Beginning Of Plastic Deformation Research Articles

Unveiling the microstructure of LES manufactured parts for sustainable productive business

The Light Engineering Sector (LES) is regarded as the bloodline of Bangladesh’s economy for simultaneously creating jobs, poverty alleviation, and strengthening foreign currency reserves by reducing dependency on imported products for local industries. But this promising industry is now facing roadblocks mainly due to poor quality under service loading. Though many new and advanced manufacturing techniques are now available, conventional sand casting is the most widely used manufacturing technique utilized by the local spare parts manufacturer. It is well established that the mechanical properties (strength, toughness) of a material are determined by its microstructure. To date, the microstructure characterization of LES products has not been subject to much scrutiny. Therefore, in this work, an attempt has been made to systematically investigate the microstructure of an LES-manufactured marine propeller using an Optical microscope (OM) and Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS). A locally made marine propeller supplied by ACBD company was used for this study. It was found that the microstructure of both materials mainly consists of pores, needle-shaped particles, and Chinese script-like features embedded in the aluminum matrix. Based on EDS results it may be concluded that the needle-shaped particle is mainly composed of Si and Al. Moreover, the average pore size observed was about 5109.5 μm. Use of horse-and-buggy technology, and lack of knowledge of the casting process such as properties of molding sand, the composition of raw material, overall metallurgical control of the casting process, etc. might have resulted in these casting defects. As the conventional particle fracture mechanism for ductile fracture starts once the maximum load is applied, the presence of pores may exhibit premature growth soon after the beginning of plastic deformation. Therefore, the presence of large pores may exhibit premature growth immediately at the beginning of plastic deformation, thereby drastically reducing the mechanical properties under service loading. In summary, both the government and the private sector should involve themselves in strong and focused dissemination research activities with the support of universities so that suitable modifications and modernization can be adopted in the existing system.

Microstructural evolution and yield strength of a novel precipitate-strengthened Fe-based superalloy during thermal aging at 700 °C

Microstructures and compressive yield strength of a novel γ′-strengthened Fe-based superalloy with about 55 wt% iron are investigated after aging at 700 °C for various durations. We found that the γ′ particle always displays a spherical shape and its size increases only from 9 ± 1.6 to 51 ± 14.0 nm with increasing the aging time up to 10,000 h. Meanwhile, the distribution of carbides at grain boundaries is always discontinuously, and no topologically close-packed phases are visible in the alloy during aging. Compressive deformation tests disclose that the yield strength of the experimental alloy at 700 °C increases firstly and then decreases with particle size. Microstructural observations uncover that the predominant deformation mechanism changes from shearing of γ′ particles by weekly-coupled dislocations to shearing by strongly-coupled dislocations and then to Orowan looping with enlarging particle size at the beginning of plastic deformation. Based on these results, it is deemed that the evolution of microstructure and deformation mechanism with aging time leads to the changes in the yield strength during thermal aging.

In-situ investigation of strengthening and strain hardening mechanisms of Cu-added medium-Mn steels by synchrotron-based high-energy X-ray diffraction

A novel Cu-added medium-Mn steel with a chemical composition of Fe–0.27C–9.1Mn–1.86Al–3.3Cu (wt.%) was designed and subjected to intercritical annealing (IA) temperature range from 620 °C to 680 °C for 1 h. The ultimate tensile strength (UTS) increases and the yielding strength (YS) decreases with the IA temperature increasing. The YS of 824 MPa, UTS of 1222 MPa, total elongation (TE) of 55%, and product of strength and elongation (PSE) of 67.2 GPa·% are achieved after IA at 660 °C. Transmission electron microscopy confirmed that Cu-rich nanoparticles precipitate in the ferrite. The in-situ high-energy X-ray diffraction (HE-XRD) experiments show that at the beginning of plastic deformation, both austenite and ferrite bear the applied load. The load is mainly undertaken by martensite with effective transformation-induced plasticity (TRIP) effect triggered. The YS of ferrite is significantly higher than that of austenite. The individual contribution of solid solution strengthening, grain refinement strengthening, dislocation strengthening, and precipitation strengthening in ferrite and austenite is analyzed. The discrepancy between the YS of ferrite and austenite is mainly attributed to the precipitation strengthening due to the Cu-rich nanoparticles precipitation. The moderate mechanical stability and the collaboration of TRIP and twinning-induced plasticity (TWIP) effects of austenite contributed to the enhanced strain hardening capability and resulted in large ductility.

