Decrease In Total Elongation Research Articles

Microstructure evolution and work hardening behavior of medium/high carbon Fe–Mn–C TWIP steel during tensile deformation at 298 K and 223 K

In this paper, a comparative investigation was conducted on the tensile properties, microstructure evolution, and work hardening behavior of Fe18Mn0.6C and Fe18Mn1.0C TWIP steels at 298 K and 223 K by tensile tests and multi-scale microscopic characterization. With decreasing deformation temperature, two steels exhibit increased yield strength and ultimate tensile strength. However, the total elongation decreases for both, with Fe18Mn0.6C steel showing a more pronounced decrease. The work hardening behavior of two steels at 298 K and 223 K was analyzed. It is observed that the work hardening rate of both steels is higher at 223 K compared to 298 K, attributed to enhanced dislocation accumulation and the formation of deformation twins or martensite at lower temperatures. Particularly, Fe18Mn1.0C steel exhibits a continuous upward stage in work hardening at any temperature, driven by its higher dislocation density. On the other hand, Fe18Mn0.6C steel at 233 K shows a relatively stable trend in work hardening rate, attributed to the presence of martensite generated during deformation, which enhances its work hardening ability. Furthermore, the contribution of different strengthening mechanisms to the total flow stress was evaluated. The calculation results highlight dislocation strengthening as the primary source of flow stress in two steels at different temperatures. These findings provide theoretical support for understanding the mechanical properties of Fe–Mn–C TWIP steel at low temperature and optimizing the material design.

Investigation on estimation of fracture resistance of SS 316 LN welds subjected to various aging conditions and test temperatures

In this study, estimation of fracture resistance (K1c) was carried out for SS 316 LN welds subjected to various aging conditions. The welds were aged at 370, 475 and 550 °C and for 1000-20,000 h durations and were tested at room temperature (RT), 380 and 550 °C. Tensile properties and J-R curves were established for SS 316 LN welds after aging treatments at all test temperatures. Decrease in total elongation was observed after 20,000 h aging durations for all aging temperatures and all test temperatures, compared to as weld condition. From J-R curves, J1c values and equivalent Kj1c values were determined. Decrease in J1c was observed after 20,000 h aging durations for all aging temperatures at all test temperatures. Based on previously proposed model, an attempt was made to estimate fracture resistance (K1c) from tensile properties. Estimation of K1c was carried out for aged welds, including as weld condition at three temperatures. Estimated K1c values were within 1-30 % of the determined fracture toughness values. K1c estimation was also applied for similar type of austenitic stainless steel welds. It is summarized that previous methodology of K1c estimation is applicable for austenitic stainless steel welds.

Influence of heat treatment on the tensile properties of Ti–15Mo additively manufactured by laser metal deposition

A bidirectional powder deposition strategy was employed to additively manufacture Ti–15Mo wt% using laser metal deposition. The as-built alloy was subsequently subject to post-fabrication heat treatment. Microstructural characterisation was conducted in the heat-treated state and also after uniaxial tensile deformation. X-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy were employed in the heat-treated state. Electron backscatter diffraction was used in investigating the deformed microstructure. Columnar β grain refinement was achieved by fragmentation from a combined contribution from precipitated phases and deformation induced products. The three distinct microstructural zones, namely the fusion, remelted and heat affected zones, observed in each deposited layer of the as-built microstructure were retained after sub-β-solvus heat treatment but completely erased in the super-β-solvus microstructure. Accommodation of plastic deformation in β matrix was by a combination of slip and primary α′′ martensite which formed preferentially at grain boundaries. Elastic modulus decreased from 86.85 ± 0.45 GPa in the as-built alloy to 72.8 ± 0.65 GPa after heat treatment. Ultimate tensile strength of 1168± 1.12 MPa from the heat-treated sample represents only a marginal increase from that of the as-built sample of 1099± 2.3 MPa. This was accompanied by a small decrease in total elongation.

