Room Temperature Tensile Tests Research Articles

Asymmetry and deformation mechanism in stress relaxation of 7B50 alloy

Stress relaxation behavior, deformation mechanism, and microstructure evolution of retrogression and reaging 7B50 aluminum alloy (7B50-T7751) have been experimentally investigated with different stress directions in this study. The stress relaxation aging tests with various initial stress levels and directions were carried out at 140 ℃ for up to 16 h. Room temperature tensile tests and transmission electron microscopy (TEM) observation tests were performed subsequently. The results show that the stress relaxation behavior under tension and compression exhibits significant asymmetry. The stress relaxed and stress relaxation rates under tension are relatively higher as compared to compression, as smaller and more dispersed dislocations under tensile stress reduce the threshold stress. The stress direction contribute the opposite effect on strength evolution during stress relaxation. Furthermore, the stress relaxation mechanism of the material has been analyzed based on creep stress exponent and threshold stress. In the low-stress range, dislocation climbing controls the deformation mechanism under tensile stress, while dislocation slip under compressive stress. As the stress increases, dislocation slip controls the deformation mechanism under tensile stress, while grain boundary slip under compressive stress. A physically-based stress relaxation constitutive model considering the dislocation theory and stress relaxation mechanism is established. The predicted results of the model are in good agreement with the experimental data, indicating that the model can effectively describes stress relaxation behavior under complex stress conditions.

Effect of Al or Cu Content on Microstructure and Mechanical Properties of Zn Alloys Fabricated Using Continuous Casting and Extrusion

The effect of Al or Cu content on the microstructure and mechanical properties of continuous casting and extrusion Zn alloys has been studied by a room temperature tensile test, X-ray diffraction, and scanning electron microscope. With the increase in Al content, the microstructure of continuous casting and extrusion Zn alloys slightly coarsens, and the lamellar eutectic structure increases. The changes in the above structural factors result in a slight decrease in strength and a significant increase in the elongation of Zn-Al alloys. The strength of Zn alloys increases as the Cu content increases due to the increased content and size of the second phase in the Zn alloys. This means that the mechanical properties of Zn alloys can be adjusted by a continuous casting and extrusion process, and the improvement of equipment capacity can improve the structure and morphology of the alloys.

An approach for identifying the deformation-induced strain from tensile tests and x-ray diffraction measurements

ABSTRACT Non-uniform plastic deformation and phase transformation introduce residual stress (RS) in a material. The present study focuses on the influence of tensile strain on RS development in 304L austenitic stainless steel, wherein strain-induced martensite (SIM) transformation influences deformation behaviour. Room temperature tensile tests were interrupted at various tensile strains between the material yield and ultimate tensile strength at 5.21 × 10−4 and 1.3 × 10−2 s−1 strain rates. Microstructural characterisation of as-received and deformed samples by electron backscatter diffraction provides evidence of SIM formation and shear bands. Analysis of tensile flow and work hardening behaviour is discussed in correlation to the microstructural and dislocation density variations. In addition, RS in the austenite and martensite phases was measured using the x-ray diffraction method, and the RS balance that occurs between the longitudinal and transverse components and the two phases after tensile deformation is presented. It is shown that a combined analysis of RS and x-ray diffraction peak width could be utilised to find the deformation level in tensile fractured, press brake bent, and roll bent samples.

Semi-solid rheo-diecast Mg-xGd-3Y–1Zn-0.4Zr (wt%) alloys and their comparison with conventional HPDC alloys

