Yield Drop Phenomenon Research Articles

Dislocation density-based modeling of the yield drop phenomenon in nickel-based single crystal superalloy

The phenomenon of yield drop, characterized by a decrease in flow stress after initial yield, has been observed in various nickel-based superalloys. Despite numerous proposed physical mechanisms, there is still a lack of a meso-mechanism-based constitutive model to explain this phenomenon. In this study, the tensile behavior of a nickel-based single crystal superalloy (DDX), was investigated at different strain rates and a temperature of 900 °C. It was observed that the yield drop phenomenon in DDX became more pronounced with increasing strain rate. To predict the yield drop phenomenon during tensile processing, an improved strength law based on continuum dislocation density theory was considered in the crystal plasticity framework. The proposed constitutive model was implemented using nonlinear iteration and incorporated into a finite element analysis software. The simulation results exhibited a good agreement between the experimental data and the stress–strain curve in the vicinity of the yield drop region, affirming the predictive aptitude of the proposed model in elucidating the yield drop phenomenon at various strain rates.

The modified relaxation plasticity model and the non-monotonic stress–strain diagram

The constitutive models at high loading rates are usually investigated by introducing additional and empirical rate-dependent components into classical models initially developed for quasi-static cases. In addition, the dynamic stress–strain curves of many materials are non-monotonic. The yield drop phenomenon can be observed in series of experiments and often neglected in the simulation of stress–strain diagrams. The structural-temporal approach proposed by Petrov and Morozov provides a new viewpoint for understanding the dynamic response of materials. They indicate that in the limit case of low-rate loading it transfers into the classical quasi-static models by neglecting certain insignificant components. This work aims to derive a constitutive model from the structural-temporal approach rather than using a direct empirical fitting approach. In this paper, the incremental version of the relaxation plasticity model (IRP model) is introduced within the framework of the structural-temporal approach. Possible model scenarios are discussed and compared with the well-known Johnson–Cook model. Theoretical results under uniaxial loading conditions are obtained and compared with experimental data for the aluminium alloy 6082-T6, API 5L X70 steel, titanium alloy Ti–6Al–4V, and aluminium alloy 2519A. It is shown that the IRP model describes the non-monotonic behaviour of the stress–strain relationship and performs better than the Johnson–Cook model for mentioned materials.

Effects of processing parameters on fabrication defects, microstructure and mechanical properties of additive manufactured Mg–Nd–Zn–Zr alloy by selective laser melting process

Mg–3Nd–0.2Zn–0.4Zr (NZ30K, wt.%) alloy is a new kind of high-performance metallic biomaterial. The combination of the NZ30K Magnesium (Mg) alloy and selective laser melting (SLM) process seems to be an ideal solution to produce porous Mg degradable implants. However, the microstructure evolution and mechanical properties of the SLMed NZ30K Mg alloy were not yet studied systematically. Therefore, the fabrication defects, microstructure, and mechanical properties of the SLMed NZ30K alloy under different processing parameters were investigated. The results show that there are two types of fabrication defects in the SLMed NZ30K alloy, gas pores and unfused defects. With the increase of the laser energy density, the porosity sharply decreases to the minimum first and then slightly increases. The minimum porosity is 0.49 ± 0.18%. While the microstructure varies from the large grains with lamellar structure inside under low laser energy density, to the large grains with lamellar structure inside & the equiaxed grains & the columnar grains under middle laser energy density, and further to the fine equiaxed grains & the columnar grains under high laser energy density. The lamellar structure in the large grain is a newly observed microstructure for the NZ30K Mg alloy. Higher laser energy density leads to finer grains, which enhance all the yield strength (YS), ultimate tensile strength (UTS) and elongation, and the best comprehensive mechanical properties obtained are YS of 266 ± 2.1 MPa, UTS of 296 ± 5.2 MPa, with an elongation of 4.9 ± 0.68%. The SLMed NZ30K Mg alloy with a bimodal-grained structure consisting of fine equiaxed grains and coarser columnar grains has better elongation and a yield drop phenomenon.