Synergistic effects of hybrid (HA+Ag) particles and friction stir processing in the design of a high-strength magnesium matrix bio-nano composite with an appropriate texture for biomedical applications

In this work, the effects of hybrid (HA + Ag) particles and triple-pass friction stir processing on the microstructure, texture, hardness, and tensile behavior of magnesium matrix bio-nano composite were investigated. The results showed that the mean grain size of samples was in the range of 1–5 μm owing to the occurrence of dynamic recrystallization and suppression of grain growth by second phase particles. All samples exhibited uniform dispersion of particles in the magnesium matrix caused by triple-pass FSP. However, some agglomerations were visible in the microstructure of AZ91/nHA nanocomposite. The average grain size of the AZ91/nHA/smAg sample (1.4 μm) was smaller than that of the AZ91/nHA/mAg sample (2.1 μm), which was attributed to the formation of higher content of MgxAgy precipitates in the AZ91/nHA/smAg composite. By performing the FSP, the content of Mg17Al12 was significantly decreased due to the dissolution of beta into the alpha caused by the breakup effect of mechanical stirring and temperature increase of samples. The AZ91/nHA/smAg sample had the highest texture parameter for the {101‾1} orientation as the high corrosion resistance texture. This was due to the promoting the non-basal slip caused by the dissolution of smAg particles in the magnesium matrix. After the FSP, the microhardness distribution of AZ91, AZ91/nHA, AZ91/nHA/mAg, and AZ91/nHA/smAg samples tended to be uniform and the average hardness was improved owing to the fragmentation of beta particles, grain refinement, and homogeneous dispersion of second phase particles. Compared with the AZ91/nHA/mAg sample, an increase in ultimate tensile strength (291.7 MPa), and a decrease in total elongation (5.6%) and energy absorption (12.3 J/cm3) were observed in the AZ91/nHA/smAg sample due to the formation of a higher content of the silver-rich precipitates in the AZ91/nHA/smAg sample during cooling caused by the higher solubility of silver submicron particles. The fracture surfaces of all processed samples consisted of a large number of fine equiaxed dimples (ductile fracture) owing to the grain refinement and the presence of fine second phase particles.

The role of thickness on the fracture behavior of Al/Mg–Li/Al composite processed by cold roll bonding

This paper aims to study the microstructure, fracture behavior, and other mechanical properties of a three-layered Al/Mg–Li/Al composite fabricated by the cold roll bonding (CRB) process. Alloying Mg with Li gave a double-phase magnesium-lithium alloy with high ductility, strengthened by bonding to two Al layers. By conducting CRB, the thickness of the composite reduced from 3 mm in the first stage to 1 mm in the last one, and in between, the mechanical and microstructural properties in five different thicknesses (2, 1.8, 1.6, 1.2, and 1 mm) were investigated. Grains were elongated in the rolling direction (RD) according to the pictures prepared by the optical microscope, and this elongation grows as the thickness of the composite reduces. Studying the mechanical properties, including the UTS, total elongation, and fracture behavior in two directions: rolling direction and traverse direction (TD), showed that UTS increases as the thickness decreases and is higher for TD samples in comparison to RD ones. Total elongation decreases by reducing the thickness, but for RD samples, it was lower than TD ones. The results show that both applied strain and thickness affect the fracture toughness simultaneously. The highest amount of fracture toughness does not belong to the sample with the highest thickness or the one that undergoes the highest strain during the CRB. For RD samples, the highest fracture toughness was recorded at the thickness of 1.6 mm, while for TD ones, it was at the thickness of 1.2 mm. SEM images of the fractured layers confirm the presence of dimples which are the reason for the ductile fracture, and this ductility reduces as the thickness goes down.

