Deformation Mechanism Evolution Research Articles

Temperature-dependent deformation mechanisms of γ′ phases in a newly developed NiCoCr-based superalloy

The γ′-strengthened NiCoCr-based superalloys are extensively used in aerospace, energy, and chemical industries. This work focuses on tensile properties and evolution of deformation mechanism in a newly developed NiCoCr-based superalloy, designated K439B, at temperatures ranging from 25 °C to 1000 °C. The results demonstrate that the deformation mechanisms of this alloy are temperature-dependent. Slip bands and strongly-coupled dislocation pairs shear γ′ precipitates at 25 °C, resulting in high yield strength and work hardening rate. At 600 °C and 700 °C, the Lomer-Cottrell (L-C) locks are observed, and stacking faults shearing γ′ precipitates become the primary deformation mechanism. At temperatures reaching 800 °C, the yield strength exhibits an anomalous increase originating from the formation of Kear-Wilsdorf (K-W) locks. When the temperature exceeds 800 °C, the primary deformation mechanism is transformed into dislocations bypassing γ′ through the Orowan mechanism. The present study elucidates the deformation mechanism of this novel designed superalloy, thereby furnishing a theoretical foundation for the further development of the alloy system.

Evolution of microstructure and deformation mechanisms in a metastable Fe42Mn28Co10Cr15Si5 high entropy alloy: A combined in-situ synchrotron X-ray diffraction and EBSD analysis

In this work, a combination of in-situ high synchrotron X-ray diffraction and electron backscattered diffraction were used to systematically investigate the activation and evolution of the deformation mechanisms in an as-cast Fe42Mn28Co10Cr15Si5 metastable high entropy alloy deformed until fracture at room temperature. This work unveils the critical role of the dual-phase γ-f.c.c. / ε-h.c.p. microstructure on the deformation response of the alloy. The different deformation modes, i.e., slip, transformation induced plasticity (TRIP) and transformation induced twinning (TWIP), were seen to initiate at different loading stresses and then to overlap. Quantitative microstructural characterization, which included the evolution of the phase fraction, stress partitioning, dislocation density, c/a ratio and lattice strain for different planes, was performed to elucidate the role of each phase on the macroscopic mechanical response of the metastable high entropy alloy. Furthermore, the magnitude of the different strengthening contributions has been quantified for the first time.

Effect of phase content on mechanical properties and deformation mechanism in TC21 titanium alloy: An in-situ tensile investigation

This work fabricated a series of microstructures consisting of different equiaxed α (αeq) and β phase contents to investigated the effect of phase content on mechanical properties and deformation mechanism, including slip initiation, slip transfer, stress-induced martensite (SIM) and crack nucleation of TC21 titanium alloy. The results show that, with the decrease of αeq phase content, the YS of the alloy firstly decreases and then increases, the UTS gradually increases, and the EL gradually decreases. The significant variation of mechanical properties is closely related to the deformation mechanism of microstructures with different αeq phase contents. In sample with high αeq phase content, αeq phase is deformed by slip prior to β phase, and with the increase of strain, β phase can coordinate deformation by slip initiation and exhibits a better ability to coordinate plastic deformation. In samples with medium αeq phase content, the stability of β phase decreases, slip in αeq grain and SIM in β grain can be initiated simultaneously. However, the formation of SIM would harden β phase, and with the increase of strain, it would restrain the ability of β phase to further coordinate plastic deformation. In samples with low αeq phase content, the stability of β phase decreases sharply, which could induce the precipitation of fine-lamellar secondary α (αs) and acicular martensite α'' in β grain during annealing. These precipitated αs and α'' can not only inhibit the formation of SIM, but also dramatically reduce the ability of β phase to coordinate plastic deformation. Meanwhile, the evolution of deformation mechanism in different samples can significantly influence the crack nucleation location (slip band or αeq/β interface) during deformation. This paper analyzes in detail the effect of αeq phase content on mechanical properties of TC21 titanium alloy based on deformation mechanism, it would contribute to the establishment of a relationship among phase content, deformation mechanism and mechanical properties.