Enhancing work hardening and ductility in additively manufactured β Ti: roles played by grain orientation, morphology and substructure

A metastable β Ti alloy was additively manufactured by laser powder bed fusion (LPBF). Tensile testing along the build direction of the as-LPBF material (LPBF-0°) revealed significant work softening immediately following yielding with no uniform deformation. By contrast, substantial work hardening and uniform elongation well over 10% were achieved perpendicular to the build direction (LPBF-90°). Similar effects were obtained in the build direction after super transus heat treatment (LPBF-0°+HT) although the strength was slightly lowered. In addition, the yield drop phenomenon observed in both orientations of the as-LPBF materials disappeared after HT. The enhanced work hardening ability, and thus ductility, can be attributed to increased interactions of slip bands/slip bands owing to additional {112}<111> slip systems becoming operative in LPBF-0°+HT and LPBF-90° while LPBF-0° was dominated by {110}<111> only. The other variations after HT may be related to the coarsening of grain structure and removal of specific substructures in the as-LPBF microstructure.

Simultaneous enhancement of strength and ductility by aging treatment in fine-grained Mg-13Gd alloy

Ideal structural materials should have both high strength and good ductility. Aging treatment is an effective way to increase the strength of Mg alloys, but usually sacrifices their ductility. Here we have successfully achieved the synergetic enhancement of strength and ductility of the extruded Mg-13Gd alloy via aging treatment, and obtained a Mg alloy with a tensile yield strength of 400 MPa and an elongation of 15%. The synergetic improvement of strength and ductility is mainly attributed to the formation of high-density nanoscale β′ precipitates and a certain proportion of precipitation free zone in the microstructure dominated by dynamically recrystallized fine grains. The yield drop phenomenon in the tensile curves is sensitive to the existence of movable pinned-dislocations within grains. It is expected that the discussion in this study will be helpful to developing Mg alloys with high strength and ductility via aging treatment.

Heterogeneity and Homogeneity in 2/4 N Multilayered Al Fabricated by Accumulative Roll Bonding and Annealing

Multilayered Al sheets having alternative stacking of 2 N Al (99.21 wt.% purity, N is short for nine) and 4 N Al (99.993 wt.% purity, N is short for nine) with various layer thicknesses from 16 to 1 μm were fabricated by 6 to 10 accumulative roll bonding (ARB) cycles, followed by annealing. The effects of layer thickness and annealing treatment on mechanical properties were studied. In all samples, the grain sizes in the 4 N layers were larger than those in the 2 N layers due to the purity difference. The grain size and mechanical properties saturated after 6 ARB cycles, but the layer thickness gradually decreases with increasing cycles and accordingly, more homogeneous microstructures were obtained with higher cycles. After annealing, heterogeneous structures were retained in 6-cycle sample, while most homogenous structures were observed in 10-cycle sample. Yield-drop phenomenon widely observed in the homogeneous samples disappeared in the heterogeneous 6-cycle sample after annealing at 225 and 250 °C.

Microstructural and mechanical behavior of a CoCrFeNiCu4 non-equiatomic high entropy alloy

High entropy alloy (HEA)-based alloy design is experiencing a conceptual broadening from equiatomic alloys to non-equiatomic alloys. To provide experimental basis for designing Cu-rich non-equiatomic HEAs, in the current study, a dual phase (Cu-rich and CoCrFeNi-rich phases) face-centered cubic CoCrFeNiCu4 alloy was systematically investigated. We provided initial and experiment-based understanding of the behavioral change of the alloy during a variety of thermal cycles and thermomechanical processing. The current results indicate that, during heating, preferred precipitation of Cu-rich particles occurs, leading to more pronounced compositional differences between the two constituent FCC phases and increased relative volume fraction of the Cu-rich phase. The Alloy exhibits a continuous melting and discontinuous solidification of the Cu-rich and CoCrFeNi-rich phases. After being cold-rolled to ∼ 90 % thickness reduction, the alloy exhibits a recrystallization temperature higher than 800 °C. Annealing at 300 and 500 °C led to strength reduction and/or ductility decrease; further increasing annealing temperature monotonically caused softening and ductilization due to decreased density of pre-existing dislocations. The yield-drop phenomena observed for the 900 °C- and 1000 °C-annealed specimens are associated with the locking of pre-existing dislocations by some “atmosphere”, the nature of which warrants further elucidation.

