Hollomon Parameter Research Articles

Effect of Cr elimination on flow behavior and processing map of newly developed ECO-7175 aluminum alloy during hot compression

ECO-Al alloys are introduced as a game-changer for the aluminum industry and it is of utmost importance to determine the role of alloying elements in their processing characteristics. In this study, the effects of Cr on the hot deformation behavior of newly-developed ECO-7175 alloy were investigated. ECO-7175 samples with and without Cr were hot-compressed using a Gleeble simulator (temperature range of 350−500 °C and strain rates of 0.001−1 s−1). The results were used to study the constitutive equations, the processing maps, and the microstructural evolution of the alloys. In Cr-containing alloy, the analysis of the deformation activation energy reveals that the rate-controlling mechanisms of the deformation change gradually from self-diffusion of Al (or diffusion of Mg in Al) to diffusion of Cr in Al by decreasing the Zener−Hollomon parameter. The analysis of the processing maps of Cr-containing alloy shows that the dynamic recrystallization (DRX) zone is limited to the deformation at high temperatures and low strain rates and expands with increasing applied strain. On the other hand, it is found that the self-diffusion of Al (or Mg in Al) is the only rate-controlling mechanism during hot deformation of Cr-free alloy in all processing conditions and its DRX zone is independent of the plastic strain.

Generalized diagrams and equations of recrystallization of cold-deformed steel St

The recrystallization processes in steel St.3 inthe ferrite state were studied. Samples with diameter of 8 mmand with height of 10 mmwere deformed by compression at 20 °Cfor 20 to 80 %, annealed at 400 – 735 °Cfor a period from 5 minutes to 10 hours, and cooled in the air. On the samples, the grain size was determined in longitudinal sections (with respect to the compression axis). After separation of the entire array of experimental data (degree of deformation ε, temperature T and time τ of annealing, grain size D) into 3 groups (no recrystallization, beginning and end of the primary recrystallization), the equations of hyperplanes best sharing these groups were found by the method of discriminant mathematical analysis. Recrystallization is not observed if the temperature is below 465 °C, or if the degree of deformation is lower than 20 % for any combination of other parameters. The deformed structure completely recrystallizes if the experimental points are in the parameter range: T > 550 °C, ε > 40 %, τ > 30 min. The largest grain refinement (up to 7 – 10 μm) was obtained after deformation with a maximum degree (80 %). The first critical (physical) degree of deformation, after which the size of the recrystallized grain is larger than the original one, is absent. The second critical (technical) degree of deformation is 25 – 35 % for temperatures of 530 – 735 °C. At such degrees grain refinement was observed in comparison with the initial deformed state. Mathematical relation between the size of the recrystallized grain and the experiments’ parameters was analyzed in two ways: according to Arrhenius in the form D=AεNτMexp (-Q/RT) , and according to Hollomon with linear temperature dependence (D ~ T). The Arrhenius solution gave the following equation: log(D) = 2,08 – 0,33log(ε) + 0,023log(τ) – 967,31 1/T. Therefore, activation energy of the recrystallization process is found to be ~18,000 J/mol. In case of the Hollomon analysis, it was proposed to use the function РН = T/1000 [СН – log(τ) + log(ε)] as the Hollomon parameter, and the Hollomon constant of CH should be found by numerical methods. For these conditions, the equation D = –21,317 – 0,034T + 0,0032log(τ) T – 0,0032log(ε)T was obtained. The accuracy of both descriptions, defined as the sum of deviations squares of the measured grain sizes from calculated, is equal to ~3,3 μm or (when normalized to an average value) ~20 %.

Precise Flow Stress Analysis for the Occurrence of Dynamic Ferritic Transformation and Dynamic Recrystallization of Austenite in Low Carbon Steel

There have been previous attempts to observe the occurrence of dynamic ferritic transformation at temperatures even above Ae3 in a low-carbon steel, and not only in steels, but recently also in titanium alloys. In this study, a new approach is proposed that involves treating true stress-true strain curves in uniaxial compression tests at various temperatures, and different strain rates in 0.1C-6Ni steel, which is a model alloy used to decelerate the kinetics of ferrite transformation from austenite. The initial flow stress up to peak stress was used to analyze the change in dynamic softening phenomena, such as dynamic recovery, dynamic recrystallization, and dynamic transformation. It is worth mentioning that for predicting the occurrence of dynamic transformation, flow stress before reaching peak stress is much more sensitive to the change in the dynamic softening rate due to dynamic transformation, compared to peak stress. It was found that the occurrence of dynamic ferritic transformation could be successfully obtained even at temperatures above Ae3 once the deformation condition was satisfied. This deformation condition is a function of both the strain rate and the deformation temperature, which can be described as the Zener - Hollomon parameter. In addition, the driving force of dynamic ferritic transformation might be much less than that of the dynamic recrystallization of austenite at a given deformation condition. By applying this technique, it is possible to predict the occurrence of dynamic transformation more sensitively compared with the previous analysis method using peak stress during deformation. Key words: low-carbon steel, hot deformation, dynamic recrystallization, dynamic transformation, flow stress analysis