This work systematically investigated the microstructure and mechanical properties of Mg-xGd-3Y–1Zn-0.4Zr (wt%) alloys prepared via rheo-diecasting (RDC) in comparison to those of conventional high pressure die casting (HPDC) alloys, and revealed their differences in microstructure, casting defects, and mechanical properties. The microstructure of the as-cast RDC Mg-xGd-3Y–1Zn-0.4Zr (wt%) alloys is primarily composed of α-Mg and Mg3(Gd,Y,Zn) eutectic phases. Of the four alloys studied, the GWZ831K and GWZ1431K alloys exhibit the best elongation (EL: 6.4±1.3 %) and the highest strengths (UTS: 270±9 MPa; YS: 201±5.7 MPa), respectively. The GWZ831K alloy exhibits a quasi-cleavage fracture during room-temperature tensile tests, whereas the GWZ1431K alloy demonstrates a brittle fracture. The phase composition of conventional HPDC alloys is fundamentally the same as that of RDC alloys, but their grains and eutectic phases differ in morphology and are smaller in size. Oxide films are frequently found in conventional HPDC castings, but neither oxide films nor pore defects are observed in RDC alloys. Compared to conventional HPDC alloys, RDC alloys have better elongation but lower strength. The lower strength of RDC alloys compared to HPDC alloys can be attributed to larger grains and eutectic phases, along with fewer stacking faults (SFs), resulting in limited strengthening effects of grain boundaries, eutectic second phases and SFs. In contrast, RDC alloys exhibit superior elongation compared to HPDC alloys due to the elimination of oxide films and pore defects, as well as the excellent ductility of the α-Mg matrix that can effectively coordinate deformation.

New insights on microstructural parameters controlling mechanical properties of rolled molybdenum

The aim of this paper is to understand microstructural evolution and their key effects on mechanical properties of rolled molybdenum (Mo) from as-received sintered sheet. A batch of pure Mo sheets with various thickness reductions have been characterized in detail. The results reveal that the microstructure evolves by three stages including grain refinement, dynamic recrystallization (DRX) and grain elongation. For the rolled Mo, the fractions of low angle grain boundaries (LAGBs) are much higher than that of as-sintered Mo but do not increase monotonically with increasing thickness reductions. The occurrence of DRX results in the decrease of LAGBs at the intermediate stage while a larger rolling reduction results in grain boundaries bursting with high angle characters at the later stage. All rolled sheets are textured with dominant orientation locating at γ-fiber and θ-fiber line. Room-temperature tensile tests demonstrate that all rolled Mo sheets are much stronger and more ductile (ultimate tensile strength, UTS = ∼650–1000 MPa and total elongation, TE = ∼10–35%) than as-sintered Mo (UTS = ∼400 MPa and TE = 0), but a larger thickness reduction unexpectedly results in the deteriorated ductility which can be attributed to the increased number of HAGBs. The elongated microstructure with refined grains and the high fraction of LAGBs induced by rolling are considered to be the dominant parameters responsible to the improved mechanical properties of Mo. We suggest that rolling parameters should be optimized carefully to avoid the detrimental effect of crack initiation from the increase in high angle grain boundaries (HAGBs).

Effect of boron on the microstructure and performance of HIP-prepared high boron steels

High boron steels containing 2 wt% and 3.3 wt% boron were fabricated using hot isostatic pressing (HIP). The enhanced shielding performance of the steel with higher boron content was quantitatively evaluated through Monte Carlo simulations using software Fluka. The steels were studied with XRD and the phase ratio were calculated using Rietveld refinement method. The microstructure of the materials was investigated using electron backscatter diffraction (EBSD). Comprehensive thermal property measurements, complemented by Hamilton thermal conductivity model analysis, were conducted on high boron steels. The results revealed that the decreased overall thermal conductivity in these materials is primarily attributed to two factors: the increased volume fraction of the boride phase and the enhanced interconnectivity among boride particles. These findings provide crucial insights into the thermal behavior of high boron steels. Room temperature tensile tests indicate that high boron content can cause the fracture mode of the material to transition from ductile to brittle. This study provides a detailed analysis of the effects of boron content on various properties of high boron steels, which can serve as shielding materials in fusion reactors. Advances were proposed for optimizing the thermal performance of high boron steels in this study.