Microstructure evolution and underlying mechanisms during the hot deformation of 718Plus superalloy

718Plus superalloy was investigated to obtain a detailed and fundamental understanding of the microstructure evolution and the underlying mechanisms. Isothermal compression tests were conducted, and different strains were utilized to trace the whole deformation process at the selected deformation conditions. The results showed that no yield drop phenomenon (YDP) occurred at all deformation conditions involved in this research except a very slight one at 1020 °C with the strain rate of 0.1 s−1 The heterogeneity of initial microstructure was responsible for the absence or weakening of YDP. Deformation conditions and the Taylor factor differences between adjacent grains were found both strongly affected the distribution of dislocation density and local misorientation, as well as the formation of low angle grain boundaries (LAGBs), which finally influenced the dynamic recrystallization (DRX) behaviors and the underlying mechanisms. Moreover, discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) behaviors as well as the underlying mechanisms were discussed in detail. Subgrain rotation would also be observed during the DDRX process, which may lead to a misjudgment of CDRX if only focused on misorientation gradient. Hence, based on the morphology examination, misorientation gradient and misorientation angle evolution, progressive subgrain rotation labeled as CDRX was confirmed to be an important assistant nucleation mechanism during the deformation of the studied 718Plus superalloy at both sub-solvus and super-solvus temperatures.

Operation of solute-drag creep in an AlCoCrFeMnNi high-entropy alloy and enhanced hot workability

Strong evidence for creep controlled by solute-drag processes, such as the yield-drop phenomenon, a stress exponent of 3 and a sluggish tendency to form subgrains, was found during hot compressive deformation of a cast BCC AlCoCrFeMnNi high-entropy alloy with coarse grains; these characteristics are typically observed in conventional metals that exhibit solute-drag creep. These three features were not observed in the FCC CoCrFeMnNi alloy with coarse grains, indicating that the addition of aluminum causes solute-drag creep to arise as the rate-controlling deformation mechanism. Compared with the FCC Al0.5CoCrFeMnNi matrix (with a small amount of BCC phase), the AlCoCrFeMnNi exhibited more distinct characteristics of solute-drag creep. This is likely because the Al concentration in the matrix of AlCoCrFeMnNi is notably higher than that in the matrix of Al0.5CoCrFeMnNi (17.8 vs. 6.5 at.%). Also, the solute-drag creep extended to an increased strain rate and a decreased temperature in AlCoCrFeMnNi because the critical stress for dislocations to break away from the solute atmospheres increases as the Al solute concentration in the matrix increases. This outcome improves hot workability of AlCoCrFeMnNi according to the processing maps because the power dissipation efficiency increases and the flow instability regime shrinks at high strain rates and low temperatures.

Dynamic deformation of fiber-metal laminates depending on its metal thickness

Applying peculiarities of irreversible deformation of fiber-metal laminates, a new analytical model is proposed. Incorporating the temporal parameters of a fiber and metal matrix enables the prediction of different deformation dependencies of glass- and carbon- reinforced fiber metal laminates depending on the metal thicknesses and strain rates. The dynamic effect of the yield drop phenomenon of glass- fiber metal laminates based on the relaxation model of plasticity for a heterogeneous material is plotted. The proposed model is capable of effectively predicting fiber metal laminates behaviour under both quasi-static and dynamic loading.

A study on anomalous yield drop behaviour in modified beta titanium alloys

ABSTRACTThe yield drop phenomenon observed in the Ti–15V-3Al–3Sn-3Cr (Ti–15–3) beta-titanium alloy and its anomalous behaviour in the boron and carbon added Ti–15–3 alloys have been studied. While the base and the carbon containing alloys exhibit yield drop, the boron containing alloy with smaller grain size than base alloy does not appear to show this phenomenon. Tensile tests were interrupted at different stress levels followed by analyses of slip lines and sub-structural characteristics using scanning and transmission electron microscopes to understand this anomalous yield point phenomenon. Infrared thermal imaging technique was used to map the strain localisation and the spatiotemporal evolution of deformation along the gauge length of the specimens during the tensile tests. Deformation in these alloys initiates only in a few grains. Pile-up of dislocations in these grains subsequently triggers the formation of dislocations in other grains and their rapid multiplications. The spreading of deformation by the generation of dislocations from pile up dislocations in one grain to neighbouring un-deformed grains and their rapid multiplication to new regions influence the yield drop phenomenon and its characteristics. It is shown in this study that microscopic instability in the grain level is a necessary, but not the sufficient condition for the manifestation of macroscopic instability during tensile deformation in polycrystalline materials. The presence of boride particles at grain boundaries restricts the slip transfer across the grains as well as the spreading of deformation to new regions, which causes the suppression of yield drop in the boron containing alloy.