Austenite transformation and work hardening of medium manganese steel

The austenite transformation behavior and mechanical properties of medium manganese steel subjected to tensile strain were studied by electron backscatter diffraction, X-ray diffraction and tensile tests. The results show that the austenite phases are mainly distributed on the grain boundary in the duplex microstructure of austenite and ferrite, and it is easy for the big-size austenite to transform at small beginning tension strain following the mechanisms of both austenite (fcc) → e-martensite (hcp) → α-martensite (bcc) and austenite (fcc) → α-martensite (bcc). Both yield strength and tensile strength increase with the increase in pre-strain, and the total elongation decreases, while the value of pre-strain plus total elongation almost keeps constant. During tensile deformation, transformation from austenite into martensite improves work-hardening rate remarkably.

Strain Rate Dependent Mechanical Properties of TWIP Steel

The effect of strain rate on the work-hardening behavior of high-manganese twinning-induced plasticity (TWIP) steel has been investigated. The influence of adiabatic heating and deformation rate on the mechanical properties was studied by quasi-static and dynamic tensile tests with synchronous temperature and strain measurements. TWIP steel has excellent strain-hardening behavior under both quasi-static and dynamic loading conditions. Strain rate has negligible effects on yield and tensile strength, but the uniform and total elongation decreases under dynamic tests. TWIP steel has excellent energy absorption (EA) capacity of above 55 kJ/kg at all strain rates compared to dual-phase steels, transformation-induced plasticity steel and ferritic steels. However, the EA of TWIP steel is slightly lower compared to austenitic stainless steels. A rise in temperature due to adiabatic heating has led to the increase of stacking fault energy, thereby resulting in a change of twinning behavior or the promotion of dislocation glide under dynamic loading.

Influence of microstructure and pre-straining on the bake hardening response for ferrite-martensite dual-phase steels of different grades

This work aims to investigate the effects of pre-straining and microstructure on bake hardening (BH) response of dual phase (DP) steels. Four kinds of different grades of commercial DP steels, DP340/600, DP420/780, DP500/780 and DP550/980, were pre-strained in tension to 0.5%, 1%, 2%, 4%, 6% and 8%, then baked at 170°C for 20min followed by restraining, or just baked at 170, 190, 210, 230, 250°C for 20min and at 250°C for 40min followed by straining without pre-straining. Electron backscattered diffraction (EBSD) measurements and temperature-dependent internal friction measurements were conducted to characterize geometrically necessary dislocations (GNDs) and analyze solute carbon content and interactions between point defects and dislocations in DP steels. The results show that for all grades of DP steels investigated, BH values increase to peak values at pre-straining ranging from 0.5% to 2% and then decline with further pre-straining. At BH condition of 170°C/20min, peak BH values with pre-straining are 33MPa for DP340/600, 34MPa for DP420/780, 78MPa for DP500/780 and 90MPa for DP550/980 respectively. Pre-straining can cause increase in tensile strength and decrease in total elongation after baking, especially for DP500/780 and DP550/980. DP steels without pre-straining can reach very high BH0 values by applying either higher temperature or longer holding time. Solute carbon content in ferrite controls the speed of BH response in DP steels. Microstructures of higher volume fraction of martensite, smaller martensite islands and smaller ferrite grains can produce higher BH values.

Effect of Martensite Fine Structure on Mechanical Properties of an 1100 MPa Grade Ultra-high Strength Steel

An 1100 MPa grade ultra-high strength steel with different martensite fine structures, characterized by prior austenite grain size, martensite packet size, block width and lath width, was studied by various heat treatment processes. The result shows that with decreasing prior austenite grain size, both the packet size and block width decrease, while the lath width has virtually no change. Accordingly, both strength and toughness increase, while total elongation decreases. The yield strength has a Hall-Petch type relationship with the prior austenite grain size, packet size and block width, and the block width may be regarded as a key factor influencing strength. On the other hand, the ductile to brittle transition temperature (DBTT) is found to be more related to the packet size, which may be considered as a dominant factor influencing toughness.