Quantitative investigation on the {10–12} twinning-detwinning behavior and twinning transfer of an extruded Mg-2.8Y sheet during compression-tension loading via quasi-in-situ EBSD observation

Abundant {10–12} twins appeared in an extruded Mg-2.8Y (wt. %) sheet with rare-earth (RE) texture under compression along the transverse direction (TD). Twinning-detwinning behavior during compression-tension loading was quantitatively investigated via quasi-in-situ EBSD observation. The evolution of deformation mechanisms was quantitatively studied by calculation of the plastic strain fraction of twinning, detwinning, and dislocation slip. Twinning and detwinning were the dominant deformation modes in compression and reverse tension, respectively, owing to their relatively higher Schmid factors (SF) and lowest activation stresses (τ). Twin variants with SF ranking 1st and 2nd were activated preferentially while detwinning with low SF ranking showed high activation. The impact of texture and load path on variant selection was systematically discussed. Moreover, short twin chains only passing through two or three grains caused by twinning transfer were observed. Grain boundary misorientation angle (GBMA) and geometric compatibility factor (m’) were introduced to analyze the crystallographic characteristics of twinning transfer. GBMA ≤34.5° or m’ ≥ 0.80 is a necessary condition for most twinning transfer events. RE texture as well as the nearly uniform distribution of GBMA gave rise to short twin chains and restricted the formation of long twin bands. The relationship between GBMA and m’ was also discussed.

Effects of bimodal basal texture on bending stress and microstructure evolution of Mg−2.6Nd−0.55Zn−0.5Zr alloys

AbstractThe extrusion direction (ED) and transverse direction (TD) samples were cut from Mg−2.6Nd− 0.55Zn−0.5Zr alloy sheet, and the effects of bimodal basal texture on the bending stress and microstructure evolution were investigated. Results showed that the ED sample with weak basal texture possessed much lower bending stress than the TD sample. This anisotropy is attributed to the role of different textures on the evolution of deformation mechanisms and microstructures. For the ED sample with weak basal texture, the dominant deformation mechanisms were basal slip in the majority with minor supplement of tension twin in the inner region while basal slip and prismatic slip in the outer region. In comparison, the TD sample with strong basal texture activated more tension twin in the inner region and more prismatic slip in the outer region. The activation of tension twin promoted the development of 〈0002〉//compression direction type texture with the confined ±30° angle range in the inner region and reduced the emergence propensity of high dislocation density and low angle grain boundaries. The massive activation of tension twin allowed the TD sample to maintain a high strain-hardening capacity, which was the fundamental reason for the high bending stress in this sample.

Microstructure and high-temperature tensile behavior of modified H13 steel with the addition of tungsten, molybdenum, and lowering of chromium

H13 can hardly meet the requirements of advanced manufacturing industries due to its insufficient strength at elevated temperatures. In the present work, modified CXN02 and CXN03 based on H13 with more molybdenum, tungsten content, and less chromium content were prepared. The yield strength of CXN03 is 1714 MPa at 25 °C, which is 106% of CXN02 and ∼111% of H13. With the test temperature increase, the yield strength of all materials decreased. As the test temperature raised to 650 °C, the yield strengths of CXN02 and CXN03 became similar to about 700 MPa, which is about 64% higher than that of H13. As-tempered CXN03 has a finer average grain size, higher dislocation density, and larger volume fraction of precipitations than as-tempered CXN02. The additional W and Mo in CXN03 allowed more M6C to present in as-annealed CXN03, thus more undissolved carbides that could pin prior austenite grains exist in as-quenched CXN03, refining the grains. The yield strength at room temperature was correlated with microstructure by a mathematical model. Calculations revealed that the higher strength of CXN03 is mainly attributed to the higher dislocation density and finer grain size. The brittle-to-ductile transition temperature is around 400 °C for both materials, and the deformation mechanism evolution with the temperature increase from 25 to 650 °C is briefly discussed.

Evolution and control of deformation mechanisms in micro-grooving of Zr-based metallic glass

The evolution of deformation mechanisms with increasing depth of cut (DOC) in micro-grooving of Zr-based metallic glass was deeply investigated in this study for the first time. The micro-grooving characteristics of metallic glass were initially studied by nanoscratch test, which verified that the material removal mode abruptly changes from extrusion mode to cutting mode with increase of DOC. Based on the free-volume theory, a dynamic model coupling stress, temperature and free volume concentration was used to simulate the change of deformation characteristics with DOC. The results showed that the deformation mechanism of metallic glass is closely related to free volume flow, which is significantly affected by average strain rate and free volume flow coefficient in deformation zone and can be tuned with different DOC. Considering that free volume flow is determined by its flow coefficient and concentration, we proposed to control the deformation characteristics of metallic glass in micro-grooving by heat treatment and verified its effectiveness experimentally. • Materials removal mode of metallic glass changes with the increase of depth of cut. • The evolution of deformation mechanisms was studied using a dynamic model. • Free volume flow affects the evolution of deformation mechanisms significantly. • Deformation characteristics of metallic glass were experimentally modulated.