Physically-Based Modeling and Characterization of Hot Flow Behavior in an Interphase-Precipitated Ti-Mo Microalloyed Steel

In this contribution, a series of hot compression tests was conducted on a typical interphase-precipitated Ti-Mo steel at relatively higher strain rates of 0.1~10 s−1 and temperatures of 900~1150 °C using a Gleeble-2000 thermo-mechanical simulator. A combination of Bergstrom and Kolmogorov–Johnson–Mehl–Avrami models was first used to accurately predict the whole flow behaviors of Ti-Mo steel involving dynamic recrystallization, under various hot deformation conditions. By comparing the characteristic stresses and material parameters, especially at the higher strain rates studied, the dependence of hot flow behavior on strain rate and deformation temperature was further clarified. The hardening parameter U and peak density ρp exhibited an approximately positive linear relationship with the Zener–Hollomon (Z) parameter, while the softening parameter Ω dropped with increasing Z value. The Avrami exponent nA varied between 1.2 and 2.1 with lnZ, implying two diverse nucleation mechanisms of dynamic recrystallization. The experimental verification was performed as well based on the microstructural evolution and mechanism analysis upon straining. The proposed constitutive models may provide a powerful tool for optimizing the hot working processes of high performance Ti-Mo microalloyed steels with interphase precipitation.

Theoretical Conversions of Different Hardness and Tensile Strength for Ductile Materials Based on Stress–Strain Curves

Based on the power-law stress–strain relation and equivalent energy principle, theoretical equations for converting between Brinell hardness (HB), Rockwell hardness (HR), and Vickers hardness (HV) were established. Combining the pre-existing relation between the tensile strength (σ b ) and Hollomon parameters (K, N), theoretical conversions between hardness (HB/HR/HV) and tensile strength (σ b ) were obtained as well. In addition, to confirm the pre-existing σ b -(K, N) relation, a large number of uniaxial tensile tests were conducted in various ductile materials. Finally, to verify the theoretical conversions, plenty of statistical data listed in ASTM and ISO standards were adopted to test the robustness of the converting equations with various hardness and tensile strength. The results show that both hardness conversions and hardness-strength conversions calculated from the theoretical equations accord well with the standard data.

Microstructure and Vickers-hardness of 20CrMnTiH steel during hot compression testing

ABSTRACTThe isothermal hot compression tests of 20CrMnTiH steel were carried out by using Gleeble-3500 thermo-simulation-machine at deformation temperatures ranging from 1123 to 1423 K and at strain rates ranging from 0.01 to 1 s−1. The microstructural characteristics after hot compression under various deformation conditions were described with optical microscope and EBSD method, meanwhile the Vickers-hardness (HV) corresponding to the samples was measured. The strain-induced fine and uniform recrystallised grains instead of the original coarse microstructure and higher hardness were obtained with increasing strain rate and decreasing deformation temperature. The average misorientation angle got increased and an intense α-fibre texture was dominated with increasing temperature, and then the angle decreased when the temperature increase to 1423 K. By introducing Zener–Hollomon (Z) parameter, the relationship of recrystallised grain size (D) and HV under different Z values were generated. The relationship of 20CrMnTiH steel recrystallised grain size (D) and HV was formulated based on the analysis of the experimental data, which is in agreement with Hall–Petch relationship.

Identification of the Weakest Metallurgical Zone on Fracture Behavior of an Undermatched Welded Joint

In the particular case of welded structures, the tearing resistance is strongly dependent on the mismatch of welded joint. In this context, the fracture behavior of an undermatched electron beam (EB) welded joint on thick plate of aluminum alloy 6061-T6 used for structural components of experimental nuclear reactors was investigated through experimental and numerical approaches. The local constitutive behavior of the various zones of the welded joint is characterized by means of a new measurement prototype and the corresponding Hollomon parameters are determined. Toughness tests are then carried out on Compact Tension specimens for different configurations of initial crack. The transferability of the identified fracture toughness at initiation of each metallurgical zone is studied through J1c tests on Single Edge Notched Tension specimens. For a reliable interpretation of toughness tests, Finite Element analysis is performed to account for the multimaterial effect not considered in a standard analysis. From these experimental and numerical results, a contrast in tensile and fracture behavior is highlighted which makes difficult the identification of the weakest metallurgical zone of the welded joint on fracture behavior.