The Influence of Cr Addition on the Microstructure and Mechanical Properties of Fe-25Mn-10Al-1.2C Lightweight Steel

The influence of Cr addition on the microstructure and tensile properties of Fe-25Mn-10Al-1.2C lightweight steel was investigated. The characteristics of the microstructures and deformation behavior were carried out through X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and room temperature tensile testing. Fe-20Mn-12Al-1.5C steel without Cr exhibited a fully austenitic single phase. With the addition of Cr, the volume fraction of ferrite continuously increased. When the content of Cr exceeded 5 wt%, the precipitation of Cr7C3 carbides was observed. In the steel with 5 wt% Cr, the quantity of κ carbides remarkably decreased, indicating that the addition of 5 wt% Cr significantly inhibited the nucleation of κ-carbides. As the Cr content increases from 0 wt% to 5 wt%, the austenite grain sizes were 8.8 μm and 2.5 μm, respectively, demonstrating that Cr alloying is an effective method of grain refinement. Tensile strength increased slightly while elongation decreased with increasing Cr content. As the Cr content exceeded 5 wt%, the yield strength increased but the elongation drastically decreased. The steel with 2.5 wt% Cr achieved a synergistic improvement in strength and ductility, exhibiting the best tensile performance.

Avoiding microstructural instabilities in high tungsten containing superalloys by adjusting Al content

The effects of Al contents on the microstructures and mechanical properties of superalloy with high tungsten content were studied. During solidification, Al was found to be enriched in the residual liquid, which led to the transformation of eutectic morphology from reticular to bulk-like. When the Al content increased to 8 wt%, β-NiAl phase formed in the inter-dendritic region, Thermo-Calc calculations confirms such the experimental phenomenon. The absolute value of the lattice misfit between the γ/γ' phases and the elastic strain energy increases, then the size of the γ' phase increases when the Al concentration rises from 7 wt% to 8 wt%. When the Al content increased to 7 wt%, the tungsten-rich α-W phase is precipitated and the volume fraction is 1.20%. The volume fraction of the α-W phase increases to 7.84% when the Al content reached 8 wt%, which is consistent with the Thermo-Calc calculation results. After the room temperature tensile and stress rupture life test at 975 °C / 235 MPa, it revealed that the precipitation of hard and brittle α-W and β-NiAl phases with high Al content, which resulted in the mechanical properties decreased due to the crack initiation at multiple locations and then interconnected. The higher density of γ' phase with 6 wt% Al makes it difficult for dislocations to bypass. The site of crack initiation is decreased due to the absense of brittle phase. The stress rupture life can reach 49.21 h of the alloy.

Effect of recrystallization on aging embrittlement and high temperature creep properties of AFA steel

As a kind of typical precipitate strengthened steel, alumina-forming austenitic (AFA) stainless steels usually suffer obvious aging embrittlement issue. Recrystallization treatment was performed by cold rolling followed with heat treatment at 1080 ℃ for a Fe-25Ni-15Cr-3.5Al based AFA steel to evaluate the effect of recrystallization on aging embrittlement. The materials before and after recrystallization were aged at 700 ℃ for 120 h to evaluate the effect of recrystallization on precipitation behavior. After recrystallization, the average grain size decreased and the density of grain boundaries increased obviously. The size of precipitates along grain boundaries was smaller and more uniform after recrystallization, and the precipitation of Cr rich phase was inhibited. Room temperature tensile test, Charpy impact, and creep behavior at high temperature of samples with different treatment states were measured and compared. The aging embrittlement of AFA steel was alleviated obviously after recrystallization. The creep resistant was also improved for sample treated by recrystallization and aging. The relevant mechanisms were discussed.