Temperature Dependent Deformation Mechanisms of a High Nitrogen-Manganese Austenitic Stainless Steel

Now at Department of Materials Science and Engineering, 6531-G Boelter Hall, University of California, Los Angeles, CA 90095-1595, USA The influence of temperature on the deformation behaviour of a Fe-16.5Cr-8Mn-3Ni-2Si-1Cu-0.25N (wt%) austenitic stainless steel alloy was investigated using transmission electron microscopy and X-ray diffraction measurements. Recrystallized samples were deformedunder tension at -75°C, 20°C, and 200°C and the microstructures were characterized after 5% strain and after testing to failure.Deformation to failure at -75°C resulted in extensive transformation induced plasticity (TRIP) with over 90%
.
-martensite. The sample deformed to 5% strain at -75°C shows that the austenite transformed first to
0-martensite which served to nucleate the
.
-martensite. Transformation induced martensite prohibits localized necking providing total elongation to failure of over 70%. At room temperature, in addition to some TRIP behaviour, the majority of the deformation is accommodated by dislocation slip in the austenite. Some deformation induced twinning (TWIP) was also observed, although mechanical twinning provides only a small contribution to the total deformation at room temperature. Finally, dislocation slip is the dominant deformation mechanism at 200°C with a corresponding decrease in totalelongation to failure. These changes in deformation behaviour are related to the temperature dependence on the relative stability of austenite and martensite as well as the changes in stacking fault energy (SFE) as a function of temperature.

Effect of Grain Refinement on Tensile Properties of Ti-15-3 Alloy

【Titanium alloys have been attractive due to a high ratio of strength to weight as well as good corrosion resistance. However, strengthening causes a decrease in ductility in Ti alloys, as is usual in other alloys. For enhanced strength without ductility reduction, grain refinement and tensile properties were investigated as functions of thickness reduction of cold rolling and annealing condition in Ti-15V-3Cr-3Sn-3Al alloy with a ${\beta}$ single phase. The average grain size of the specimen, which was cold-rolled by 90% and annealed at 700 $^{\circ}C$ for 5 min, was decreased to approximately 19 ${\mu}m$ . The grain refinement of 63 μm to 19 ${\mu}m$ increased yield stress by 90 MPa without a significant decrease in total elongation. The Ti-15-3 alloy exhibited very low work hardening during tensile test at a crosshead speed of 2 mm/min. This result was discussed based on dynamic recovery associated with dislocation annihilation in grain boundaries.】

TWIN ASTIR: First tensile results of T91 and 316L steel after neutron irradiation in contact with liquid lead–bismuth eutectic

The TWIN ASTIR irradiation program [1] was aimed at determining the separate and possibly synergistic effects of a liquid lead–bismuth eutectic (LBE) environment and neutron irradiation. The materials in one capsule were irradiated in contact with LBE to a dose of about 1.5 dpa at a temperature between 460 and 490 °C and subsequently tested in liquid lead–bismuth eutectic environment at temperatures between 200 °C and 450 °C. This paper discusses the tensile results of T91 and 316L with and without irradiation while in contact with liquid lead–bismuth eutectic. The ferritic–martensitic T91 was softened by 50–100 MPa while the austenitic 316L steel showed a slight decrease in total elongation. Although no irradiation hardening was observed at the elevated irradiation temperature, the ferritic–martensitic T91 steel was prone to a certain extent of liquid metal embrittlement at 200 °C, 350 °C and 450 °C which was not affected by irradiation. The decrease in total elongation of the 316L is thought to be due to the neutron irradiation effect.

Liquid metal embrittlement susceptibility of ferritic–martensitic steel in liquid lead alloys

