Decrease In Uniform Elongation Research Articles

Microstructure Evolution and Tensile Properties of 304L Stainless Steel Subjected to Surface Mechanical Attrition Treatment

A gradient nanostructured layer (GNsL) was generated on the two sides of bulk sample in 304L stainless steel by means of the surface mechanical attrition treatment. The microstructure of the GNsL was characterized via TEM observation. The prominent microstructural features involve the intersection of multi-system twin operation, subdividing the original grains into blocks, a martensite transformation mainly occurring at the interface of the twins as well, and the randomly orientated nanocrystallites at the top of surface. After annealing at 750°C for 10 min, recovery had occurred and the dislocation density was much reduced. The vast majority of the grains at the top surface were in the nanocrystalline/ultrafine range, with some recrystallization regions. The uniaxial tensile tests were performed to obtain the mechanical property of bulk samples with GNsL. The yield strength was about 2 times higher than that of the coarse-grained counterpart, but with a decrease in uniform elongation and elongation to failure as well. The relationship between the microstructure and mechanical property was discussed in detail.

Effect of Si on Mechanical Property of Galvannealed Dual Phase Steel

The mechanical properties of galvannealed dual phase steels bearing Si were investigated, and then the effect of Si on work hardening behavior during tensile deformation was discussed. Both tensile strength and yield strength were increased with increasing Si content, especially increasing ratio of tensile strength was larger than that of yield strength. On the other hand, the decrease of uniform elongation was very small with increasing Si content. Therefore, tensile strength and total elongation balance was improved by Si addition. High Si (1.2%Si) steel exhibited higher work hardening rate than low Si (0.01%Si) steel in all strain region. Dislocation cell structures were observed at 8% tensile strain in low Si steel, while dislocation cell structures were scarcely observed and instead, tangled dislocations were dominantly observed in high Si steel. Furthermore, high Si steel exhibited higher the increasing ratio of dislocation density in ferrite during tensile deformation than low Si steel. These results indicated that the formation of dislocation cell structure was retarded by Si addition, which led to the increase of work hardening rate and resulted in the improvement of tensile strength and total elongation balance.

Bake hardening behavior of TRIP and DP steels

The bake hardening (BH) behavior of transformation-induced plasticity (TRIP) and dual-phase (DP) steels after different prestrains was studied. The experimental results indicate that TRIP steel exhibits good BH ability while DP steel does not, and prestrain displays a strong effect on the BH values of both steels. The comparison of microstructures of the two steels showed that the hard second phase in the matrix might be harmful to the BH ability. For deformed specimens, baking resulted in a loss of uniform elongation, but there was no obvious decrease in uniform elongation for unprestrained specimens.

Effect of Temperature and Strain Rate on Dynamic Properties of Low Silicon TRIP Steel

Abstract The dynamic tensile test of 0.11C-0. 62Si-1.65Mn TRIP steel was carried out at different strain rates and test temperatures. The results show that both temperature and strain rate affect the retained austenite transformation. At high strain rates, the uniform elongation decreases, whereas the total elongation and energy absorption increase. The tensile strength is less strain rate sensitive. With raising test temperature, the tensile strength is reduced and the mechanical properties generally deteriorate, especially at 110 °C. However, excellent mechanical properties were obtained at 50 °C and 75 °C.

Continuous Grain Refinement Using Severe Torsion Straining Process

This study presents a new rapid continuous process for grain refinement in metallic materials through severe plastic deformation (SPD). The new process, designated in this study the severe torsion straining process (STSP), is applicable to a wide range of alloys based on aluminum, magnesium and copper including carbon steel. This process consists of producing a local heated zone in a rod and cooling both sides of the heated zone by spray water while rotating one end with the other. Thus, torsion strain is introduced in the local heated zone. The STSP can be continuous because the straining is achieved while the rod is shifted along the longitudinal axis of the rod. Furthermore, the process requires no die, suggesting a potential for commercialization of grain refinement through SPD. In this study, STSP was applied to an Al-Mg alloy and a Mg-Al-Zn alloy. It is shown that STSP is effective for both alloys so that the grain size is reduced to ~1.5 μm for the Al alloy and ~0.9 μm for the Mg alloy. Tensile testing showed that the strength is increased with a minimal decrease in uniform elongation. There is a critical ratio between rotation speed and moving speed, which defines the feasibility of STSP operation without breaking the rod. The grain size tends to be lowered as the ratio is close to the critical value.

Microstructural evolution of both as-irradiated and subsequently deformed microstructures of 316 L stainless steel irradiated at 30–160 °C at LANSCE

Specimens of 316L stainless steel were irradiated to 0.5–10.3dpa at 30–80°C with a mixture of 500–800MeV protons and spallation neutrons at the Los Alamos Neutron Science Center (LANSCE). Tensile test results of irradiated 316L reported earlier had showed hardening and embrittlement with increasing irradiation dose, with significant irradiation hardening occurring at a dose of as low as 0.5dpa. Transmission electron microscope (TEM) examination of the irradiated microstructure of 316L showed black-spot damage (small loops) and somewhat larger faulted Frank loops to produce the hardening. There was an initial decrease in uniform elongation at low dose levels from 49% (unirradiated) to 30% at 1.1dpa, followed by a second, rather abrupt contribution to ductility loss at higher doses (∼2.5dpa) from 21% at 2.5dpa to 0.5% at 3dpa. This second drop in ductility was not accompanied by any visible new or enhanced microstructural development. In the current study additional transmission electron microscope investigation was conducted on both as-irradiated and irradiated plus subsequently deformed 316L in the vicinity of the second abrupt ductility loss (∼2.5dpa). The steel was observed to deform mainly by twinning and no brittle phases were found in the deformation microstructure. It is proposed that gas accumulation with increasing dpa, especially of hydrogen, may be a contributor to this second abrupt decrease in uniform elongation. Although the retained gas (helium and hydrogen) levels approached ∼0.6at.% total at the highest exposure level, no discernible cavities were observed.