Influence of post-welding heat treatment on microstructure and peculiarity of 10Cr9Mo1VNb boiler steel welded by hot wire TIG

Hot wire tungsten noble gas (HW-TIG) soldering tests are conducted on 10Cr9Mo1VNb boiler steel with multi-layer and multi-pass, and the soldered joints are subjected to post-weld heat treatment (PWHT) at different temperatures and times from 740°C to 790°C and 1 hour to 4 hours. The influence of PWHT on the microstructure and peculiarity of the joints are analyzed by using hardness, metallography, room temperature tensile testing, Charpy impact testing, SEM, and other experimental methods. The results show that the PWHT can transform the martensite formed in the welding process into tempered martensite, which can significantly improve the uniformity of the microstructure and peculiarity of each area of the joint. When subjected to heat treatment at 740°C, both strength and ductility improve with the increase of heat treatment time, while the plasticity gets a slight decrease. However, when subjected to heat treatment at 790°C, strength, plasticity, and ductility decrease with the increase of time, mainly due to the carbide coarsening such as M23C6 in the joint.

Liquid metal compatibility of pre-oxidized FeCrAl in flowing Sn

A flowing Sn thermal convection loop (TCL) experiment was conducted in FeCrAlMo (Kanthal alloy APMT) tubing for 1000 h with a peak temperature of 400 °C. Oxide dispersion strengthened (ODS) Fe-(10–12)Cr-6Al and APMT specimens were exposed and all were pre-oxidized to form an external alumina scale. Unlike static exposures at 400° and 500 °C where pre-oxidation prevented dissolution, large mass losses were observed in the TCL hot leg after the specimens were cleaned in Li. The observed pitting suggests that localized failure of the pre-formed alumina surface oxide caused significant metal loss. Specimen mass gains were not observed on the cold leg implying that mass transfer did not occur and the small levels of Fe and Cr in the Sn after the experiment implies that Sn-rich phases may have formed. Post exposure room temperature tensile testing showed limited impact of the Sn exposure on the 12Cr ODS FeCrAl alloy, comparable to a 1000 h anneal in Ar at 400 °C. These results suggest that pre-oxidation is not sufficient to protect FeCrAl alloys in contact with flowing liquid Sn at 350°-400 °C.

Tensile behavior of the designed SiCf/Ti2AlNb/Ti17 composites

SiCf/Ti2AlNb-reinforced Ti17 (SiCf/Ti2AlNb/Ti17) composites were fabricated through hot isostatic pressing. The designed SiCf/Ti2AlNb/Ti17 composites exhibited excellent tensile properties, with tensile strengths of 1657 MPa at room temperature and 1830 MPa at 450 °C. The microstructure, fracture morphology, and longitudinal section of the composites were characterized by X-ray diffraction, scanning electron microscopy, and electron backscatter diffraction techniques. The fracture process of SiCf/Ti2AlNb/Ti17 composites during room-temperature tensile tests was as follows: the SiCf/Ti2AlNb reaction layer first fractured, the crack extended into the Ti2AlNb matrix, the Ti2AlNb matrix crack extended to the adjacent reaction layer and promoted fiber fracture, the crack extended to the Ti2AlNb/Ti17 interface, ultimately the Ti17 capsule fractured. The fracture process of SiCf/Ti2AlNb/Ti17 composites during 450 °C tensile tests was as follows: the SiCf/Ti2AlNb reaction layer first fractured, multiple fiber fractures occurred, and the crack in the reaction layer entered the Ti2AlNb matrix, the crack extended to the Ti2AlNb/Ti17 interface, eventually the Ti17 capsule fractured. The room-temperature tensile fracture was fitted to the LLS model, while the tensile fracture at 450 °C was fitted to the GLS model. The different fracture processes indicated that the SiCf/Ti2AlNb/Ti17 composites were suitable for exploiting fiber strength at 450 °C and therefore exhibited excellent tensile strength.