Abnormal three-stage plastic deformation in a 17-4 PH stainless steel fabricated by laser powder bed fusion

A 17-4 PH stainless steel has been additively manufactured (AM) using laser powder bed fusion (LPBF). Different from traditional counterpart, the AM 17-4 PH steel exhibits a peculiar dual-phase microstructure containing large amount of retained austenite (66 vol%) and the rest martensite. This metastable structure is highly susceptible to surface polishing, thus widely scattered data of phase constitution are reported in many previous papers. It is found that the thermal martensite transformation is suppressed by the ultra-fine microstructure produced by LPBF, thus giving rise to a large amount of austenite retained. And as a consequence, plastic deformation of the AM 17-4 steel displays an obvious three-stage behavior: (I) yield plateau, (II) severe work hardening, and (III) final necking. Underlying mechanisms are revealed, respectively. At the beginning of plastic deformation, because there is lacking of further strengthening mechanism upon yielding in the fine-grained structure, a plastic instability caused by local softening occurs and leads to the formation of complex Lüders band. When the accumulated mechanical energy plus the intrinsic chemical energy exceeds activation energy of martensite transformation, further plastic deformation results in severe strain-induced martensite transformation (SIMT). The SIMT endows the AM 17-4 steel with extremely high work hardening ability in stage II, consequently, flow stress rises sharply from 490 MPa to the maximum level of 1090 MPa, and most of the retained austenite is exhausted in this stage. Afterward, the work hardening rate drops below the true stress, an increase in strain produces no additional load increase, local necking occurs and leads to final rupture of the AM 17-4 steel.

Work hardening behavior of hot-rolled metastable Fe50Co25Ni10Al5Ti5Mo5 medium-entropy alloy: in situ neutron diffraction analysis

ABSTRACT Metastability engineering is a strategy to enhance the strength and ductility of alloys via deliberately lowering phase stability and prompting deformation-induced martensitic transformation. The advantages of the strategy are widely exploited by ferrous medium-entropy alloys (MEAs) that exhibit phase transformation from metastable face-centered cubic (FCC) to hexagonal close-packed (HCP) or body-centered cubic (BCC) martensite and a significant increase in work hardening. Fe50Co25Ni10Al5Ti5Mo5 (at%) MEA is an example of such materials, which shows ~1.5 GPa of tensile strength assisted by exceptional work hardening from the deformation-induced BCC martensitic transformation. In this work, the martensitic transformation and its effect on the mechanical response of the MEA were studied by in situ neutron diffraction under tensile loading. Strain-induced BCC martensite started forming rapidly from the beginning of plastic deformation, reaching a phase fraction of ~100% when deformed to ~10% of true strain. Lattice strain and phase stress evolution indicate that stress was dynamically partitioned onto the newly formed BCC martensite, which is responsible for the work hardening response and high flow stress of the MEA. This work shows how great a role FCC to BCC martensitic transformation can play in enhancing the mechanical properties of ferrous MEAs.

Study of work-hardening behavior of high manganese steel during compression

High manganese steels are well used for their excellent hardening properties. In this study, we investigate the mechanical responses of high manganese steel in a compressed condition. During compression deformation, twin crystal is first generated inside some of the grains with orientation advantage, and then all of the grains are twinned. Selective twinning on some grains leads to the formation of textural structures. When the plastic deformation is low, twins appear independently and throughout the grain. After the true strain is more than 0.5, the twins appear as many small parallel short bars. Dislocation density does not increase significantly at the beginning of plastic deformation but increases rapidly while the flow stress no longer increases. Simultaneously, the twinning and the increased density of dislocations increase hardness, with a maximum value of 630 HV at a true strain of 0.6, during plastic deformation.

Dislocation glide and mechanical twinning in a ductile VNbTi medium entropy alloy

• {112}<111> mechanical twinning was activated in an equiatomic refractory VNbTi medium entropy alloy at room temperature. • Twinning nucleation mostly occurred at the beginning of plastic deformation, and a twinning-kinking mixed microstructure was formed. • Dislocation glide dominates the subsequent deformation, and kinking was helpful to relax the stress concentration. • The synergistic activation dislocation glide, twinning and kinking provide a new path to design ductile refractory medium-/high-entropy alloys. An equiatomic VNbTi medium-entropy alloy with outstanding tensile properties and unique deformation behavior is reported. The screw dislocation glide, deformation twinning, and dislocation accumulation induced kink bands are identified as three deformation mechanisms that contribute to a large elongation above 20%. The {112}<111> twins are activated at the beginning of the yield stage accompanied by sudden stress-drop and pronounced acoustic emission. Dislocations dominate subsequent tensile deformation, and the prevalent multiplanar dislocation slip promotes the formation of complex dislocation configurations (e.g., debris, dipoles, and loops) and dense dislocation networks. The twin bands and kink bands can further impede the dislocation motion meanwhile effectively alleviate stress concentration. The synergistic activation of these deformation mechanisms provides new opportunities to design ductile refractory medium- and high-entropy alloys.

