Low Strain Rate Deformation Research Articles

Effects of interstitial solute atoms on the very low strain-rate deformations for an IF steel and an ultra-low carbon steel

In order to investigate the effects of interstitial solute atoms on very low strain-rate deformation of steel by using an interstitial free (IF) steel and an ultra-low carbon (ULC) steel, creep tests and crosshead-arresting tests were conducted at room temperature. The concentrations of solute carbon and solute nitrogen for the ULC steel were 8 and 12 ppm, respectively, and were about 20 times higher than those of the IF steel. The static tensile tests showed that the tensile properties in the two steels were quite similar. However, the room temperature creep test and the crosshead-arresting test showed that their deformation behaviors were significantly different. The very low strain-rate deformation of the ULC steel was associated with the so-called dynamic strain aging, which appeared at a critical strain rate of 5.8 × 10 −8 s −1.

The effect of alpha platelet thickness on plastic flow during hot working of TI–6Al–4V with a transformed microstructure

The effect of alpha platelet thickness on the plastic flow of Ti–6Al–4V with a transformed microstructure was established by conducting isothermal, hot compression tests at hot working temperatures on samples with identical crystallographic texture and beta grain size. Microstructures containing alpha laths/platelets ranging in thickness from approximately 0.4 to 10μm were produced by various sequences consisting of hot rolling and heat treatment. Constant-strain-rate and strain-rate-jump compression tests were conducted at subtransus temperatures of 815, 900, and 955°C in the strain rate regime between 10−3 and 10s−1. The rate-jump tests suggested that plastic flow is controlled by a power-law creep (dislocation glide/climb) mechanism in all cases except the low-strain rate deformation of material with the thinnest alpha laths. All of the constant-strain-rate compression tests yielded flow curves consisting of a peak stress at low strains (≤0.03), extensive flow softening, and a steady-state flow stress at large strains. The peak stress results indicated a significant Hall–Petch dependence on alpha lath/platelet thickness at the two lower test temperatures. The magnitude of this dependence was predicted by the classical Eshelby expression for grain-size strengthening. In addition, a first-order analysis demonstrated that the observed flow softening is of the same magnitude as that which would be associated with the loss of Hall–Petch strengthening (due to alpha–beta interfaces) during hot working.

Comparison between high and low strain-rate deformation of tantalum

To understand the constitutive behavior of tantalum, compression tests are performed over the range of strain rates from 0.0001/s to 3000/s, and at temperatures from 296 to 1000 K. The flow stress is seen to be representable as the sum of a thermal, an athermal, and a viscous drag component. At high strain rates (3000/s), the thermal component is observed to be expressible in terms of the temperature and the strain rate, whereas the athermal component is independent of these variables. At lower strain rates, however, such a separation of the effects of the strain, strain rate, and temperature on the flow stress is not easily achieved. At high enough temperatures, i.e., temperatures above which the thermal component is essentially zero, viscous drag appears to have a significant effect on the flow stress.

Comparison between high and low strain-rate deformation of tantalum

Comparison between high and low strain-rate deformation of tantalum

Static recovery and recrystallization microstructures in sheared octachloropropane

Octachloropropane (C 3C1 8) was sheared using a press mounted on an optical microscope and then allowed to adjust its microstructure statically, at the deformation temperature. Depending on strain rate and deformation temperature, the post-deformational changes in microstructure are strikingly different. After low temperature-high strain-rate deformation, fast growth of new strain-free grains on the boundaries of deformed grains results in the obliteration of grain-shape foliation and intracrystalline deformation features, and the development of a foam texture. After high temperature—low strain-rate deformation, on the other hand, grain-shape foliation and grains with subgrain boundaries tend to survive the adjustment. Lattice preferred orientation is maintained after the post-deformational adjustment at both deformation conditions and thus remains a good indicator of deformation.

Low strain rate deformation study in a two-phase TiAl based intermetallic alloy

Low strain rate deformation at an intermediate temperature in a two-phase TiAl based intermetallic alloy was studied by means of compression testing and TEM observation. Dislocation networks occurred at the initial deformation stage when the strain was small. When specimens were deformed for a longer period of time with larger strains, discontinuous subboundaries formed within the lamellae of the γTiAl α 2-Ti 3Al structure. The formation of the subboundaries refined the original lamellae and so resulted in improved plasticity during low strain rate deformation.