Impact of the initial microstructure and the loading conditions on the deformation behavior of the Ti17 titanium alloy

In this work, the impact of the microstructure and the loading conditions on the mechanical behavior of a β-rich Ti17 titanium alloy is investigated. For this purpose, two different initial microstructures are considered : (i) a two-phase lamellar α + β microstructure and (ii) a single-phase equiaxed β-treated microstructure. First, compression tests are performed at different strain rates (from $$10^{-1}$$ to 10 s−1) and different temperatures (from 25 to $$900\,^\circ $$C) for both microstructures. Then, optical microscopy, scanning electron microscopy, EBSD and X-ray diffraction analyses of deformed specimens are carried out. Whatever the loading conditions are, the flow stress of the as-received α + β Ti17 is higher than that of the β-treated Ti17. Also, because of a higher strain-rate sensitivity, the β-treated Ti17 is less prone to shear banding. At low temperatures (i.e., $$T \le 450\,^\circ $$C), the deformation behavior of both the as-received α + β and the β-treated Ti17 is controlled by strain hardening. For the β-treated Ti17 alloy, martensitic transformation is systematically detected in this temperature range. The softening behavior of the as-received α + β Ti17 observed at high temperatures is due to the joint effect of dynamic recrystallization, dynamic transformation, adiabatic heating and morphological texture evolution. For the β-treated Ti17 alloy, when the temperature exceeds $$700\,^\circ $$C, stress–strain curves display a yield drop phenomenon, which is explained by dynamic recrystallization.

Interaction of carbon with microstructural defects in a W-Re matrix: An ab initio assessment

The interaction of carbon atoms with point defects and the core of edge and screw dislocations with Burgers vector a0/2⟨111⟩ in W and a W-Re matrix is studied by means of ab initio calculations. The structure and energetics of the ground-state atomic configurations are presented and rationalized. It is found that di-vacancies, which are thermally unstable in pure W according to the state-of-the-art ab initio calculations, can nucleate at C and Re-C complexes, which fill the gap in the explanation of the emergence of nanovoids observed experimentally under irradiation. Also, on the basis of the recent experimental evidence and our calculations, the temperature ranges for the manifestation of the yield drop phenomenon, which is related to the obstruction of dislocation motion due to their decoration by impurities such as carbon, are revealed.

Pronounced yield drop phenomenon at high temperatures in Al-Mg alloys with high contents of Mg (5–13 wt%)

The high-temperature yield drop phenomenon in Al-Mg alloys with high contents of Mg (5–13 wt%) was examined in a wide range of strain rates under compression. Yield drop was highly pronounced when the added amount of Mg was beyond 10 wt%. For example, at 1 s−1 at 698 K, the yield drop ratio between the upper yield stress and the lower yield stress was as high as 1.43 in the Al-13Mg alloy, while the yield drop ratios for the Al-5Mg, Al-7Mg and Al-10Mg alloys were 1, 1.1 and 1.1, respectively. Analysis of the deformation mechanism associated with the yield drop phenomenon suggests that dynamic deformation model based on a sudden increase in the dislocation density during initial straining is more suitable in explaining the observed yield drop behavior than the static theory based on the critical stress for the breakaway of dislocations from the solute atmosphere.

Hydrogen-enhanced orientation dependence of stress relaxation and strain-aging in Hadfield steel single crystals

The effects of electrochemical hydrogen charging on the stress relaxation and strain-aging of 〈111〉- and 〈001〉-oriented single crystals of Hadfield steel were studied under tension at room temperature. An orientation dependence of stress relaxation and strain-aging-assisted yield-drop phenomena was observed for hydrogen-free 〈111〉- and 〈001〉- oriented single crystals. Hydrogen charging up to five hours increased the stress relaxation rate for slip-associated deformation in 〈001〉-oriented single crystals and decreased it for twinning-assisted deformation in 〈111〉-oriented specimens. The present results demonstrate that different mechanisms dominate the interaction of hydrogen with dislocations, stacking faults and twins.