Effect of strain on microstructural development during uniaxial compression of metastable beta Ti–10V–2Fe–3Al alloy

The effect of strain during uniaxial compression on mechanical behavior of Ti–10V–2Fe–3Al alloy produced by blended elemental powder metallurgy and containing a metastable β matrix with ~0.1 fraction of α phase was investigated. A detailed scanning transmission electron microscopy study revealed the initiation of slip and deformation-induced α′′ martensite and ωD formation at the beginning of plastic deformation in the stress plateau region of stress-strain curve. {332}<113> twinning occurred at a later stage. Deformation-induced ωD formed in the β matrix and within {332}<113> twins, but was consumed by other deformation products at strains exceeding 0.2. Formation of {112}<111> twins within {332}<113> twins and along their interfaces with the matrix was uncovered. The rate of development of deformation products among the β phase grains was non-uniform with many being fully transformed to α′′ martensite by 0.4 strain, whereas others experienced deformation conditions related to stress plateau region (<0.1 strain) and were only partially transformed. Formation of deformation-induced nanoscale ωD at the α′′ martensite/β interface was found only in the sample with more stable β matrix due to 0.26 fraction of α being present. This was related to a higher rigidity of the β phase resulting from the higher levels of alloying elements. This induced the triggering of β→ω transformation by stresses arising from α′′ martensite formation.

A modified Johnson-Cook model for dynamic behavior of spray-deposition 17 vol.% SiCp/7055Al composites at high strain rates

In this study, the dynamic impact tests of spray-deposited 17 vol% SiCp/7055Al composites at various strain rates were performed with a Split Hopkinson Pressure Bar (SHPB). In these tests, the strain rate was 392 s−1–2002 s−1, and the temperature was 293 K–623 K. Subsequently, the Johnson-Cook (JC) was used to describe the flow behaviors under high speed impact deformation, and its effectiveness was assessed. Results show that the stress values predicted by the JC model could be inconsistent with the experimental ones. A modified JC constitutive model of 17 vol% SiCp/7055Al composites was developed by modifying the strain rate hardening term and considering coupling effects of strain, temperature and strain rate. According to the comparison between the experimental data and the results assessed with the modified JC model, the proposed model could assess the stress-strain values more accurately, especially in the beginning of plastic deformation. This indicates that the composites exert the joint effects of strain rate hardening and temperature softening during high-speed impact deformation.

Plastic flow behavior of twinning induced plasticity steel from low to warm temperatures

This work investigates the tensile behavior of twinning induced plasticity (TWIP) steel under room to warm temperatures. Stress-strain flow curves have been obtained from steady-state uniaxial tensile tests performed at different temperatures, that is, from 20 to 800°C, and different engineering strain rates, 0.003 and 0.5s−1. The yield and ultimate tensile strength, elongation at fracture, reduction of area, strain hardening exponent and strain hardening rate have been defined to describe the plastic behavior of TWIP steel. It has been found that the plastic flow behavior of TWIP steel is characterized by negative strain rate sensitivity and strain hardening, at temperatures from 20 to 300°C, until the twinning mechanism occurs. At these temperatures, dynamic strain aging starts at the beginning of plastic deformation, with the appearance of fluctuations in the work hardening rate and local strain rate. Pronounced local serrations also appear at 300°C and 0.003s−1. Plastic deformation is mainly driven by dislocation gliding at 550 and 800°C, and mechanical twins are absent. As a result, negative strain rate sensitivity and local serrations disappear in the stress-strain curves. Creep also contributes notably to plastic deformation at 800°C, as was also observed on the fracture surfaces of tensile samples. Mechanical twins were only visible for the lower temperatures. The grains in the sample tested at 800°C and 0.003s-1 were fully recrystallized.

HIGH SPEED SYNCHRONOUS RELUCTANCE DRIVES FOR MOTOR SPINDLES

This article presents a new design for setting up high speed synchronous reluctance motors (SynRM) in motor spindle applications for up to 30,000 rpm. The transversal laminated SynRM rotor package is supported by thin steel sheets and a shaft nut to exert a constant axial clamping force. Both, numerical and practical investigations are carried out using FEM-analysis and a test bench for centrifugal force load tests. The objective is to find out at which speed the elastic deformation changes into the plastic deformation. The beginning of plastic deformation is a failure criterion in electric motor construction.

