Low-temperature Deformation Mechanisms Research Articles

Low-temperature superplastic deformation mechanism of ultra-fine grain Ti–6Al–4V alloy by friction stir processing

The ultra-fine grain Ti–6Al–4V alloy sheet was successfully achieved by friction stir processing (FSP). The low-temperature superplastic deformation mechanism of Ti–6Al–4V alloy under the strain rate of 3 × 10−4 s−1 at 600 °C was studied by scanning electron microscope, electron backscatter diffraction and superplastic tensile test. It is found that the grain size of Ti–6Al–4V alloy by FSP is refined from 7.48 μm to 0.82 μm, and there is no preferential crystallographic orientation of the uniform grain. After FSP, the β phase of the base material (BM) in the grain boundary is refined and homogenized. During the superplastic tensile processing, with the increase of strain, the grain is refined continuously, and the α→β phase transition is induced by the dislocations pile-up at the β grain boundary, and the content of β phase increases. The grain near the fracture was refined to 0.35 μm, and the stress concentration led to the formation of cavities near the β phase, which eventually caused the fracture. The dynamic recrystallization in superplastic tensile processing is mainly geometric dynamic recrystallization and discontinuous dynamic recrystallization. The elongation of superplastic tensile (1033%) is 19.7 times higher than that of BM (50%). The order of superplastic deformation mechanism in the low-temperature superplastic deformation of the ultra-fine grain Ti–6Al–4V alloy sheet is as follows: intragranular dislocation slip and diffusion, grain boundary sliding, and phase boundary sliding. Phase boundary sliding is the main deformation mechanism for low-temperature superplastic deformation.

Microstructure and Mechanical Properties of TC4 Titanium Alloy at the Temperature of 77K

Titanium alloy has the advantages of low thermal conductivity, a small expansion coefficient and being non-magnetic, making it an ideal low-temperature structural material. In this paper, the typical TC4 titanium alloy in industrial titanium alloy is selected as the research object. The microstructure deformation law and mechanical behavior of TC4 titanium alloy at liquid nitrogen temperature are mainly investigated, and compared with the microstructure and properties at room temperature. The macroscopic and microscopic deformation mechanism of the simultaneous increase in elongation and hardening index of titanium alloy at low temperature is revealed, which provides a basic basis for the low-temperature deformation mechanism and strengthening and toughening design of titanium alloy. Based on the uniaxial tensile tests at room temperature (298 K) and low temperature (77 K), the effects of low temperature on the yield strength, elongation, tensile strength and work hardening curve of titanium alloy were compared and analyzed. The strength/plasticity synergistic improvement of TC4 titanium alloy under low-temperature deformation was found. At low temperature, the yield strength, tensile strength and elongation of TC4 titanium alloy are improved compared with room temperature. The tensile strength increases from 847.93 MPa at 298 K to 1318.70 MPa at 77 K, and the elongation increases from 21.8% at 298 K to 24.9% at 77 K. The grain morphology, grain orientation, dislocation density and fracture morphology of titanium alloy under room temperature and low-temperature tensile conditions were studied by SEM and EBSD. The results of fracture morphology characterization at room temperature and low temperature show that TC4 titanium alloy exhibits ductile fracture characteristics and a large number of dimples are formed on the fracture surface. The dimple depth at low temperature is shallower than that at room temperature and the overall surface is more flat. Compared with room temperature deformation, the deformation process of TC4 titanium alloy in a low-temperature environment produces stronger dislocation pile-up and forms a large number of twins, but the grain rotation is more significant, which effectively alleviates the stress concentration and delays the initiation and propagation of cracks at grain boundaries.

Low-temperature impact toughness and deformation mechanism of CT20 titanium alloy