The susceptibility of the ferritic–martensitic steels T91 and EUROFER97 to liquid metal embrittlement (LME) in lead alloys has been examined under various conditions. T91, which is currently the most promising candidate material for the high temperature components of the future accelerator driven system (ADS) was tested in liquid lead bismuth eutectic (LBE), whereas the reduced activation steel, EUROFER97 which is under consideration to be the structural steel for fusion reactors was tested in liquid lead lithium eutectic. These steels, similar in microstructure and mechanical properties in the unirradiated condition were tested for their susceptibility to LME as function of temperature (150–450 °C) and strain rate (1 × 10 −3–1 × 10 −6 s −1). Also, the influence of pre-exposure and surface stress concentrators was evaluated for both steels in, respectively, liquid PbBi and PbLi environment. To assess the LME effect, results of the tests in liquid metal environment are compared with tests in air or inert gas environment. Although both unirradiated and irradiated smooth ferritic–martensitic steels do not show any or little deterioration of mechanical properties in liquid lead alloy environment compared to their mechanical properties in gas as function of temperature and strain rate, pre-exposure or the presence of surface stress concentrators does lead to a significant decrease in total elongation for certain test conditions depending on the type of liquid metal environment. The results are discussed in terms of wetting enhanced by liquid metal corrosion or crack initiation processes.

Effects of temperature and strain rate on the mechanical properties of T91 material tested in liquid lead bismuth eutectic

The effects of temperature and strain rate on the susceptibility of one of the most promising candidate materials for cladding and high temperature components for ADS, Ferritic/Martensitic steel (T91), to liquid metal embrittlement have been investigated in the temperature interval 150–450 °C, at strain rates between 1 × 10 −3 s −1 and 1 × 10 −6 s −1. The effect of the liquid lead bismuth eutectic has been evaluated by comparison between tests in the liquid metal and in Ar with 5% H 2. Although the untreated T91 material did not undergo liquid metal embrittlement and there was no apparent change in trends of the tensile properties as function of temperature or strain rate due to the liquid metal, the pre-exposed T91, which was exposed to oxygen poor LBE at 450 °C for 4000 h and tested at 450 °C did show a decrease in total elongation. The total elongation of the untreated T91 specimens in liquid lead bismuth and in H 2/Ar-gas mixture followed the same dependence on the temperature up to 245 °C. At temperatures above 375 °C no results are available from testing in controlled gas environment however reference data measured on the same batch of steel in air are used for comparison. Varying the strain rate did not cause any deterioration in the tensile properties of the material in liquid metal compared to those in gas atmosphere. The results are discussed in terms of self-healing mechanism and crack initiation processes.

Degradation in tensile and creep properties of 2.25Cr–1Mo steel by long term service in plant

Degradation in tensile and creep properties has been investigated for 2·25Cr–1Mo steel, after long term service at 577°C for 1·9 × 105 h. Creep tests were carried out at 550–690°C for up to about 10 000 h for the long term serviced material. The results are compared with those for virgin material tested for up to 100 000 h. The creep rupture time is shorter but creep ductility is larger for the long term serviced material than for the virgin material at high stress and short time conditions. The difference between the two materials becomes decreased with decreasing stress and increasing time. Microstructure evolution during long term service causes a softening and promotes dynamic recovery or recrystallisation during subsequent creep, which accelerates the onset of acceleration creep. This results in a higher minimum creep rate and a shorter rupture time for the long term serviced material than for the virgin material. The deviation from Monkman–Grant relationship is correlated with a decrease in total elongation.

A study of internal hydrogen embrittlement of steels

A novel procedure for hydrogen charging and studying the Internal Hydrogen Embrittlement (IHE) of steels is described here. A cylindrical notched tensile sample with an extended end is employed for hydrogen charging. The extended portion of the sample forms the cathode in an alkaline bath and a constant uni-axial tensile load is applied during hydrogen charging. The stress gradient set up by the notch, which is not in contact with the electrolyte, enhances the hydrogen concentration at various trapping sites of the matrix beyond the solubility limit. Subsequent to charging, the specimen is kept under the same load as that during charging, for another 24 h to stabilize the population of hydrogen within the specimen matrix. At the end of this stage, the specimen is tensile tested to failure at room temperature. Two different steels namely maraging and mild steels have been chosen to study the effect of hydrogen ingress on mechanical properties. While an increase in tangent modulus (linear portion of the stress–strain diagram), yield strength, work hardening rate and ultimate tensile stress (UTS) has been observed on hydrogenation, a decrease in total elongation has been noticed for both the steels studied. Fractographic investigation has revealed that the fracture mode is predominantly ductile dimple (failure by micro-void coalescence) in both the materials and that the hydrogen reduces the size of the dimples. The observations of this investigation are significant in two respects: firstly, it demonstrates the efficacy of a hydrogen charging method for steels which can introduce hydrogen to a level much higher than its solubility limit and secondly, it reports for the first time enhancement of modulus and work hardening by hydrogen charging. These observations have been rationalized on the basis of current understanding on the effect of hydrogen on plastic properties and hypothesis of the models of IHE. It is suggested that the trapping of hydrogen by dislocations and other structural features of the matrix and the mutual interactions of their strain fields can account for the observed effects on yield strength, tangent modulus, work hardening rate, UTS and ductility.