Effects of sulphur on hot ductility of niobium containing low carbon steels during low strain rate deformation

The effects of sulphur on hot ductility of niobium steels, in which cracking susceptibility on the continuously cast slab surface is highest, have been studied by means of hot tensile testing with particular emphasis on the segregation of sulphur atoms to the matrix/grain boundary Nb(C,N) precipitate interfaces. When low manganese niobium steels are solution treated at high temperature and then deformed at temperatures ranging from the low temperature γ to the γ/α duplex phase regions, two troughs appear in the ductility versus strain rate curve, accompanied by intergranular fracture of γ, at strain rates of ~1 s−1 and 10−3−10−4 s−1. The loss of ductility at high and low strain rates is caused by dynamic precipitation of iron rich (Fe,Mn)S and Nb(C,N) particles, respectively, both within γ grains and on the γ grain boundaries. The dependence on sulphur content is obvious at high strain rates, but it is found that the loss of ductility owing to Nb(C,N) precipitation is also reduced by decreasing the sulphur content to less than 10 ppm. This can be explained by the reduced segregation of sulphur atoms to the grain boundary Nb(C,N) precipitate/matrix interfaces, leading to suppressed decohesion and consequent nucleation of microvoids which result in ductile intergranular fracture of γ.MST/1425

Effects of sulphur on hot ductility of niobium containing low carbon steels during low strain rate deformation

The effects of sulphur on hot ductility of niobium steels, in which cracking susceptibility on the continuously cast slab surface is highest, have been studied by means of hot tensile testing with particular emphasis on the segregation of sulphur atoms to the matrix/grain boundary Nb(C,N) precipitate interfaces. When low manganese niobium steels are solution treated at high temperature and then deformed at temperatures ranging from the low temperature γ to the γ/α duplex phase regions, two troughs appear in the ductility versus strain rate curve, accompanied by intergranular fracture of γ, at strain rates of ~1 s−1 and 10−3−10−4 s−1. The loss of ductility at high and low strain rates is caused by dynamic precipitation of iron rich (Fe,Mn)S and Nb(C,N) particles, respectively, both within γ grains and on the γ grain boundaries. The dependence on sulphur content is obvious at high strain rates, but it is found that the loss of ductility owing to Nb(C,N) precipitation is also reduced by decreasing the sulphur content to less than 10 ppm. This can be explained by the reduced segregation of sulphur atoms to the grain boundary Nb(C,N) precipitate/matrix interfaces, leading to suppressed decohesion and consequent nucleation of microvoids which result in ductile intergranular fracture of γ.MST/1425

Effect of two-stage strain-rate test on the superplastic behavior of an Al-Li-Cu-Mg-Zr alloy—Microstructural characterization

The effect of two-stage strain rate tests on the mechanical behavior of a warm-rolled Al-Li-Cu-Mg-Zr alloy during superplastic deformation has been studied. The results showed that a two-stage strain rate test produces much higher elongation than that of single strain rate tests. The optimum deformation parameters correspond to the proper combination of strain rate hardening and strain hardening during the second stage of superplastic deformation. The paper examines the effect of two-stage strain rate tests on microstructural changes during superplastic deformation of an Al-Li-Cu-Mg-Zr alloy, and relates these changes to the superplasticity, as well as the effect of the first stage strain rate on the deformation-induced recrystallized grain size and the conversion rate from the subgrain structure to the high angle grain boundary structure. The results show that the higher first stage strain rate results in a faster deformation-induced recrystallization rate and a finer recrystallized grain size. The microstructure formed using the optimum first stage deformation parameters is characterized by stable fine recrystallized grains and less cavitation. Both a higher first stage strain rate and first stage strain may result in severe cavitation. The grain growth of the fine grains results in high strain hardening, necking resistance and great elongationsmore » during the second stage low strain rate deformation.« less

Deformation of rapidly solidified dispersion strengthened titanium alloys

The influence of erbium on the behavior of titanium and titanium-aluminum based alloys has been investigated over a range of strain rates and temperatures (25 to 775°C). Data from hotextruded bulk specimens indicate that, although oxide dispersion strengthening can be large under certain conditions, the strengthening is minimal in fine-grain material subjected to low strain rate deformation at high temperatures. Both microstructural observations and an analysis of the flow data indicate profuse grain boundary sliding under these conditions. Grain coarsening anneals, which also coarsen the dispersoids and increase their population along grain boundaries, result in significantly improved high temperature flow strengths, especially in the high temperature/low strain rate regime.

