and Ag phases were thinned continuously with increasing numbers of revolutions. After 20 revolutions, the alternating Cu and Ag phases were significantly refined and became fibrous with dimensions as thin as 5 nm. The strain hardening behavior of the Cu-Ag alloy was characterized after different numbers of HPT revolutions, and a saturation microhardness was attained. It is shown that the tensile fracture mode changed from necking to fully brittle shearing with increasing numbers of revolutions, and some shear offsets with sizes of ∼5-20 μm were observed on the fracture surfaces. Based on the abnormal saturation microhardness value of the eutectic alloy, the strengthening mechanisms of various Cu-Ag alloys are discussed.     

高能球磨制备Fe-Cu纳米晶过饱和固溶体

对Fe-20at%Cu合金粉末进行了高能球磨,并利用XRD对Fe-Cu二元合金粉末在球磨过程中的物相变化进行了分析。结果表明,球磨30 h后形成了Fe(Cu)纳米晶过饱和固溶体。热力学计算分析指出,Fe-Cu二元系不具有形成过饱和固溶体的热力学驱动力。高能球磨在Fe-Cu二元互不溶体系中扩展固溶度的驱动力是动力学驱动。在随后的退火过程中,纳米晶过饱和固溶体发生分解。     

Influence of the thermal treatment on the deformation-induced precipitation of a hypoeutectic Al-7 wt% Si casting alloy deformed by high-pressure torsion

A hypoeutectic Al-7 wt% Si alloy was subjected to high-pressure torsion (HPT) at room temperature using an imposed pressure of 6 GPa and torsional straining through five revolutions. Different thermal treatments, prior to the HPT processing, resulted in reducing supersaturated Si concentration in comparison to the as-cast material. Microstructural parameters and microhardness were evaluated in the present work. Processing by HPT produced significant grain refinement with grain sizes of about 200-400 nm. Furthermore, fine deformation-induced Si precipitates from the supersaturated solid solution were observed, which are very useful in retaining the fine microstructure during HPT processing. The microhardness increase was outstanding, with values for processed samples twice higher (84 HV) than that for the as-cast Al-7 wt% Si alloy (44 HV). The results demonstrate that the refining and strengthening of the Al matrix by HPT processing depend on the available supersaturated solid solution during the processing.     

The high-entropy alloys with high hardness and soft magnetic property prepared by mechanical alloying and high-pressure sintering

The equiatomic multiprincipal CoCrFeCuNi and CoCrFeMnNi high-entropy alloys (HEAs) were consolidated via high pressure sintering (HPS) from the powders prepared by the mechanical alloying method (MA). The structures of the MA'ed CoCrFeCuNi and CoCrFeMnNi powders consisted of a face-centered-cubic (FCC) phase and a minority body-centered cubic (BCC) phase. After being consolidated by HPS at 5 GPa, the structure of both HEAs transformed to a single FCC phase. The grain sizes of the HPS'ed CoCrFeCuNi and CoCrFeMnNi HEAs were about 100 nm. The alloys keep the FCC structure until the pressure reaches 31 GPa. The hardness of the HPS'ed CoCrFeCuNi and CoCrFeMnNi HEAs were 494 Hv and 587 Hv, respectively, much higher than their counterparts prepared by casting. Both alloys show typical paramagnetism, however, possessing different saturated magnetization. The mechanisms responsible for the observed influence of Cu and Mn on mechanical behavior and magnetic property of the HEAs are discussed in detail.     

Microstructure and hardness of W-25Re alloy processed by high-pressure torsion

The evolution of microstructure and microhardness was studied in a commercial tungsten-25% rhenium (mass fraction) (W-25Re) alloy processed by the high pressure torsion (HPT) procedure under a pressure of 7.7 GPa up to 10 revolutions at different temperatures. The results show that the samples processed by 10 revolutions at room temperature could have the smallest grain size at around 0.209 μm. High saturation hardness (HV ～1200) could be achieved after the rapid strengthening stage for samples processed by 10 revolutions both at room temperature and at 573 K. Microstructural observation and analysis from Hall–Patch relationship could reveal that grain refinement and grain boundaries strengthening are the main factors of hardening mechanism in W-25Re alloy. It is also demonstrated that sintered W-25Re sample may have brittle phase separation phenomenon after HPT processing.