Role of Austenite Stability in Elevated Temperature Mechanical Properties of Gas Metal Arc-Directed Energy Deposition Austenitic Stainless Steels

A gas metal-directed energy deposition process was used to fabricate builds using two commercial weld fillers used in power generation applications, 16-8-2 and 316H. Microstructure stability and mechanical properties were investigated through room-temperature and elevated temperature tensile testing and creep testing at 650°C, 750°C, and 825°C. 16-8-2 exhibited reduced austenite stability which resulted in athermal martensite formation after aging at 650°C for 1000 h and strain-induced martensite formation during room-temperature tensile testing. 316H exhibited relatively higher austenite stability due to increased alloying content, resulting in no athermal martensite or strain-induced martensite. Due to lower austenite stability, ferrite formed during creep at 650°C in 16-8-2, which resulted in reduced creep life and lower creep ductility compared to 316H. At 750°C and 825°C, when ferrite is no longer thermodynamically stable, 16-8-2 exhibited longer creep life and similar creep ductility as 316H. The formation of ferrite in 16-8-2 appears to have a greater impact on creep performance than the formation of embrittling topologically close-packed phases like the σ phase, as 316H exhibited superior creep performance while predicted to form 14 vol.% σ phase at 650°C.

Evaluation of Microstructure and Mechanical Properties of TiAl/HI-TEMP 820/ SCM440H Materials Manufactured through Vacuum Brazing according to Process Temperature

The TiAl alloy is attracting attention as a lightweight and heat-resistant material, because of its high specific strength, excellent high-temperature formability, and fatigue strength. However, its applications are limited by its high unit price and low room temperature ductility. To overcome this issue, dissimilarly bonded materials have been extensively employed. This involves joining a brittle metal to a low-cost metal that possesses excellent plasticity, using various dissimilar bonding techniques. In this study, TiAl/HI-TEMP 820/SCM440H materials were fabricated using a vacuum brazing process under different temperature conditions. After the brazing process, the microstructure of the interfacial area revealed seven distinct layers resulting from chemical reactions between the base metals and the filler metal. These reaction layers consisted of a Ni solid solution, intermetallic compounds (Ti<sub>3</sub>Al, TiNi<sub>2</sub>Al, Ti<sub>2</sub>Ni, FeNi), and borides (CrB, TiB<sub>2</sub>, FeB). To analyze the effect of brazing temperature on the relationship between the microstructure and mechanical properties at the interface of TiAl/HI-TEMP 820/SCM440H materials, conventional uniaxial tests and nanoindentation tests were performed. The measured nanohardness exhibited a significantly large distribution for each reaction layer, with the highest hardness values observed in the intermetallic compounds and borides layers. Additionally, room temperature tensile tests confirmed that fractures initiated in the highhardness and brittle intermetallic compounds and borides layers.

Microstructural evolution and high-temperature strengthening mechanisms of the IN 738LC superalloy prepared by selective laser melting

Defect-free SLM-IN 738LC superalloy samples were fabricated using selective laser melting (SLM). The as-built samples exhibit a cellular structure with fine carbides, γ′ phases, and high-density dislocations at the cell boundary and nano-size γ′ phases in the interior, resulting in excellent deformation resistance and favorable mechanical properties at room temperature, with a yield strength (YS) of 970 MPa, an ultimate tensile strength (UTS) of 1263 MPa, and an elongation (EL) of 33%. This study investigates the evolution behavior of the γ′ phase at different solid solution treatment (SHT) temperatures and its impact on alloy strength and toughness. The 1020 + AHT sample exhibits a uniform distribution of coarse γ′ phases. The samples subjected to 1070 + AHT and 1120 + AHT display a bimodal distribution of γ′ phases. When the SHT temperature was further increased, fine spherical γ′ phases are evenly distributed in the 1160 + AHT and 1200 + AHT samples. Moreover, the γ/γ′ eutectics at the grain boundaries gradually dissolve with increasing SHT temperature, whereas the size and fraction of carbides increase. The room temperature tensile test of the 1070 + AHT sample demonstrated a favorable balance between strength and plasticity (YS = 1217 MPa, UTS = 1472 MPa, EL = 9.25%). This is attributed to the synergistic strengthening effect of the bimodal distribution of the γ′ phase. The temperature-dependent deformation behavior of the 1070 + AHT sample was analyzed at 23 °C, 800 °C, 900 °C, and 1000 °C. At 23 °C, numerous dislocations accumulated around the γ/γ′ interfaces. When the temperature increased to 800–900 °C, the dislocations shearing γ′ phases is activated and the alloy remained high strength. At 1000 °C, both the dislocation shearing and dislocation bypassing mechanisms coexist, resulting in a decrease in the strength of the alloy and an increase in its plasticity. This work provides scientific and theoretical support for SLM-IN 738LC parts with favorable properties.

