Cu-2.5 at%Al合金単結晶の高温定常変形

Flow stress and substructural change during the transient deformation of Armco iron and silicon steel

Polycrystals of Cu-Sn solid solutions containing 1 to 4 at%Sn were deformed in tension at temperatures of 823, 873 and 923 K and at strain rates of 1.7×10−5∼8.3×10−4 s−1. The results obtained are as follows:(1) In the steady state deformation stage the strain rate is represented by the relation-ship of \dotε∝σn, where n=3.1∼3.7 in high tin content alloys and n=4.7∼5.7 in low tin content alloys. The steady state deformation behaviour changes from a pure metal type (n∼5) to an alloy type (n∼3) with increasing tin content.(2) In the high concentration alloys showing an alloy type behaviour a work softening phenomenon was always observed. The high temperature deformation in these alloys is controlled by the dragging of solute atomosphere for dislocation motion.(3) In the low concentration alloys showing a pure metal type behaviour, the parameter m*, the effective stress sensitivity of dislocation glide velocity, was found to be 1.0∼2.5. In many cases except the region of high strain rate, the dislocation motion is viscous.(4) The steady state strain rate is independent of the stacking fault energy when n∼3, but depends on it when n∼5.(5) In the case that the dislocation velocity is proportional to the effective stress the method of determining the internal stress by stress relaxation test was examined. The results obtained by this method was approximately in accord with those by the dip test using an extrapolation technique.

Plastic deformation process of Ti alloys depends on the competition of active deformation modes: slip and twinning. The specific deformation mechanism depends on the availability of the specific deformation modes as well as on the critical resolved shear stress for slip and the twin activation stress. Effort has been made to provide information on the different slip systems other than prismatic slips, which get activated during the channel die compression process. It is found that, though at the initial stage of deformation, prismatic slip is prevalent, but other slip systems, especially pyramidal types, also appear at the later stage of deformation. Transmission electron microscopy results confirm the presence of pyramidal slips beside prismatic ones. It is also observed that twining does not take place in those grains having the most stable orientation as per the hexagonal close packed system. Information on the critical resolved shear stresses of different glide systems for T40 is also provided.

Effect of annealing treatment on microstructure and mechanical properties of lightweight steels micro-alloyed with vanadium and niobium

We have investigated the gettering of transition metals in multicrystalline silicon wafers during a phosphorus emitter diffusion for solar cell processing. The results show that mainly regions of high initial recombination lifetime exhibit a significant lifetime enhancement upon phosphorus diffusion gettering. Nevertheless, transition metal profiles extracted by secondary ion mass spectrometry in a region of low initial lifetime reveal significant gradients in Cr, Fe, and Cu concentrations towards the surface after the emitter diffusion, without exhibiting a significant enhancement in the lifetime. In a region of higher initial lifetime, however, diminutive concentration gradients of the transition metal impurities are revealed, indicating a significantly lower initial concentration in these regions. From spatial maps of the dislocation density in the wafers, we find that lifetime enhancements mainly occur in regions of low dislocation density. Thus, it is believed that a generally higher concentration of transition metals combined with an impurity decoration of dislocations in regions of high dislocation density limit the initial lifetime and the lifetime after the phosphorus diffusion, in spite of the notable gettering of transition metal impurities towards the surface in these regions. Furthermore, after a hydrogen release from overlying silicon nitride layers, we observe that only regions of low dislocation density experience a significant lifetime enhancement. This is attributed to impurity decoration of the dislocations in the regions of both high dislocation density and high transition metal impurity concentration, reducing the ability of hydrogen to passivate dislocations in these regions.

Elucidating the deformation behavior of as-cast B2 CoTi intermetallic using in-situ tensile testing

Apparent and effective creep parameters in single crystals of a nickel base superalloy—II. Secondary creep

In situ neutron diffraction revealing the achievement of excellent combination of strength and ductility in metastable austenitic steel by grain refinement

Evolution of microstructure and strength of a high entropy alloy undergoing the strain-induced martensitic transformation

Deformation heterogeneity in copper oligocrystals using high-resolution stereo DIC