Modeling of Irradiation Embrittlement of Reactor Pressure Vessel Steels

A model to describe the change in the inelastic and fracture properties of reactor pressure vessel steels due to neutron irradiation in the ductile region (i.e., irradiation embrittlement) is developed. First, constitutive equations for unirradiated elastic-viscoplastic-damaged materials are developed within the framework of the irreversible thermodynamics theory. To take into account the effect of hydrostatic pressure on the nucleation and growth of microvoids, properly defined dissipation potential is used. Then, the effect of irradiation on the material behavior is incorporated into the proposed model as a function of neutron fluence Φ by taking into account the interaction between irradiation-induced defects and movable dislocations. As regards the damage strain threshold pD, the mechanism of void nucleation due to pile-up of dislocations at the inclusions in the material is proposed first under unirradiated-condition, and then the effect of irradiation on the mechanism is formulated. In order to demonstrate the validity of this model, it is applied to the case of uniaxial tensile loading of a low alloy steel A533B cl. 1 for the pressure vessel use of light-water reactors at 260°C. The resulting model can describe the increase in yield stress and ultimate tensile strength, the decrease in total elongation and strain hardening, and the strain rate dependence of yield stress due to neutron irradiation. [S0094-4289(00)00901-4]

Influence of thermal aging on tensile and impact bending properties of the steel grades OPTIFER and F82H mod.

The low activation martensitic chromium steels OPTIFER Ia, Ib, II, III and IV and F82H mod. were subjected to aging by annealing between 550°C and 650°C for up to 10,000 h. In general, this caused a decrease in tensile strength by up to 10%. The decrease in total elongation depended on the thermal treatment of the material. Impact bending strength was hardly affected by aging. With one exception, all tested steels turned out to be sufficiently resistant to aging.

Effect of Neutron Irradiation on Yield Stress and Ductility of Reduced-Activation Martensitic 9Cr–WVTa Steels

The tensile properties of reduced-activation martensitic 9Cr-IWVTa and 9Cr-3WVTa steels for fusion reactors were investigated over the temperature range from room temperature to 873 K after neutron irradiation in the Japan Materials Testing Reactor at 538 K to fast neutron fluences of 2×10 23 and 3×10 24 n/m 2 (E>1 MeV). A conventional 9Cr-1MoVNb steel was also examined for comparison. The irradiation caused an increase in yield stress and a decrease in total elongation. Irradiation-produced defect clusters could not be seen directly by transmission electron microscopy. The increase in yield stress caused by irradiation Δσ y was proportional to the 1/4 power of the neutron fluence

Dislocation Pinning and Channeling in a Neutron-Irradiated 10 mass%Cr-30 mass%Mn Austenitic Steel

The deformation behavior of a 10 mass%Cr-30 mass%Mn-0.1 mass%C austenitic steel was investigated over the temperature range from room temperature to 873 K after neutron irradiation in JMTR at 573 K to a fast neutron fluence of 8.5×10 22 n/m 2 (E>1 MeV). A high density of irradiation-produced dislocation loops with the size of about 5 nm and 10-30 nm was observed. Irradiation caused an increase in yield stress with a prominent yield point and a decrease in total elongation near room temperature. These behaviors became diminished with increasing test temperature