Viscous Glide Creep Research Articles

Creep behavior of 50at%Ni 25at%Ti 25at%Hf high temperature shape memory alloy under constant load

In the last three decades, aerospace industry has required shape memory alloys with higher phase transformation temperatures. In response to this technical challenge, High Temperature Shape Memory Alloys that can be actuated at higher temperatures were developed. However, high temperature actuation applications also lead to observe much higher dislocation motion and thus plastic deformation due to creep and phase transformation. In this study, 50at%Ni 25at%Ti 25at%Hf high temperature shape memory alloy was used for the investigation of creep deformation under tensile loads corresponding to stress magnitudes between 200 MPa and 500 MPa. Deformation behavior of the alloy in austenitic phase was investigated by means of creep deformation experiments at 450 °C, 550 °C, and 600 °C. 50at%Ni 25at%Ti 25at%Hf alloy has similar creep behavior as NiTi despite higher loading conditions. Almost no noticeable creep strain values were observed under 200 MPa and 300 MPa at 550 °C and 600 °C. Creep parameters such as the stress exponent and the activation energy values were calculated using power law creep model and viscous glide creep was found to be the main mechanism in 50at%Ni 25at%Ti 25at%Hf high temperature shape memory alloy.

Dynamic recrystallization and hot deformation mechanisms of a eutectic Al0.7CoCrFeMnNi high-entropy alloy

The deformation mechanisms and microstructural evolution of an Al0.7CoCrFeMnNi high-entropy alloy (HEA) composed of dual phases (FCC phase: 56.6 vol% and BCC phase: 43.4 vol%) have been studied during hot compressive deformation in the temperature range between 1173 and 1373 K and in the strain rate range between 10−3 and 10 s−1 up to a strain of 0.8. Among the AlxCoCrFeMnNi HEAs (with x = 0, 0.5, 0.7 and 1.0) from earlier studies and in the present study, the Al0.7CoCrFeMnNi alloy produced the finest grain size (1–46 µm) and the highest fraction of dynamically recrystallized grains (39–99%) after hot deformation, implying that the most effective grain refinement can be achieved from a eutectic microstructure. Continuous dynamic recrystallization (CDRX) was the main DRX mechanism in both the FCC and BCC phases, but the kinetics of CDRX was faster in the BCC phase, resulting in formation of a higher fraction of dynamically recrystallized grains in the BCC phase under the same deformation condition. The stress exponent associated with plastic flow at low strain rates was approximately 3. The steady-state constitutive relation linking the flow stress and the strain rate could be well interpreted in terms of either viscous glide creep with antiphase boundary interactions (when the BCC phase is assumed to be the B2 phase at high temperatures) or viscous glide creep with Cottrell-Jaswon interactions (when the BCC phase is assumed to be the disordered BCC (A2) phase at high temperatures).

Nano-structure evolution of secondary Al3(Sc1−xZrx) particles during superplastic deformation and their effects on deformation mechanism in Al-Zn-Mg alloys

The nano-structure evolution of secondary Al3(Sc1−xZrx) particles during high-strain-rate (0.01 s−1) superplastic deformation at 500 °C was investigated by high resolution transmission electron microscopy. The results show that by increasing the true stains from 0.69 to 2.40, the mean radii of spheroidal secondary Al3(Sc1−xZrx) particles increase from 9.8 ± 3.4 nm to 16.4 ± 6.8 nm, and the lattice misfit value increases from 1.04% to 1.30%. Before deformation, Al3(Sc1−xZrx) nano-particles are completely coherent with Al(α) matrix. As the deformation proceeds, the accumulated sever plastic deformation introduces misfit dislocations at the interface between particles and matrix. Superplastic flow deformation data indicate that with the increase of true strains, the strain rate sensitivity of Al-Zn-Mg alloy decreases from 0.29 to 0.19, and the deformation activation energy increases from 112 to 121 kJ/mol. However, for Al-Zn-Mg alloy with secondary Al3(Sc1−xZrx) nano-particles, the strain rate sensitivity increase from 0.33 to 0.45, and the deformation activation energy decreases from 107 to 84 kJ/mol. Based on the microstructural results and the established constitutive equations at different strains, it can be concluded that at the working hardening stage, two kinds of alloys are both controlled by dislocation viscous glide creep mechanism. At the dynamic softening stage, dislocation creep mechanism operates in Al-Zn-Mg alloy, whereas grain boundary sliding mechanism is dominant in Al-Zn-Mg-Sc-Zr alloy. Secondary Al3(Sc1−xZrx) nano-particles play an important role in accelerating the cooperative grain boundary deformation and only affect dynamic softening deformation mechanism of Al-Zn-Mg alloys.