Texture–Microstructure Correlation in Hot-Rolled AZ31

The paper presents a correlation between microstructure and crystallographic texture in hot-rolled AZ31 magnesium alloy. The microstructure and texture evolution are investigated using optical microscopy, field emission scanning electron microscopy, X-ray diffraction, respectively. The results indicated discontinuous dynamic recrystallization, twinning dynamic recrystallization, and particle stimulated nucleation as the main mechanisms during hot deformation. The average size of grains decreased from 19 to 1 μm as the rolling reduction increased from 0 to 60%. A large area fraction of new grains was observed at 40% thickness reduction. At the beginning of plastic deformation, basal slip and extension twinning increased the intensity of {0001} basal fiber texture. Dynamic recrystallization had a considerable effect on weakening the basal texture. Interestingly, some unusual highly stable components including $$ (01\bar{1}3)[7\bar{8}13],(01\bar{1}3)[\bar{2}110] , $$ and $$ (01\bar{1}3)[14\bar{5}\bar{3}] $$ existed at all deformation conditions and their intensity remained nearly unchanged.

The Influence of Structure and Phase Composition of Titanium Alloy on Superplastic Deformation

The paper presents research into the behavior of high-temperature plastic deformation of titanium alloy Ti–4.74 wt.% Al–5.57 wt.% Mo–5.04 wt.% V alloy (VT22) depending on its structure and phase composition. It is shown that the formation of fine-grain structure in this alloy is not a sufficient condition for realizing superplastic deformation. It is found that the formation of the ultra-fine grain structure in VT22 alloy leads to the temperature decrease down to 823 K at the beginning of plastic deformation. This allows achieving the percent elongation of the alloy specimens over 1300% within the temperature range of 973–1073 K and at a 6.9·10–3 s–1 initial tensile rate. The grain boundary sliding is considered to be the main deformation mechanism.

Martensite phase stress and the strengthening mechanism in TRIP steel by neutron diffraction

Two TRIP-aided multiphase steels with different carbon contents (0.2 and 0.4 mass%) were analyzed in situ during tensile deformation by time-of-flight neutron diffraction to clarify the deformation induced martensitic transformation behavior and its role on the strengthening mechanism. The difference in the carbon content affected mainly the difference in the phase fractions before deformation, where the higher carbon content increased the phase fraction of retained austenite (γ). However, the changes in the relative fraction of martensitic transformation with respect to the applied strain were found to be similar in both steels since the carbon concentrations in γ were similar regardless of different carbon contents. The phase stress of martensite was found much larger than that of γ or bainitic ferrite since the martensite was generated at the beginning of plastic deformation. Stress contributions to the flow stress were evaluated by multiplying the phase stresses and their phase fractions. The stress contribution from martensite was observed increasing during plastic deformation while that from bainitic ferrite hardly changing and that from γ decreasing.

Unusual Plastic Deformation Behavior in Lath Martensitic Steel Containing High Dislocation Density

To understand the strengthening mechanism of a metallic material with high dislocation density, the plastic deformation behavior of lath martensite was studied by means of in situ neutron diffraction measurements during tensile deformations using a 22SiMn2TiB steel and a Fe-18Ni alloy. The characteristics of dislocation were analyzed and were discussed with the relation of stress-strain curves. The dislocation densities (ρ) induced by martensitic transformation during heat-treatment in both materials were found to be originally as high as 1015 m-2 order, and subsequently to increase slightly by the tensile deformation. The parameter M value which displays the dislocation arrangement dropped drastically at the beginning of plastic deformation in both materials, indicating that the random arrangement became more like a dipole arrangement.

Work Hardening, Dislocation Structure, and Load Partitioning in Lath Martensite Determined by In Situ Neutron Diffraction Line Profile Analysis

A lath martensite steel containing 0.22 mass pct carbon was analyzed in situ during tensile deformation by high-resolution time-of-flight neutron diffraction to clarify the large work-hardening behavior at the beginning of plastic deformation. The diffraction peaks in plastically deformed states exhibit asymmetries as the reflection of redistributions of the stress and dislocation densities/arrangements in two lath packets: soft packet, where the dislocation glides are favorable, and hard packet, where they are unfavorable. The dislocation density was as high as 1015 m−2 in the as-heat-treated state. During tensile straining, the load and dislocation density became different between the two lath packets. The dislocation character and arrangement varied in the hard packet but hardly changed in the soft packet. In the hard packet, dislocations that were mainly screw-type in the as-heat-treated state became primarily edge-type and rearranged towards a dipole character related to constructing cell walls. The hard packet played an important role in the work hardening in martensite, which could be understood by considering the increase in dislocation density along with the change in dislocation arrangement.

