Conversion of the hot torsion test results into flow curve with multiple regimes of hardening

The method of Fields and Backofen has been commonly used to reduce the data obtained by hot torsion test into flow curves. The method, however, is most suitable for materials with monotonic strain hardening behaviour. Other methods such as Stüwe’s method, tubular specimens, differential testing and the inverse method, each suffer from similar drawbacks. It is shown in the current work that for materials with multiple regimes of hardening any method based on an assumption of constant hardening indices introduces some errors into the flow curve obtained from the hot torsion test. Therefore such methods do not enable accurate prediction of onset of recrystallisation where slow softening occurs. A new method to convert results from the hot torsion test into flow curves by taking into account the variation of constitutive parameters during deformation is presented. The method represents the torque twist data by a parametric linear least square model in which Euler and hyperbolic coefficients are used as the parameters. A closed form relationship obtained from the mathematical representation of the data is employed next for flow stress determination. Two different solution strategies, the method of normal equations and singular value decomposition, were used for parametric modelling of the data with hyperbolic basis functions. The performance of both methods is compared. Experimental data obtained by FHTTM, a flexible hot torsion test machine developed at IROST, for a C–Mn austenitic steel was used to demonstrate the method. The results were compared with those obtained using constant strain and strain rate hardening characteristics.

The effect of prestrain and bake hardening on the low-cycle fatigue properties of TRIP steel

The scope of this study was to examine the effects of plane strain prestrain, induced via cold-rolling, and subsequent automotive paint bake hardening cycle on both tensile and fatigue properties of a hot rolled TRIP780 multiphase steel. Strain-life data has been generated for as-received (0% prestrain), 10% and 20% prestrained samples, in both baked and unbaked conditions. Cold rolling  increased the number of strain reversals to failure at high cyclic strain amplitudes with no effect at low strain amplitudes. Bake hardening increased the number of reversals to failure at high cyclic strain amplitudes. The prestrained material exhibited partial cyclic softening, with some residual strength increase. The residual strength increase was attributed to the austenite to martensite transformation that occurred during the prestraining process.

Effect of pre-straining and bake hardening on the microstructure and mechanical properties of CMnSi trip steels

The effects of pre-straining and bake hardening on the mechanical behaviour and microstructural changes were studied in two CMnSi TRansformation-Induced Plasticity (TRIP) steels with different microstructures after intercritical annealing. The TRIP steels before and after pre-straining and bake hardening were characterised by X-ray diffraction, optical microscopy, transmission electron microscopy, three dimensional atom probe and tensile tests. Both steels exhibited discontinuous yielding behaviour and a significant strength increase with some reduction in ductility after pre-straining and bake hardening treatment. The following main microstructural changes are responsible for the observed mechanical behaviours: a decrease in the volume fl:action of retained austenite, a increase in the dislocation density and the formation of cell substructure in the polygonal ferrite, higher localized dislocation density in the polygonal ferrite regions adjacent to martensite or retained austenite, and the precipitation of fine iron carbides in bainite and martensite. The mechanism for the observed yield point phenomenon in both steels after treatment was analysed.

Effect of bake-hardening on the structure-property relationship of multiphase steels for the automotive industry

The effect of a bake-hardening (BH) treatment on the microstructure and mechanical properties has been studied in C-Mn-Si TRansformation Induced Plasticity (TRIP) and Dual Phase (DP) steels after: (i) thermomechanical processing (TMP) and (ii) intercritical annealing (IA). The steels were characterized using X-ray diffraction, transmission electron microscopy (TEM) and three-dimensional atom probe tomography (APT). All steels showed high BH response. however, the DP and trip steels after IA/BH showed the appearance of upper and lower yield points, while the stress-strain behavior of the trip steel after TMP/BH was still continuous. This was due to the higher volume fraction of bainite and more stable retained austenite in the TMP/BH steel, the formation of plastic deformation zones with high dislocation density around the "as-quenched” martensite and “TRIP” martensite in the IA/BH DP steel and IA/BH TRIP steel, respectively.

Effect of pre-straining and bake hardening on the microstructure and mechanical properties of CMnSi trip steels

The effects of pre-straining and bake hardening on the mechanical behaviour and microstructural changes were studied in two CMnSi TRansformation-Induced Plasticity (TRIP) steels with different microstructures after intercritical annealing. The TRIP steels before and after pre-straining and bake hardening were characterised by X-ray diffraction, optical microscopy, transmission electron microscopy, three dimensional atom probe and tensile tests. Both steels exhibited discontinuous yielding behaviour and a significant strength increase with some reduction in ductility after pre-straining and bake hardening treatment. The following main microstructural changes are responsible for the observed mechanical behaviours: a decrease in the volume fraction of retained austenite, an increase in the dislocation density and the formation of cell substructure in the polygonal ferrite, higher localized dislocation density in the polygonal ferrite regions adjacent to martensite or retained austenite, and the precipitation of fine iron carbides in bainite and martensite. The mechanism for the observed yield point phenomenon in both steels after treatment was analysed.