Effects of temperature and strain rate on the tension-compression asymmetric plastic deformation behavior of ME20M magnesium alloy: Experiments and modeling

The tension-compression asymmetry of a rare-earth-containing alloy sheet, ME20M, is experimentally investigated along the rolling direction over a wide strain-rate range from 0.001 to 2500 s−1 and at temperatures from 213 to 488 K. The asymmetry of yield stress in tension and compression decreases with increasing temperature and decreasing strain rate. The strain-rate sensitivity of the tension-compression asymmetry during the plastic deformation stage increases with increasing temperature. It is observed that the compressive strain-hardening rate is much higher than the tensile one due to more {101-2} tension twins occurring in the compressive plastic deformation. A viscoplastic self-consistent (VPSC) model is employed to simulate the tensile and compressive stress-strain responses, deformation texture, and deformation mechanism evolution over wide ranges of strain rates and temperatures. Results indicate that the VPSC model is capable of capturing the macroscopic plastic deformation characteristics of ME20M alloy. Tensile plastic deformation is dominated by slip at strain rates up to 103 s−1, and basal slip and tension twinning are the main deformation modes in compressive plastic deformation. Effects of strain rate and temperature on the main plastic deformation mechanism of ME20M alloy are insignificant within the tested rate and temperature ranges.

Hot Deformation Behavior and Microstructure Evolution of a New Near β Titanium Alloy Reinforced with Trace TiCp

In this study, hot deformation behavior and microstructure evolution of a new near beta titanium alloy reinforced with trace TiCp (2 vol%) are investigated by isothermal compression test. The flow stress curves present various characteristics based on the deformation parameters. The hot deformation activation energy is determined to be 507.1 KJ mol−1, which is lower than that of the matrix alloy. Based on the dynamic material model (DMM), processing map at the strain of 0.6 indicates that flow instability mainly concentrates at 780–820 °C and strain rate higher than 0.22 s−1. Results of microstructure evolution illustrate that flow instability is characterized by the cracking, localized plastic flow, and bending of TiC particles. In the safety region, the morphology and fraction of α and β phases are closely depended on deformation conditions. At α/β field, dynamic recrystallization (DRX) of β grains dominants the deformation mechanism due to the existence of TiC particles by providing nucleation sites. Meanwhile, the DRX degree of β grains is enhanced markedly compared with that of the matrix alloy in β phase field. Finally, deformation mechanism evolution under the given processing conditions is illuminated.

Mechanical properties and deformation mechanisms of Ti-3Al-5Mo-4.5 V alloy with varied β phase stability

Evolution of deformation mechanisms and mechanical properties of Ti-3Al-5Mo-4.5V alloy with different β phase stability have been systematically investigated. β phase stability alteration is achieved through quenching temperature variation from dual α+β field (700°C) to single β field (880°C). Tensile tests at ambient temperature show that apparent yield strength of the alloy experiences an abrupt decrease followed by a significant increase from 700°C to 880°C. Work hardening behavior is characterized by transition from the initial two-regime feature to the three-stage outlook. Concurrently, the maximum working hardening rate drops from 14000MPa to 3000MPa, which is concurrent with the shrinking volume fraction of primary α phase. Detailed discussion about the relationship between deformation mechanisms and β phase stability has been outlined.

Saturated impulse for fully clamped square plates under blast loading

Saturated impulse refers to the critical value, beyond which the deflection of a beam or plate under pulse loading will no longer increase with further applied load. The present paper investigates the saturated impulse of fully clamped square plates subjected to a blast loading that is assumed to be a uniformly distributed Linearly Rising Exponentially Decaying (LRED) pressure pulse. Considering both bending moment and membrane force, the transient dynamic plastic response is predicted by a rigid, perfectly plastic (R-PP) model characterized by travelling plastic hinge lines and time-dependent velocity field. The saturated duration, saturated impulse, saturated deflection, as well as the evolution of deformation mechanism are explored. By comparing three characteristic durations, i.e. the travelling duration of the plastic hinge lines, saturated duration and loading duration, the dynamic responses of a square plate are classified into three types such that a regime map on the loading parameters’ plane is constructed accordingly. The correlation is made between the results obtained from the elastic-plastic numerical simulation and the R-PP theoretical approximation. To facilitate engineering designs, a method of replacing the LRED pressure pulse by an equivalent rectangular pressure pulse is proposed to predict the maximum deflection of fully clamped square plates.