Effect of Heat Treatment Conditions on the High Temperature Deformation of 6082-Al Alloy

High-temperature deformation of an artificially aged 6082-Al alloy was conducted in the present investigation. Tensile tests were carried out at temperatures of 623, 673 and 723 K at various strain rates ranging from 5x10-5 to 2x10-2 s-1. The behavior of the alloy is characterized by high stress exponent, n and high apparent activation energy, Qa that are higher than what is usually observed in Al and Al solid-solution alloys under similar experimental conditions, which implies the presence of threshold stress; this behavior results from dislocation interaction with second phase particles. The threshold stress, σo values were seen to decrease exponentially with temperature. By incorporating the threshold stress in the analysis, the true activation energy, Qt was calculated to be close to that of dislocation pipe diffusion in Al. Analysis of the experimental data of the alloy in terms of the Zener- Hollomon parameter vs. normalized effective stress, revealed a single type of deformation behavior with an n value of ~7. Measurements showed that the values of elongation percent at failure increase with strain rate and temperature.

Microstructural evolution of AZ61 magnesium alloy during hot deformation

The microstructural evolution of AZ61 magnesium alloy during hot compression at various temperatures was investigated. The experimental results show that dynamic recrystallisation occurs over a wide temperature range. Grains can be greatly refined through dynamic recrystallisation. The mean size of the recrystallised grains increases with a decrease of temperature or value of Z (Zener – Hollomon parameter), while the reciprocal of the recrystallised grain size has a good linear relationship with the natural logarithm of the Z value, as well as the hyperbolic term of the flow stress. Basal and non-basal segments have been found in both recrystallised grains and primary grains, whereas dislocation pileups exist only in recrystallised grains when the temperature is lower than 673 K. The occurrence of twins is dependent on temperature and strain. When the strain increases, primary twins evolve into secondary twins. However, secondary twins grow with an increase of temperature; some secondary twins evolve into subgrains.

Comparison of torsion and compression constitutive analyses for elevated temperature deformation of Al–Li–Cu–Mn alloy

Experimental alloy 2090 (Al – 2.1Li – 2.6Cu – 0.4Mn) was deformed in torsion and compression at temperatures T=300, 400, and 500°C and strain rates ε̇=10–2 to 10 s–1. The torsion and compression results were analysed using equation A(sinh ασ)n=Z=ε̇ exp(QHW/RT), where Z is the Zener – Hollomon parameter. The variation of stress exponent n, Arrhenius slope s, and activation energy QHW were calculated with variation of the stress multiplier a from 0.01 to 0.08 MPa–1. The change of α caused the stress exponent to decrease at a declining rate and Arrhenius slope s to increase linearly, thus causing the activation energy QHW to become stable above α=0.04 MPa–1. The two experimental test techniques gave very similar flow stresses and constitutive constants. At α=0.052 MPa–1 activation energy values showed reasonable consistency with other age hardenable aluminum alloys. The compression tests were also analysed using the power law expression ε̇ exp(Q′HW/RT)=Z=A″σn′.

Tensile stress-strain analysis of cold worked metals and steels and dual-phase steels

A study has been made of the applicability of the differential Crussard-Jaoul (C-J) analysis that assumes the Ludwik power relation, the modified C-J analysis based on the Swift formula, and the Hollomon analysis to uniaxially prestrained metals and steels and high strength, formable, dual-phase steels. The pure aluminum and copper metals and a series of plain carbon steels with carbon ranging from 0.10 to 1.05 pct were uniaxially prestrained by a given amount of strain under ambient temperature. A plain carbon steel with carbon of 0.10 pct was utilized in manufacturing the dual-phase steels. An empirical analysis exhibited the limited applicability of the C-J analysis for the interpretation of the stress-strain relationship of uniaxially prestrained metals and steels. The C-J analysis was also less sensitive to changes in the deformation behavior of the dual-phase steels in which the ferrite matrix and the shape and distribution of the second phase martensite were altered by three heat treatments. The modified C-J analysis was most suitable for describing work-hardening of uniaxially prestrained metals and steels. This analysis revealed that the dual-phase steels deformed in two stages. The first stage was associated with deformation of the ferrite matrix, and the second stage was associated with uniform straining of ferrite and martensite. The more generally used Hollomon curves deviated from linearity over all the uniform strain range regardless of the uniaxially prestrained metals and steels and dual-phase steels. Thus, the Hollomon parameters could not be assigned to an entire curve.