The impact toughness and deformation mechanism of near-α-type low-temperature titanium alloy CT20 at different temperatures (20 °C, 0 °C, −50 °C, −100 °C and − 196 °C) are studied. The results show that the impact toughness is as high as 90 J/cm 2 at 20 °C, and does not decrease obviously at 0 °C, −50 °C and − 100 °C, but brittle fracture occurs at −196 °C (29 J/cm 2 ). The tortuosity of the crack path decreases with decreasing temperature. At 20 °C, the grain boundary and α colony interface with high-angle grain boundary deflect the crack propagation direction effectively, forming a tortuous crack path, while crack tends to cut through α lamellae, forming a relatively straight path at cryogenic temperature. The lamellar α undergoes kink deformation at 20 °C, showing severe plastic deformation characteristics. In addition, deformation twins and geometrically necessary dislocations (GNDs) are observed near and far from the fracture surface, indicating that the plastic zone is large. As the temperature decreases, the twin density near the crack path tends to increase. However, few twins are seen in the region far from the fracture surface, and GND densities are low at −196 °C, implying that the plastic zone is small. In a word, the synergistic effect of tortuous crack path, dislocation slip and deformation twinning enable CT20 to achieve high impact toughness at 20 °C. At −196 °C, the plastic deformation capacity of the microstructure decreased significantly, resulting in the formation of a small plastic zone and brittle fracture. • α colony interface deflects crack propagation direction effectively at 20 °C, while crack tends to cut through α lamellae at cryogenic temperature. • The synergistic effect of tortuous crack path, dislocation slip and twinning enables CT20 to achieve the highest impact toughness at 20 °C. • The coordinated deformation of dislocation slip and twins keeps the impact toughness at a high level in the temperature range of 0 °C ~ -100 °C. • The plastic deformation ability decreases significantly, resulting in the formation of a small plastic zone and brittle fracture at −196 °C.

Low-Temperature Deformation Mechanism and Strain-Hardening Behaviour of Laser Welded Dual-Phase Steels

This paper analysed the change in microstructure after laser welding DP800 and DP1000, the effect of the laser welds on low temperatures deformation, and strain hardening behaviour when loaded at temperatures between −40 °C and 20 °C using quasi-static strain rates (1.7 × 10−2 s−1). The results showed that the fusion zone (FZ) was fully martensitic due to the rapid cooling during welding. Owing to the severity of the heat-affected zone, the joint efficiencies of DP800-DP800 and DP1000-DP1000 welds were 99.0% and 88.7%, respectively. The UTS, YS, and work hardening exponents of the welded joints increased slightly, while the strain hardening capacity of the base metals was much higher than those of the welded joints with decreasing temperatures. The evaluated work hardening exponents of the welded joints were determined using the Hollomon equation, Afrin equation, and Crussard-Jaoul analysis are in the range of 0.2–0.47, 0.24–0.59, and 0.45–0.71, respectively. The welded joints and the base metals demonstrated only stage III strain hardening, with DP800 joints exhibited excellent uniform and total elongation ranging between 8.0–8.7% and 10.4–14.2%, respectively. Fractures were located in the base metal of welded DP800 and SCHAZ of DP1000 welds, respectfully. The fracture surfaces demonstrated characteristic dimple fractures. The uniqueness of this study is found in its design, as there is currently no known literature on the low-temperature deformation mechanism and strain-hardening behaviour of similar DP800 and DP1000 welds.

Low temperature deformation mechanisms of polycrystalline CoZr and Co39Ni11Zr50 B2-type intermetallic compounds

The B2-type intermetallic compounds CoZr and Co39Ni11Zr50 were deformed in tension at low temperatures. While CoZr is ductile down to 4 K, Co39Ni11Zr50 becomes brittle below 125 K due to a martensitic phase transformation. Thermal activation analysis shows that CoZr follows the Cottrell-Stokes law indicating forest dislocation cutting as the dominant rate-controlling deformation mechanism, similar to face-centered cubic metals. The moderate ductility of both intermetallic compounds at low temperatures may qualitatively be related to a significant metallic character of bonding giving rise to a low Peierls stress estimated for primary {110}<100> slip and most likely also leads to an easier activation of secondary {110}<110> slip which was proven by transmission electron microscopy. Secondary slip is necessary for the fulfillment of the von Mises criterion for homogeneous plastic deformation of polycrystalline materials. The present results generalize the findings made on the ductile rare earth intermetallics YAg and YCu and, therefore, may help to search for other ductile systems in the broad class of intermetallic compounds.