Influence of the Loading Path on the Deformation Mechanisms of Magnesium Alloys

The evolution of deformation mechanisms in randomly textured magnesium alloy during uniaxial and biaxial mechanical tests has been monitored using concurrent application of acoustic emission and neutron diffraction methods. The influence of the loading path on both twinning and dislocation slip is discussed in detail. It is shown that both the twinning and non-basal slip are sensitive to the loading direction.

Identification of the threshold stress and true activation energy for characterizing the deformation mechanisms during hot working

For characterizing the origins of threshold stress and the evolution of deformation mechanisms in hot working region, the hyperbolic-sine constitutive analyses were conducted to determine the threshold stress σ0, the true stress exponent nt and the true activation energy Qt in AA7050 Al alloys. The origins of σ0 could be related to the four bypassing mechanisms of dislocations overcoming an obstacle. The origins of σ0 are the local climb at 603–633K, and it may change from the local climb to the detach stress with increasing strains at 663–693K. The nt and Qt after Young's modulus and threshold stress correction can be associated with the specific deformation mechanism. The deformation mechanisms at peak strains may change from the climb-controlled dislocation creep (lattice diffusion) at 603K to the Zn diffusion controlled solute drag creep which is controlled by (sub)grain boundary diffusion at 693K. The nt and Qt decrease with increasing strains may be attributed to the grain boundary self-diffusion accelerated by the fluxes of vacancies for lower temperatures (603–663K) and the atom diffusion controlled solute drag creep accelerated by the vacancies, (sub)grain boundaries, and the motion of high angle grain boundaries for higher temperatures (663–693K), respectively.

Effects of prior compression on ductility and yielding behaviour in extruded magnesium alloy AZ31

In the present paper, the effects of precompression along extrusion direction (ED) on subsequent compression perpendicular to ED were investigated in an extruded magnesium alloy AZ31. The results showed that the yield stress under compression perpendicular to ED increased if there was precompression along ED. The evolution of deformation mechanism was responsible for increase in yield stress because plastic deformation was dominated by both basal slip and {10–12} twinning under compression perpendicular to ED in samples without prestrain, but basal slip was difficult to be activated and {10–12} twinning dominated deformation in samples with precompression. However, because basal slip had no obvious contribution to plastic deformation, the ductility decreased if there was precompression along ED.

Evolution of the Plastic Deformation Mechanisms in a Compressed Mg-10Gd-2Y-0.5Zr Alloy

This paper investigates the evolution of the deformation mechanisms in a homogenized Mg-10Gd-2Y-0.5Zr alloy ingot compressed at 300-500 °C and 0.1-20 s-1. It can be found that the basal slip and mechanical twinning are the major deformation mechanisms in the alloy compressed at 300 and 0.1-20 s-1. Increasing the testing temperature to 350 °C, basal slip, non-basal slip and mechanical twinning control the plastic deformation of the alloy compressed at 0.1-20 s-1. When the testing temperatures increase further to 400-500 °C, the mechanical twinning is replaced gradually by the local shear bands which are formed by dynamic recrystallization (DRX) grains (referred as transformation bands). The transformation bands have the trend to form the typical DRX microstructure with increasing the temperatures (might be caused by increasing testing temperatures or strain rates). Besides, the transformation bands can also be found in the sample compressed at 350 °C and 20 s-1when the temperature in the deformation alloy is high enough to activate non-basal slip and form DRX grains at local zone.

Evolution of Texture and Microstructure of AZ31 Mg Alloy Sheet at High Strain Rates

The high strain rate behaviour of magnesium alloys is of great interest to automotive,aerospace and/or defence industries because some critical components should have the propermechanical properties to work under crash or impact conditions. In the current study, resultsfrom an extensive experimental program are presented. The uniaxial mechanical behaviour ofAZ31 sheet under dynamic conditions (ε=103 s-1) is analyzed and compared with that observedat low strain rates. AZ31 sheets have been tested in tension and compression using Hopkinsonbar apparatus at 25°C and 250°C. Moreover, interrupted tests were also performed in orderto relate the evolution of deformation mechanisms with strain. Finally, detailed microstructureand texture examination by electron backscatter di raction (EBSD) and neutron di ractionhas been carried out in order to elucidate the predominant deformation and recrystallizationmechanisms.