工業用純チタンの熱間変形抵抗と熱間加工後の再結晶挙動

In this paper, the deformation resistance and recrystallization behavior in high temperature working (600-850°C) with high strain rates (2-150 l/s) which covers the most hot rolling conditions have been investigated.In the usual hot rolling condition the dynamic recrystallization does not occur and the resistance to hot deformation is determined by the balance between strain hardening and dynamic recovery. The flow stress sharply increases with increase in the impurity content at higher purity condition while the hardening rate due to the increase in the impurity content becomes smaller if the range exceeds a certain value. The recrystallization and grain growth behavior of commercially pure titanium is similar to that of plain carbon steels in austenite phase. In commercially pure titanium deformation bands, which act as nucleation site for recrystallization, are more easily formed than in plain carbon steels.The activation energy for hot deformation and recrystallization lies in a range between 220-260 kJ/ mol which is nearly the same for low strain rate deformation.

Shear localization during metal cutting

The occurrence of catastrophic shear-type chips during metal cutting has been analyzed. A model which incorporates a simpl heat transfer analysis and material properties such as the strain-hardening rate, the temperature dependence of the flow stress and the strain rate sensitivity of the flow stress was developed in order to establish the tendency towards localized flow. Using data published in the literature, it was found that non-uniform flow in metal cutting is imminent when the ratio of the normalized flow softening rate to the strain rate sensitivity is equal to or greater than 5. This trend is identical with that found for flow localization and shear band formation during lower strain rate deformation as found in torsion and isothermal forging.

Anisotropie embrittlement in high-hardness ESR 4340 steel forgings

ESR 4340 steel forgings tempered to a hardness of HRC 55 exhibit a severe loss of tensile ductility in the short transverse direction which is strain-rate and humidity dependent. The anisotropy is also reflected in blunt-notch Charpy impact energy, but is absent in the sharp-crack fracture toughness. Brittle behavior is associated with regions of smooth intergranular fracture which are aligned with microstructural banding. Scanning Auger microprobe analysis indicates some intergranular segregation of phosphorus and sulfur in these regions. The anisotropic embrittlement is attributed to an interaction of nonequilibrium segregation on solidification with local equilibrium segregation at grain boundaries during austenitizing. This produces defective regions of enhanced intergranular impurity segregation which are oriented during forging. The regions are prone to brittle fracture under impact conditions and abnormal sensitivity to environmental attack during low strain-rate deformation. A relatively sparse distribution of these defects (∼10cm−3) accounts for the discrepancy between smooth bar and blunt-notch testsvs sharp-crack tests. Isotropie properties are restored by homogenization treatment. For application of these steels at extreme hardness levels, homogenization treatment is essential.

Laser shock processing of low carbon steel

The effect of laser shock processing (LSP) on the microstructure and mechanical properties of the low carbon (0.04 wt % C) steel was studied. LSP was performed with a 1.054 ^m wavelength Ndrphosphate laser operating in a pulse mode (600 ps duration and up to 200 J energy with a 3 mm diameter beam) with power densities above 10* * W/cmA Shock waves were generated by volume expansion of the plasma formed when the material was laser irradiated. Maximum shock wave intensities were obtained by not using a plasma-confining overlay. When an energy-absorbing black paint coating was used, the specimen surface yielded LSP-induced deformation without melting. The maximum pressure we were able to generate was approximately 2.5 GPa. The rise-time was about 25 ns and a generally 140 ns decay of the laser-shock wave followed. Mechanical properties of material such as surface hardness and residual stress were improved through modifying the microstructure by shock waves. The microstructure of low carbon steel was shown to be strain rate dependent; high density dislocation arrays (~10Wcm.2) were generated by LSP due to high strain-rate deformation and dislocation cell substructures were produced by the low strain rate deformation of shot peening and cold rolling. Strengthening of low carbon steel was a function of dislocation density regardless of processing method.