Superplasticity, flow and fracture mechanism in an Al–12.7Si–0.7Mg alloy

The superplastic behavior of an Al–12.7mass%Si–0.7mass%Mg alloy was investigated under different conditions. Reasonable superplastic elongations were achieved in the fine-grained (9.1μm) Al–Si–Mg alloy at temperatures ranging from 733 to 793K at initial strain rates ranging from 1.67×10–4 to 1.67×10–3s−1. A maximum elongation to failure of 379% was demonstrated with a strain rate sensitivity, m, of 0.52 and an activation energy for flow, Q, of 156.7KJ/mol at 793K at an initial strain rate of 1.67×10–4s−1, which is close to the lattice diffusion activation energy of aluminum. The dislocation activity within Al grains indicated that intragranular slip is the accommodation mechanism of grain boundary sliding. EBSD (Electron Backscatter Diffraction) results revealed that most grain boundaries were high angle boundaries and therefore indicated that boundary sliding and grain rotation occurred during deformation. A deformation mechanism map was plotted for the Al–Si–Mg alloy at 793K and it is shown that the experimental datum points are in excellent agreement with the predictions of the map. Most cavities were formed around silicon particles and the cavity formation mechanism was proposed. The observation on the fracture surface revealed the presence of filaments. The filament quantity or density increased with increasing testing temperature, which can be interpreted by the transition of dislocation viscous glide creep to grain boundary sliding mechanism at elevated temperatures. The formation of filaments was related to the deformation mechanisms and the lattice diffusion at elevated temperatures. The superplastic fracture in the Al–Si–Mg alloy exhibited a diffuse necking and was a pseudo-brittle fracture. The fracture mechanism was intergranular fracture.

Creep and microstructure in powder metallurgy 15 vol.% SiCp–2009 Al composite

The creep behavior and microstructure of powder metallurgy (PM) 15 vol.% silicon particulate-reinforced 2009 aluminum alloy (SiCp–2009 Al composite) and its matrix PM 2009 Al were investigated over six orders of magnitude of strain rate and at temperatures in the range 618–678 K. The results show that the creep behavior of PM 15% SiCp–2009 Al composite resembles that of PM 2009 Al with regard to (a) the variations in both the apparent stress exponent and the apparent activation energy for creep due to applied stress, (b) the value of the true stress exponent, (c) the value of the true activation energy for creep, (d) the interpretation of creep in terms of a threshold stress, and (e) the temperature dependence of threshold stress. This resemblance implies that deformation in the matrix governs deformation in the composite. Analysis of the creep data in terms of creep rate against an effective stress shows that the creep behaviors of the composite and unreinforced alloy are consistent with the operation of viscous glide creep at low stresses. A comparison between the creep data of the composite and those of the unreinforced matrix revealed that the composite exhibited more creep-resistant characteristics than its matrix over the entire range of applied stresses.