Improving mechanical performance of heat-resistant eutectic Al-Fe-Ni alloy by in-situ TiB2 particles

The mechanical performance and strengthening mechanism of in-situ TiB2 ceramic particles reinforced eutectic Al-Fe-Ni alloy were researched. The smaller-sized TiB2 particles are uniformly distributed inside the grain and larger-sized particles tend to gather at the eutectic grain boundary. The hardness and room temperature tensile tests show that TiB2 particles improve the comprehensive mechanical properties of the eutectic alloy. Meanwhile, the high temperature tensile strength of eutectic alloys is also enhanced by particles. The pinning effect of TiB2 particles on dislocation and eutectic boundary are the main strengthening mechanisms. The study on the microstructures and properties of TiB2 particles reinforced eutectic alloys provides guidance for enhancing the mechanical performance of eutectic Al alloys.

Effect of Cooling Rate on the Microstructure and Mechanical Property of Nickel-Based Superalloy MAR-M247.

Heat treatment is an important process for optimizing the microstructures of superalloys, and the cooling rate after solid solution treatment is one of the most critical parameters. In this work, we treated solid solution MAR-M247 alloys with water quenching, air cooling, and furnace cooling. Microstructure characterization, hardness, and room temperature tensile tests were conducted to investigate the effect of cooling rate on the microstructure and mechanical properties of MAR-M247 alloys. The results showed that the cooling rate after solid solution treatment mainly affected the precipitation behavior of the secondary γ' phase, but it had few effects on other microstructure characterizations, including grain size, γ/γ' eutectic, and MC carbide. The water-quenched sample had the highest cooling rate (400 °C/s) and hardness (400 HV) but suffered from premature fracture because of quenching cracks. A further decrease in cooling rate from 1.5 °C/s to 0.1 °C/s deteriorated hardness (384 HV to 364 HV) and yield strength (960 MPa to 771 MPa) but increased elongation (8.5% to 13.5%). Moreover, the deformation mechanism was transformed from dislocation shearing to Orowan bypassing. The decreased yield strength was mainly due to the weakened precipitation strengthening resulting from γ'-phase coarsening. The improved elongation was attributed to not only the higher work-hardening index caused by interface dislocation networks but also the more uniform deformation, which delayed necking.

Effect of heat treatment on microstructure evolution, strengthening mechanisms and mechanical properties of Zn modified Al–Mg alloys with Sc and Zr additions

In this work, a new Al–Mg–Zn alloy was designed based on thermodynamic calculations combined with the concept of cooperative strengthening and toughening. The influence of different aging processes on microstructure and mechanical properties of the designed Al–Mg–Zn alloy was studied using experimental techniques including hardness testing, room-temperature tensile testing, EBSD, and HRTEM. The results indicate that after four different aging heat treatments, fine and uniformly distributed nano T-Mg32(Al, Zn)49 phases have precipitated within the grains. Compared to single-stage aging (140 °C/36 h, 180 °C/7 h), two-stage aging (90 °C/24 h + 140 °C/18 h, 90 °C/24 h + 180 °C/5 h) resulted in a smaller size and an increased number density of precipitated phases within the grains. After the aging process of 90 °C/24 h + 140 °C/18 h, the grains exhibited the smallest precipitated phases, being of 2.77 nm in size and a number density of 1.06 × 1023 #/m³. Additionally, TEM diffraction spot analysis identified the presence of Al3(Sc, Zr) precipitates within α-Al matrix. Under the aging process of 90 °C/24 h + 140 °C/18 h, the alloy exhibited superior mechanical properties with a tensile strength of 566 MPa, yield strength of 477 MPa, and elongation of 16.5%. The primary strengthening mechanisms of the presently investigated Al–Mg–Zn alloy include precipitation strengthening, solid solution strengthening, and grain boundary strengthening. A yield strength model considering various strengthening mechanisms is used to predict the yield strength based on thermodynamic calculations and experimental data. The research findings will offer theoretical insights guiding the design of compositions and heat treatment schedule for aluminum alloys.