Mechanism of low temperature deformation in aluminium alloys

This study investigates differences in the deformation mechanisms between room temperature (296 K) and cryogenic temperatures (77 K) and their advantages for low temperature formability in alloys EN AW 1085, EN AW 5182 and EN AW 6016. Compared to room temperature behaviour, tensile tests showed an overall increase in yield strength, ultimate tensile strength and uniform elongation with differences among the principal alloy types. In general, the improved mechanical properties result from higher strain hardening rates at lower temperatures. The application of an extended Kocks-Mecking approach showed a significant reduction of the dynamic recovery and suggested higher dislocation densities upon cryogenic deformation. This was confirmed via in-situ synchrotron experiments, which also reveal a higher proportion of screw dislocations. Moreover, kernel average misorientation maps from electron backscattered diffraction and in-situ cryogenic deformation in a transmission electron microscope displayed a more uniform dislocation arrangement with a reduction of slip lines and less highly misaligned areas after deformation at lower temperatures. Supported by a detailed characterization of the microstructure and its dislocation structure, the associated fundamental mechanisms we reveal, which are at the origin of the exceptional improvement in mechanical properties, are extensively discussed.

Low temperature deformation mechanisms of single crystalline intermetallic compound YAg

YAg belongs to the family of B2-type rare earth intermetallic compounds that in polycrystalline form exhibit moderate low temperature ductility. In this work, YAg single crystals have been deformed at low temperatures down to 4 K. The activation parameters and their dependence on stress and temperature, as well as the comparison with the deformation behavior of polycrystalline YAg, indicate that the cutting of forest dislocations is the rate-controlling deformation mechanism, similar to face-centered cubic metals. The results suggest that the low yield stress and high work-hardening rate observed cause the moderate ductility at low temperatures.

Low temperature deformation mechanism of semiconductor single crystal and molding of Ge microlens array by direct electrical heating

Although deforming a silicon single crystal at a temperature of about 600 °C lower than its melting point (1414 °C) by direct electrical heating was successfully demonstrated, the mechanism has still not been fully clarified. In this paper, we propose a model for the low temperature deformation of a semiconductor single crystal by direct electrical heating. The thermographic observation during direct electrical heating reveals that the local temperature is higher at the region where dense dislocation occurred in the semiconductor single crystal by uniaxial pressing. This is interpreted in terms of the scattering of an electron by the dislocation leading to an increase in the electrical resistivity. Finally, the deformation temperature of the semiconductor single crystal apparently becomes low due to the occurrence of such hot spots. We have also demonstrated an application to mold a microlens array composed of a germanium single crystal with a focal length of 25 µm.

Low-temperature superplastic deformation mechanism in Ti–6Al–4V alloy processed by friction stir processing

The rolled Ti–6Al–4V alloy plates with ~2.3 mm in thickness were processed by Friction stir processing (FSP), and ultrafine (~0.57 μm) and equiaxed microstructure was successfully obtained in the stir zone (SZ). Superplastic tensile tests of this microstructure were conducted in the strain rate range of 1 × 10−4-3 × 10−3 s−1 with the tensile testing temperature of 550–600 °C and an extremely excellent low-temperature superplasticity (LTSP) with an elongation of 1400% was obtained at 3 × 10−4 s−1 and 600 °C. Such a large elongation was owing to the presence of a large proportion of high angle grain boundaries (90.7%) and random crystallographic orientation in the SZ of the friction stir processed (FSPed) Ti–6Al–4V alloy. Furthermore, the precipitated β phase during tensile deformation also made a contribution to the enhancement of LTSP. In addition, a slight grain growth from initial 0.57 μm in the FSPed microstructure to 1.07 μm in the gauge section of the tensile fractured sample at 3 × 10−4 s−1 and 600 °C proved the excellent microstructure stability of the FSPed Ti–6Al–4V alloy. The grain boundary sliding (GBS) was recognized as the dominant superplastic deformation mechanism. The grain rotation (GR) and dynamic recrystallization (DRX), especially the discontinuous dynamic recrystallization (DDRX), acted as the coordination mechanisms that could relax the stress concentration generated during GBS, and thereby promoted the continuous operation of GBS and the achievement of the enhanced LTSP.

Thermally activated deformation of nanocrystalline and coarse grained CoCrFeNiMn high entropy alloy in the temperature range 4.2–350 K

Mechanical properties of a nanocrystalline (∼ 60 nm) and a coarse grained (grain sizes ∼ 4 µm) CoCrFeNiMn high entropy alloys were studied in uniaxial compression in the temperature range 4.2–350 K. Temperature dependences of yield strength, flow stress and strain rate sensitivity have been registered and analyzed in the framework of two thermal activation deformation models, that of thermal activation of local barrier overcoming, and that of Peierls valley double kink formation. Microscopic parameters of dislocation interaction with the barriers for thermally activated motion are estimated and low temperature deformation mechanisms are discussed.

Low-Temperature Superplasticity and Deformation Mechanism of Ti-6Al-4V Alloy.