The work hardening behavior of metals subjected to strain path changes is different from that in monotonic deformation. Such changes can also lead to transients in flow stress and in the strain hardening rate. The origin of these phenomena is the instability of the dislocation structure produced during the initial straining. Subsequent deformation under a different path involves a restructuring of the dislocations [1]. The effect of a change in the strain path on the behavior of prestrained aluminum alloys was studied by Wilson et al. [2]. Decreased or enhanced work hardening rates were observed at the beginning of the second stage of deformation. These effects were associated either with the reorientation of internal stresses or to the dissolution of the original dislocation structure and the formation of a new one. Vieira and Fernandes [3] studied the influence of a double change in the strain path on the flow curve of copper sheets. The overall behavior depended more on the orientation relationship between the previous and the subsequent stage and less on the sequence of deformation. A high initial flow stress was usually followed by a low work hardening rate. The dislocation structure tended to correspond to the last mode of deformation. A transient flow behavior can also be caused by a change in the strain rate. The association of changes in the strain path and in the strain rate was investigated by Bate [4]. The effect of a strain rate change on the work hardening of aluminum samples was cancelled when a large change in the deformation mode was imposed. The elimination of the rate sensitivity was associated with the disruption of the cell walls due to a change in the strain path. The effects of a change in the strain path were evaluated in tension and torsion tests in this paper. The experiments were conducted in two and three stages and the results were compared with the flow curves from monotonic experiments. The material was a low carbon steel (0.245%C; 0.407%Mn; 0.155%Si; 0.0076%S). The torsion and tension specimens, 3.10 mm in radius (R) and 44.70 mm in length (L), were annealed in vacuum at 1000 ◦C for 20 min and furnace cooled to room temperature. The tests were performed at room temperature in a MTS servo hydraulic testing machine. The initial strain rate used in all experiments was 6.34× 10−2 s−1 [4, 5]. The samples were stored at∼−5 ◦C between two stages of deformation. Load (Q) and elongation (1L) were measured in tensile tests and torque (T ) and angle of twist (θ ) were measured in torsion tests. These data were converted into effective stress (σ ) and strain (e) using the following equations [6, 7]: Tension (uniform strain range):

A framework for quantifying the thermomechanical response of polymer bonded explosives (PBX) at the microstructural level is developed using a cohesive finite element method (CFEM). This framework allows the contributions of individual constituents, fracture and frictional contact along failed crack surfaces to heating to be analyzed and tracked. Digitized micrographs of actual PBX materials and idealized microstructures with various distributions of grain sizes are used in the analysis. The analysis concerns impact loading of HMX/Estane with strain rates on the order of 104 – 105 s-1. Issues studied include large deformation, thermomechanical coupling, failure in the forms of microcracks in both bulk constituents and along grain/matrix interfaces, and frictional heating. The Estane matrix is described by a thermo-elasto-viscoelastic constitutive formulation, accounting for temperature dependence, strain rate sensitivity and strain hardening. The HMX crystals are assumed to be elastic under the conditions analyzed. Energy localization leading to formation of local hot spots as potential ignition sites is primarily due to the viscoelastic dissipation in the matrix in early stages of deformation and frictional heating along crack surfaces in later stages of deformation. Microstructure-response relations that can be used in the design of soft energetic composites are established.

Activating profuse pyramidal slips in magnesium alloys via raising strain rate to dynamic level

A new procedure is demonstrated to comprehend fatigue phenomena. The internal stress and the effective stress in a material are defined to analyze the fatigue process. The internal stress is attributed to the dislocation structure developed during fatigue. From this viewpoint, fatigue tests were carried out and some characteristics such as internal stress, effective stress and hysteresis loop were measured. The internal and effective stresses were measured by means of the strain dip test.The main results are as follows:(1) The internal stress increased with the progress of fatigue.(2) The effective stress obtained during fatigue under a maximum stress larger than the fatigue limit (over-stress) was larger than that under or below the fatigue limit (under-stress).(3) During fatigue under manifold multiple-repeated stress in two stress levels (over-stress and under-stress), both the effective stress and the plastic strain range measured at the under-stress increased by the over-stress loading.(4) The effective stress depended on plastic strain rate.From those results, the concept of internal stress is anticipated to contribute not only to the understanding of fatigue mechanism but also to the establishment of a cumulative damage rule.

Assessment of activation volume in 304L SS at high temperatures