Cyclic and Biaxial Deformation Behavior for Bake-hardenable Steel Sheet

Abstract It is well known that bake hardening treatment on the bake-hardenable steel sheets leads to the increase of yield strength and appearance of yield point phenomena under uniaxial tension deformation. On the other hand, strain paths during forming process and the subsequent dent resistance are usually not monotonic nor uniaxial, so that more investigation under complex strain path and stress state are essential in order to establish its constitutive model and achieve an accurate prediction of dent resistance by numerical simulation. In the present study, the effects of bake hardening treatment on various pre-strain paths and secondary deformations after bake hardening treatment were observed. From the obtained results, the yield drop phenomena did not appear and the Bauschinger effect was observed in cyclic deformation with bake hardening treatment. On the other hand, the yield drop and the subsequent yield plateau followed by workhardening was observed in equi-biaxial tension test as similar to the result of conventional uniaxial tension test result with bake hardening treatment.

High-temperature flow characterization and microstructural evolution of Ti6242 alloy: Yield drop phenomenon

The hot deformation behavior of Ti6242 alloy holding a duplex microstructure (primary α+transformed β) was investigated. This was performed through hot compressing the experimental material at temperatures in the range of 1000°C to 1100°C, where the alloy was in its single-phase β region. The hot deformations were conducted under different strain rates of 0.003, 0.03 and 0.3s−1 up to the true strain of 0.7. The microstructural examinations reveal newly recrystallized grains, which were formed in the vicinity of initial β grain boundaries under all testing conditions. A yield drop phenomenon is observed during straining at the strain rates of 0.003 and 0.03s−1. However, a continuous yielding at early stages of deformation is characterized for the case of 0.3s−1. In the former condition, the yield drop phenomenon is considered to mask the typical flow softening behavior, which is commonly attributed to the occurrence of dynamic recrystallization (DRX). A numerical approach was employed to determine the effective alloying element in dislocation pinning and yield drop. The results indicate that the hot deformation activation energy at the upper yield points varies between 222 and 301kJ/mol. The activation energy at lower strain rates is found to be equal to that of Al diffusion in β titanium. The atom distribution maps reveal that the aluminum atoms in the form of atom clusters play an important role in yield drop phenomenon.

The Effect Of Grain Size On The Mechanical Properties Of Aluminum

Abstract Although many studies have focused on the unique plastic deformation behavior of nanocrystalline aluminum (e.g., the positive deviation from Hall-Petch relation, the unusual yield-drop phenomenon in tensile mode, etc.), the data reported by different research groups are inconsistent with each other, possibly because of different fabrication processes. In this study, aluminum samples with a wide grain-size spectrum – from a few micrometers down to 100 nanometers – are manufactured by powder metallurgy. The grain size was measured by X-ray diffraction analysis and transmission electron microscope observation. Furthermore, the tensile behavior, which varied according to a grain size, is discussed with a comparison of the theoretical models.

Tensile yield behavior of Fe–30Mn–3Al–3Si twining-induced plasticity steel

The tensile yield behavior of Fe–30Mn–3Al–3Si twining-induced plasticity steel was investigated in this paper. Only the annealed samples with medium grain sizes ranging from 1.6 to 22μm exhibited a yield-drop phenomenon. The reason for the occurrence of a yield-drop phenomenon is the reduction of dislocation density during tensile deformation.

Grain Size Dependence of Tensile Properties in Cu-Sn Thin Foils (Experimental Study)

This study was carried out to investigate the effects of grain size on mechanical properties in Cu-Sn foil with a thickness of 30 um. The grain size was varied from approximately 7 um to 50 um using heat treatment at 773 K for 2 h to 24 h in a vacuum atmosphere. Tensile test was carried out at room temperature with strain rate of 1mm/min. Typical yield drop phenomenon was observed. Mechanical properties were found to be strongly affected by microstructural features including grain size. The yield strength and tensile strength gradually decreased with increasing the grain size. The strain to fracture also decreased by grain growth. These results could be explained by not only the grain size dependence of yield strength but also the ratio of thickness to grain size dependence of yield strength.