The low-temperature superplastic tensile behavior and the deformation mechanisms of Ti-6Al-4V alloy are investigated in this paper. Through the experiments carried out, elongation to failure (δ) is calculated and a set of values are derived that subsequently includes the strain rate sensitivity exponent (m), deformation activation energy (Q) at low-temperature superplastic deformation, and the variation of δ, m and Q at different strain rates and temperatures. Microstructures are observed before and after superplastic deformation. The deformation mechanism maps incorporating the density of dislocations inside grains at temperatures of 973 and 1123 K are drawn respectively. By applying the elevated temperature deformation mechanism maps based on Burgers vector compensated grain size and modulus compensated stress, the dislocation quantities and low-temperature superplastic deformation mechanisms of Ti-6Al-4V alloy at different temperatures within appropriate processing regime are elucidated.

An improved constitutive description of tensile behavior for CP-Ti at ambient and intermediate temperatures

The tensile behavior of commercially pure titanium (CP-Ti) was investigated at temperatures ranging from 283K to 573K and strain rates ranging from 5×10−5s−1 to 5×10−3s−1. In this temperature range, the apparent activation energy (Q) is temperature dependent causing by the competing process of deformation mechanisms. In order to quantitatively describe the evolutionary process of the deformation mechanism with temperature, a quantitative index is proposed based on the variation of Q with temperature. The threshold temperature 438K is determined by the quantitative index of deformation mechanism, and this threshold temperature indicates the transition from the low temperature deformation mechanism dominated regime to the high temperature deformation mechanism dominated regime. Since the deformation mechanism is changing in the concerned temperature range, the classical Arrhenius type model is inappropriate to describe the tensile behavior of CP-Ti. To extend the application scope of the Arrhenius type model to a wider temperature range, the Arrhenius type model is improved by considering the competing process of deformation mechanisms.

Low and high temperature deformation mechanisms in TiAl alloys

The deformation mechanisms involving ordinary dislocations are studied at various temperatures, in single-phased and lamellar TiAl alloys. In situ straining and heating experiments are performed in the transmission electron microscope (TEM) as well as postmortem studies of deformed samples. It is shown that the dislocations can move by glide or climb depending on the temperature range and that some dynamic strain aging occurs at intermediates temperatures. In lamellar alloys, the crossing of interfaces is found to play a determining role.

Electron microscope characterization of low cycle fatigue in a high-strength multiphase titanium aluminide alloy

The micromechanisms controlling low cycle fatigue of a Nb-bearing TiAl alloy (TNB-V2) have been characterized by conventional and high-resolution transmission electron microscopy. Fully reversed isothermal tests were performed under strain control at temperatures of 25, 550 and 850 °C. Samples fatigued at 25 and 550 °C exhibited dense structures of ordinary dislocations, dipoles and debris that were accumulated in tangles. The dipole defects apparently serve as additional glide obstacles but may also contribute to dislocation multiplication if the local stress rises. Another important low temperature deformation mechanism is the stress-induced transformation of an orthorhombic phase, which is a significant constituent of the microstructure. The orthorhombic phase is apparently unstable under tetragonal distortion and transforms into γ(TiAl) phase. Under high-temperature fatigue the lamellar microstructure degrades by phase transformation combined with dynamic recrystallization.

Dislocation formation and twinning from the crack tip in Ni3Al: molecular dynamics simulations

The mechanism of low-temperature deformation in a fracture process of L12 Ni3Al is studied by molecular dynamic simulations. Owing to the unstable stacking energy, the [01̄1] superdislocation is dissociated into partial dislocations separated by a stacking fault. The simulation results show that when the crack speed is larger than a critical speed, the Shockley partial dislocations will break forth from both the crack tip and the vicinity of the crack tip; subsequently the super intrinsic stacking faults are formed in adjacent {111} planes, meanwhile the super extrinsic stacking faults and twinning also occur. Our simulation results suggest that at low temperatures the ductile fracture in L12 Ni3Al is accompanied by twinning, which is produced by super-intrinsic stacking faults formed in adjacent {111} planes.