Microstructure Evolution of a Medium Manganese Steel During Thermomechanical Processing

An as-cast Fe-0.2C-10Mn-3Si-3Al medium manganese steel with a ferrite plus austenite duplex microstructure was subjected to hot compression tests at deformation temperatures within two-phase (α + γ) range and various strain rates. The microstructure evolution of the experimental steel during hot deformation was investigated. The flow curves were characterized by a discontinuous yielding at the beginning of plastic deformation, followed by a weak work hardening to a peak and a subsequent mild softening stage. Two restoration processes took place during hot deformation, namely dynamic recrystallization (DRX) of austenite and continuous dynamic recrystallization of ferrite. The DRX of austenite was believed to dominate the softening stage of the flow curves. The discontinuous yielding stemmed from the existing Kurdjumov–Sachs (K–S) orientation relationship between ferrite and austenite in the initial undeformed microstructure, which gradually weakened during subsequent deformation.

Разработка научных основ и оптимизация технологических режимов нового способа протяжки литых слитков

The paper considers an idea of realization of alternating deformation in forging cast ingots and determines a rational shape of forging tools. The results of the analysis of the strained state of metal during compression in heads with a profiled surface and in smooth heads (at the second stage of deformation) are presented. The use of new tools provides the formation the metal structure in first passes by means of additional alternating deformation without a significant change of the cross-section shape of the billet. Alternating deformation increases the accumulated strain without changing the cross-section shape of the billet, stimulates the formation of the polygonized subgranular structure and refines the grain structure. Results of the strained state analysis of metal during forging in profiled and flat heads are presented. The degree of the accumulated strain in particles in the deformation zone is calculated for six points along the height of the section of the head hollow in order to estimate strain inhomogeneity in the ingot height. The accumulated strain for a specific particle is determined from the problem solution for a flat deformed state under isothermal conditions by means of Deform software. In layers adjacent to the contact zone the strain is minimal since there are areas of hindered deformation (sticking areas), the size of these areas decreases towards the depth of the metal; at the beginning of plastic deformation a free surface appears which is not under the working tool. Ways to control the technological process of forging are found for increasing homogeneity of strained state in the ingot section and stimulating the formation of the polygonized substructure during compression and the ultradisperse structure after metadynamic recrystallization during a pause.

Simulation of mechanical behavior of AZ31 magnesium alloy during twin-dominated large plastic deformation

Experiments and visco-plastic self-consistent (VPSC) simulations were used to quantify the amount of twinning and the relationship to stress–strain behavior in a textured Mg–3Al–1Zn plate. Two different compression directions were utilized to favor {1 0 1¯ 2} extension or {1 0 1¯ 1} compression twinning. {1 0 1¯ 2} twins nucleate at the beginning of plastic deformation and grow to consume the parent grains completely. During compression along the normal direction, {1 0 1¯ 1} twinning and {1 0 1¯ 1}–{1 0 1¯ 2} double twinning start at strain of 0.05, and the number of twins increases until rupture, above strain of 0.15. {1 0 1¯ 1} and {1 0 1¯ 1}–{1 0 1¯ 2} twinning also occur during compression along the transverse direction, start at strain of 0.06 and then multiply in grains totally reoriented by {1 0 1¯ 2} twins. Using suitable parameters, the VPSC model can accurately predict the occurrence of extension, compression and double-twinning as well as the flow stresses and deformed textures. According to VPSC simulations, twinning and slip have the same latent hardening parameters.

Controlling Bottom Hole Flowing Pressure Within a Specific Range for Efficient Coalbed Methane Drainage

The stress state of coal surrounding a coalbed methane (CBM) production well is affected by the bottom hole flowing pressure (BHFP). The permeability of coal shows a marked change under compression. The BHFP must be restricted to a specific range to favor higher permeability in the surrounding coal and thus higher productivity of the well. A new method to determine this specific range is proposed in this paper. Coal has a rather low tensile strength, which induces tensile failure and rock disintegration. The deformation of coal samples under compression has four main stages: compaction, elastic deformation, strain hardening, and strain softening. Permeability is optimal when the coal samples are in the strain softening stage. The three critical values of BHFP, namely, p wmin, p wmid, and p wupper, which correspond to the occurrence of tensile failure, the start of strain softening, and the beginning of plastic deformation, respectively, are derived from theoretical principles. The permeability of coal is in an optimal state when the BHFP is between p wmin and p wmid. The BHFP should be confined to this specific range for the efficient drainage of CBM wells. This method was applied to field operations in three wells in the Hancheng CBM field in China. A comprehensive analysis of drainage data and of the BHFP indicates that the new method is effective and offers significant improvement to current practices.