Room Temperature Dynamic Strain Aging in Ultrafine-Grained Titanium

Abstract

Dynamic strain aging (DSA) in coarse-grained (CG) titanium is usually observed at intermediate to high temperatures 473 K to 973 K (200°C to 700°C) and is characterized by serrations in the stress vs strain curves. In the present work, despite the absence of apparent serrations, ultrafine-grained titanium (UFG Ti) undergoes DSA at room temperature, exhibited through an abnormal increase in the elastic limit and negative strain rate sensitivity. This effect is observed at 293 K (20°C) in the strain rate interval of 10 À4 to 10 À2 s À1 , and at 203 K (À70°C) and 373 K (100°C) in a distinct strain rate range. Based on a calculated activation energy of 17.3 kJ/mol and microstructural observations by transmission electron microscopy, it is proposed that the dominant mechanism for DSA in UFG Ti involves interstitial solutes interacting with dislocations emitted from grain boundaries. The interstitials migrate from the grain boundaries along dislocation lines bowing out as they are emitted from the boundaries, a mechanism with a low calculated activation energy which is comparable with the experimental measurements. The dislocation velocities and interstitial diffusion along the dislocation cores are consistent.

High Temperature Plastic Instability and Dynamic Strain Aging in the Tensile Behavior of AISI 316 Stainless Steel

“…An average negative, value of the strain rate sensitivity in Figure 2(b) was found to be around m = -0.02, which is consistent with those reported in the literature 14,15,17 for stainless steels and ultrafine grained titanium in the DSA interval of temperatures. Another point worth noticing in Figure 2(b) is the clear existence of two well defined peaks in the ultimate stress around 300°C and 420°C for the 3.5 x 10 -4 s -1 strain rate curve.…”

Dynamic deformation and failure of ultrafine-grained titanium

“…1 that the activation volume is inversely proportional to both the strain-rate sensitivity and yield stress.The yield stress of UFG titanium is approximately twicethat of coarse-grained CP titanium. The activation volume of UFG pure titanium is~40b 3 , where b=0.289 nm is the Burgers vector in hexagonal titanium.Thisrepresents a significant reductionintheactivation volume, compared with 80b 3 for coarse-grained CP titanium [10],suggesting a change in the rate controllingmechanism.…”

“…The average grain size of the as-received UFGtitanium was ~120nm.The microstructure of cross-section of as-received titanium rod was observed by Lopeset al [10] usingTEM as shown in Fig.1. Dynamic compression tests were performed using a split Hopkinson pressure bar (SHPB).…”

“…The reason for this difference is considered to be the increased propensity to thermal softening in ECAP samples.Although both the Bai-Dodd and Grady equations predict a decrease in band thickness with increasing flow stress, the precipitous drop for the UFG titanium must be attributed to an additional mechanism for the loss of mechanical stability.Guo et al [25] reported that the thermal conductivity of nanocrystalline titanium with a grain size less than 100nm is 5.2 (m•K) -1 , which is only about one fourththat of the coarse-grained titanium 20 (m•K) -1 . The thermal softening parameter of UFG titanium is twice that of coarse-grained titanium [5,10]. From Equations5 and 6, the decrease of thermal conductivity parameter and increased thermal softening parameter explain the decrease…”

“….002 [10]. [24] reported that there was a large drop of shear-band thickness forUFG copper compared with its coarse grained counterpart.…”