Precipitation and tensile behaviors of Allvac 718Plus superalloy during long-term thermal exposure

Allvac 718Plus alloy is a promising Ni-based superalloy with excellent thermal stability and mechanical properties up to 704 °C. The γ′ coarsening, σ precipitation and the resulting degraded tensile properties at room and elevated (704 °C) temperatures during long-term thermal exposure at 704 and 760 °C are systematically investigated in this work. The results showed that a higher exposure temperature and a longer exposure time significantly accelerate the γ′ coarsening and σ precipitation. The σ particles were detrimental to the alloy by acting as the preferential nucleation sites for cracks, and the reduction in tensile strengths was mainly caused by the increasing size of γ′ precipitates during thermal exposure. Furthermore, there were significant differences between the deformation mechanisms of room-temperature and elevated-temperature (760 °C) tensile tests. Concerning the deformation mechanism of γ-matrix, planar slipping dominates during tensile testing at room temperature, while the formation of stacking faults and microtwinning are primary during tensile testing at elevated temperature (760 °C). The dominant precipitation strengthening mechanism depended on the γ′ characteristics of different thermal exposure conditions and the tensile temperature. The 704 °C/100 h exposure sample shows a dislocation shearing mechanism when tested at room or elevated (760 °C) temperature. Notably, the 760 °C/1000 h exposure sample exhibits a mixed precipitation strengthening mechanism of Orowan bypassing and superlattice stacking faults shearing when tensile tested at room temperature while showing a dislocation and superlattice stacking faults shearing mechanism when tensile tested at elevated (760 °C) temperature.

Effect of a Gradient Temperature Rolling Process on the Microstructure and Mechanical Properties of the Center of Ultra-Heavy Plates

As there is a small amount of deformation in the center during the rolling process of ultra-heavy plates, it is extremely easy to cause poor mechanical properties in the center. Increasing the deformation in the center is the most feasible method to eliminate the deformation effects in the cross-section of ultra-heavy plates. In this study, the gradient temperature rolling (GTR) process is compared with the traditional uniform temperature rolling (UTR) process. It is found that the GTR process can significantly increase the deformation in the center and thereby refine the grains.. The room temperature tensile test and instrumented Charpy impact test are used to test the strength at room temperature and impact energy at low temperature. Combined with the obtained impact load/energy displacement curve, the deformation and damage process under impact load are analyzed. The microstructure morphology and impact fracture obtained by different rolling processes in the center are analyzed by experimental methods such as OM, SEM, EBSD, etc. The prior austenite grain (PAG) boundary morphology is analyzed and the densities of grain boundaries are statistically quantified. The results showed that the strength, plasticity, and low-temperature toughness of the GTR process are improved compared to the UTR process, with increased dislocation density in the center microstructure, the density of PAG boundaries, and the density of packet boundaries. The size of the PAG in the center is refined by ~49%, the density of PAG boundaries increased by ~140%, the density of high-angle packet boundaries increased by ~39%, and the density of low-angle packet boundaries increased by ~49%. The crack propagation in the instrumented Charpy impact test of the GTR process showed stable expansion, indicating a ductile fracture compared to the semi-brittle fracture of the UTR process. The densities of PAG boundaries and high-angle packet boundaries are the most important factors affecting the strength and low-temperature toughness.