Low-temperature thermally-activated deformation and irradiation softening in neutron-irradiated molybdenum

The effect of neutron irradiation on low-temperature deformation of Mo in two heat treatments, i.e. annealed and stress-relieved, was investigated. Specimens were irradiated at reactor coolant temperature (∼80 °C) to doses ranging from 7.2 × 10 −5 to 0.28 dpa in the High Flux Isotope Reactor. Tensile tests were carried out between −50 and 100 °C at strain rates of 1 × 10 −5–1 × 10 −2 s −1. Thermal activation analysis based on tensile data was performed to understand the low-temperature deformation mechanism. Irradiation softening and reduced dependence on test temperature and strain rate of the yield stress was observed in the annealed Mo after low-dose neutron irradiation (<∼0.003 dpa). Higher dose neutron irradiation caused athermal hardening only. The stress-relieved Mo showed a weaker dependence on test temperature and strain rate of the yield stress than the annealed Mo, and the dependence of the yield stress of the stress-relieved Mo was nearly unchanged after irradiation. Comparison of the experimental values of activation parameters with the theoretical predictions of dislocation models indicates that the Fleischer model of interactions of dislocations with tetragonal strains gave a better description of the activation process than the double-kink model, which implies a scavenging effect. The reduced test temperature and strain rate dependence following irradiation may be explained by the decreased effective stress due to trapping of interstitial solute species by neutron-produced defects.

Effects of nanocrystalline and ultrafine grain sizes on constitutive behavior and shear bands in iron

The mechanical behaviors of consolidated iron with average grain sizes from tens of nanometers to tens of microns have been systematically studied under uniaxial compression over a wide range of strain rates. In addition to the well-known strengthening due to grain size refinement, grain size dependence is observed for several other key properties of plastic deformation. In contrast with conventional coarse-grained Fe, high-strength nanocrystalline and submicron-grained Fe exhibit diminished effective strain rate sensitivity of the flow stress. The observed reduction in effective rate sensitivity is shown to be a natural consequence of low-temperature plastic deformation mechanisms in bcc metals through the application of a constitutive model for the behavior of bcc Fe in this strain rate and temperature regime. The deformation mode also changes, with shear localization replacing uniform deformation as the dominant deformation mode from the onset of plastic deformation at both low and high strain rates. The evolution and multiplication of shear bands have been monitored as a function of plastic strain. The grain size dependence is discussed with respect to possible enhanced propensity for plastic instabilities at small grain sizes.

Microstructural analysis of i-AlPdMn quasi-crystals deformed between room temperature and 300 °C under confining pressure

AlPdMn quasi-crystalline specimens were deformed at low temperatures using a high-confining-pressure device. The microstructures, observed in each sample by transmission electron microscopy, depend on the deformation temperature and show that plastic deformation is accompanied by dislocation movements. A detailed analysis of the dislocation configurations shed light on the low temperature deformation mechanisms.

Low temperature deformation mechanisms of icosahedral AlPdMn quasicrystals.

ABSTRACTConfining pressure techniques, that superimpose a shear stress on an isostatic component, allowed us to plastically deform AlPdMn specimens at temperatures below the brittle-to-ductile transition temperature obtained using conventional uniaxial deformation tests at usual strain-rates. Microstructural observations associated with these low deformation temperatures are reported. They provide new insights on the elementary deformation mechanisms that control plasticity in this non-periodic structure.

Indentation size effect in polycrystalline F.C.C. metals

High purity aluminum and alpha brass samples were tested for the indentation size effect (ISE) using a combination of microhardness (high load) and nanoindentation (low load). We employed rate effects to study low temperature deformation mechanisms using nanoindentation creep and load relaxation. Based on these rate effects which are conspicuous in terms of the rate sensitivity of the hardness, ∂H /∂ ln ε ̇ eff , we calculated the activation volume, V ∗ , and compared data from indentation creep with data from uniaxial loading. The data from nanoindentation for alpha brass when plotted, V∗ vs H or τ (flow stress) , extrapolated into literature data from conventional tensile testing, while the aluminum nanoindentation data exhibited an offset. We propose some mechanisms for this offset. We demonstrate that the trend of V ∗ vs H, resulting from the ISE in a single specimen, concurs with that obtained from testing specimens with various levels of work hardening. This suggests that the ISE is driven by a dislocation mechanism. Additionally, when the results are fitted to a strain gradient plasticity model (SGP), the data at deep indents (microhardness and large nanoindentation) exhibited a straight-line behavior closely identical to literature data. However, for shallow indents (nanoindetation data), the slope of the line severely changes, decreasing by a factor of ten, resulting in a “bilinear behavior”. We also propose some possible mechanisms for this behavior.