Anisotropic wetting characteristics on submicrometer-scale periodic grooved surface

Submicrometer-scale periodic structures consisting of parallel grooves were prepared on azobenzene-containing multiarm star polymer films by laser interference. The wetting characteristics on the patterned surfaces were studied by contact angle measurements. Macroscopic distortion of water drops was found on such small-scale surface structures, and the contact angles measured from the direction parallel to the grooves were larger than those measured from the perpendicular direction. A thermodynamic model was developed to calculate the change in the surface free energy as a function of the instantaneous contact angle when the three-phase contact line (TPCL) moves along the two orthogonal directions. It was found that the fluctuations, i.e., energy barriers, on the energy versus contact angle curves are crucial to the analysis of wetting anisotropy and contact angle hysteresis. The calculated advancing and receding contact angles from the energy versus contact angle curves were in good agreement with those measured experimentally. Furthermore, with the groove depth increasing, both the degree of wetting anisotropy and the contact angle hysteresis perpendicular to the grooves increased as a result of the increase in the energy barrier. The theoretical critical value of the groove depth, above which the anisotropic wetting appears, was determined to be 16 nm for the grooved surface with a wavelength of 396 nm. On the other hand, the effect of the groove wavelength on the contact angle hysteresis perpendicular to the grooves was also interpreted on the basis of the thermodynamic model. That is, with the wavelength decreasing, the contact angle hysteresis increased due to the increase in the number of energy barriers. These results may provide theoretical evidence for the design and application of anisotropic wetting surface.

Anisotropic mechanical properties of nickel foams fabricated by powder metallurgy

In the present study, porous nickel foam samples with pore sizes of 20 μm and 150 μm and porosities of 60 % and 70 % were fabricated by the space-holding sintering method via powder metallurgy. Electron scanning microscopy (SEM) and Image-Pro Plus were used to characterise the morphological features of the porous nickel foam samples. The anisotropic mechanical properties of porous nickel foams were investigated by compressive testing loading in different directions, i.e. the major pore axis and minor pore axis. Results indicated that the nominal stress of the nickel foam samples increases with the decreasing of the porosity. Moreover, the foam sample exhibited significantly higher nominal stress for loading in the direction of the major pore axis than loading in direction of the minor pore axis. It is also noticeable that the nominal stress of the nickel foams increases with the decreasing of the pore size. It seems that the deformation behaviour of the foams with a pore size in the micron-order differs from those with a macro-porous structure.

Ion exchange, anisotropic structure and thermal stability of polypyrrole films

The thermal stability of electrochemically prepared polypyrrole (PPy) films with p-toluene sulfonate (pTS) or perchlorate (CIOP₄⁻) counter ion (PPy/pTS and PPy/ClO₄⁻) is improved by simple treatment with aqueous sulfuric acid, sodium sulfate or sodium bisulfate. The degree of stabilization achieved depends on the solution, temperature and duration of treatment. PPy/pTS is easily stabilized and thick films (43μm) retain 90 % of the initial conductivity after long period (300 h) at 150 °C, while thinner films (12 μm) retain slightly less (70 %). A model for the conductivity decay has been proposed. Although the mechanism for improved stability is not yet clear it is apparent that the level of ion exchange and the original polymer microstructure are important. The early stages of ion exchange are not symmetrical and diffusion is facilitated at the electrode side of the film. Furthermore, X-ray diffraction shows no evidence of morphological change after treatment of thick PPy/pTS but in thin PPy/pTS and PPy/ClO₄⁻ films an additional peak is indicative of more ordered structure following treatment. These observation may imply that there is a higher density of crosslinks and branching at the growth side than at the electrode side of the film.

Characterization of the bake-hardening behavior of transformation induced plasticity and dual-phase steels using advanced analytical techniques

The bake-hardening (BH) behavior of TRansformation Induced Plasticity (TRIP) and Dual-Phase (DP) steels after intercritical annealing (IA) has been studied using transmission electron microscopy, X-ray diffraction and three dimensional atom probe tomography. It was found for the DP steel that carbon can segregate to dislocations in the ferrite plastic deformation zones where there is a high dislocation density around the "asquenched" martensite. The carbon pinning of these dislocations, in turn, increases the yield strength after aging. It was shown that bake-hardening also leads to rearrangement of carbon in the martensite leading to the formation of rod-like low temperature carbides in the DP steel. Segregation of carbon to microtwins in retained austenite of the TRIP steel was also evident. These factors, in combination with the dislocation rearrangement in ferrite through the formation of cells and microbands in the TRIP steel after pre-straining, lead to the different bake-hardening responses of the two steels.

Study of phase transformation and work hardening phenomenon during drilling of Ti-5553 and Ti-64

Titanium 5553 is a recently developed modification of Russian near-β titanium alloy VT-22 which has applications and potential particularly in the aerospace industry for such key components as landing gear. However, indications are that Ti-5553 has poorer machinability characteristics than other Ti alloys and a comprehensive and far-reaching analysis is a necessary research imperative. This paper presents the result of phase transformation and work hardening during drilling of Ti-5553 compared with Ti-64. The aim of this research work is to optimise the machining condition for Ti-5553, in which the β to a phase transformation, together with material work hardening could be fully understood. Analysis of machinability indicators, such as subsurface micrograph and hardness of drilled samples and drilling forces and torques, demonstrated that Ti-5553 generally has poorer machinability characteristics than Ti-64 and to some extent this variation has been quantified to allow for further and more detailed investigation.