Effect of temperature on transitions in creep mechanisms in class-A alloys

Alloy class materials exhibit viscous glide controlled creep, where dislocations are locked by solute atoms thereby decreasing their glide velocity while dislocation annihilation by climb still occurs. The creep-rate here varies as cube of the applied stress (stress exponent, n = 3) while the activation energy for creep becomes equal to that for solute atom diffusion. At high enough stress, the dislocations get freed from solute atom locking and transition to climb controlled creep is noted with n ∼ 5 while at still higher stresses, power-law breakdown is observed. For relatively small grain-sizes, viscous creep mechanisms such as Nabarro–Herring or Coble creep usually occur at low stresses. A close examination of the model equations reveals that at temperatures below a critical temperature, a transition from diffusional viscous creep to dislocation climb occurs without the intermediate viscous glide creep. Recent experimental results on a Zr–Nb sheet clearly follow these formulations exhibiting narrower intermediate region with decreasing temperature.

Shear deformation behavior of Al–5% Mg in a semi-solid state

Shear tests have been carried out over a wide temperature range of 753–893 K, including temperatures below and above the solidus temperature, for an Al–5 wt% Mg alloy. The deformation behavior in the semi-solid state is divided into two regions; one is in a semi-solid state containing a discontinuous liquid and the other is in a semi-solid state containing a continuous liquid. An analysis of the activation energy suggests that the deformation in the latter region is associated with the lubricated flow. Deformation in the former region is likely to be a transition from the viscous glide creep in the solid state to the lubricated flow in a semi-solid state containing a continuous liquid.

Deformation behavior of a Ni−30Al−20Fe−0.05Zr intermetallic alloy in the temperature range 300 to 1300 K

The deformation behavior of an extruded Ni-30 (at. pct) Al−20Fe−0.05Zr intermetallic alloy was studied in the temperature range of 300 to 1300 K under initial tensile strain rates varying between about 10−6 and 2×10−3 s−1 and in constant load compression creep between 1073 and 1300 K. The deformation microstructures of the fractured specimens were characterized by transmission electron microscopy (TEM). Three deformation regimes were observed: Region I consisted of an athermal regime of low tensile ductility (less than 0.3 pct) occurring between 400 and 673 K, where the substructure consisted of slip bands in a few grains. Exponential creep was dominant in region II between 673 and 1073 K, where the substructure changed from a mixture of dislocation tangles, loops, and dipoles at 673 K to a microstructure consisting of subgrains and dislocation tangles at 973 K. The tensile ductility was generally about 2.0 to 2.5 pct below 980 K in this region. A significant improvement in tensile ductility was observed in region III, which occurred between 1073 and 1300 K. An analysis of the data suggests that viscous glide creep with a stress exponent,n, of about 3 and high-temperature dislocation climb withn≈4.5 where the two dominant creep mechanisms in this region depending on stress and temperature. The average activation energy for deformation in this region was about 310±30 kJ mol−1 for both processes. In this case, a transition from viscous glide creep to dislocation climb occurred when σ/E<1.7×10−4, where σ is the applied stress andE is the Young’s modulus.

Surface void nucleation under the power-law creep condition in an Al-3 at. pct Mg solid solution alloy

In this study, under the viscous glide creep condition of Al-3 at. pct Mg solid solution alloy, which is the power-law creep condition with stress exponent n=3, [9,10] surface void nucleation due to GBM (grain boundary migration) and evaporation of magnesium is investigated

Creep behavior of fine grained Cu-2.8 Al-1.8 Si-0.4 Co alloy

The elevated temperature creep properties of a fine grained copper alloy (CDA 638) was investigated between 673 and 798 K at strain rates ranging from 10 −6 s −1 to 2 × 10 −2 s −1. The mechanical data were analyzed in order to establish the stress, temperature, and grain size dependence of the strain rate. The double logarithmic plot of stress vs strain rate at constant temperature and grain size revealed four distinct regions. It is suggested that the rate controlling deformation mechanisms for these four regions are, in terms of increasing strain rate values: region I, dislocation climb controlled creep; region II, a transition from climb-controlled to viscous glide controlled creep; region III A, viscous glide controlled creep, and; region IIIB, a transition region from viscous glide creep to obstacle-limited glide in the so-called “Power Law” breakdown regime.