Yield and Plastic Deformation of Mg-Alloy AZ31 at Elevated Temperatures

The yield and plastic deformation behaviour of wrought Mg-alloy AZ31 sheets were investigated at elevated temperatures from ambient to 300°C. It is found that the 0.2% proof stress and ultimate strength decrease linearly with increasing temperature, and the Young’s modulus goes down in the same way. Whereas the ductility, expressed by elongation, increases with temperature, in contrast to the decrease in uniform elongation. The plastic deformation anisotropy, represented by the ratio of transverse and thickness strain of the plate sample, decreases as the temperature increases, and at about 225°C, coincidentally, reaches to unity. The activation volumes for yield stress and ultimate strength are discussed with a thermal activation law-Arrhenius Equation.

Strain Rate and Temperature Dependent Plastic Deformation Behavior of Low-Carbon TRIP Steel

Low-carbon TRIP steel has high strength and excellent formability and is one of the key materials to meet the demand for weight reduction of automobile body parts. However, in order to disseminate the materials, the clarification of a complex plastic deformation mechanism depending on the strain rate and temperature and formulation of a proper constitutive equation is essential. The present investigation experimentally clarifies the quantitative effects of the strain rate and temperature on the material characteristics by means of tensile tests under strain rates from 1.7×10-4 to 1.7 x 10-1 and temperatures ranging from -50 to 250°C. The deformation and transformation behaviors substantially change in the range of 100 to 150°C. With regard to the relatively the low temperatures up to 100°C, retained austenite transforms into martensite. The increase in strain rate causes a rise in the flow stress of the ferrite phase and an increase in temperature, which suppresses the martensitic transformation. As a balancing effect, the stress-strain relationship expresses a high degree of strain rate independency. In the later stage of uniform deformation, the effective TRIP effect declines under the high strain rate and the uniform elongation decreases. However, the tensile strength is almost unchanged due to the very small amount of work hardening. Meanwhile, for the high temperature range above 150°C, the retained austenite transforms into bainite which requires a longer transformation time, which yields the strong strain rate dependency of the deformation behavior and the tensile strength and uniform elongation increase as the strain rate decreases.

Effects of temperature and strain rate on tensile properties and activation energy for dynamic strain aging in alloy 625

Alloy 625 ammonia cracker tubes were service exposed for 60,000 hours at 873 K. These were then subjected to a solution-annealing treatment at 1473 K for 0.5 hours. The effects of temperature and strain rate on the tensile properties of the solution-annealed alloy were examined in the temperature range of 300 to 1023 K, employing the strain rates in the range of 3×10−5 s−1 to 3×10−3 s−1. At intermediate temperatures (523 to 923 K), various manifestations of dynamic strain aging (DSA) such as serrated flow, peaks, and plateaus in the variations of yield strength (YS) and ultimate tensile strength (UTS) and work-hardening rate with temperature were observed. The activation energy for serrated flow (Q) was determined by employing various methodologies for T<823 K, where a normal Portevien-Le Chatelier effect (PLE) was observed. The value of Q was found to be independent of the method employed. The average Q value of 98 kJ/mol was found to be in agreement with that for Mo migration in a Ni matrix. At elevated temperatures (T≥823 K), type-C serrations and an inverse PLE was noticed. The decrease in uniform elongation beyond 873 K for 3×10−5 s−1 and 3×10−3 s−1 and beyond 923 K for 3×10−4 s−1 strain rates seen in this alloy has been ascribed to reduction in ductility due to precipitation of carbides and δ phase on the grain boundaries.

Embrittlement of RAFM EUROFER97 by implanted hydrogen

About 380 μm thick specimens of low-activation martensitic stainless steel EUROFER97 were homogeneously implanted with protons below about 70 °C to concentrations up to about 1200 appm. Tensile tests were performed at 25 and 200 °C. The tests at 25 °C showed an increase of yield stress and ultimate tensile strength and a decrease of uniform elongation and elongation to fracture, while effects at 200 °C were strongly reduced. Scanning electron microscopy showed virtually no change of the ductile, transgranular fracture mode by the implanted H. A slight decrease of necking was observed only at the highest concentrations. The results are compared to previous measurements on F82H-mod and to literature results on tensile tests after He implantation and neutron irradiation. F82H-mod specimens were also implanted under applied tensile stress to concentrations up to 1900 appm. Straining was ascribed to accumulation of atomic defects, but no fracturing occurred. Thermal desorption measurements are included on H-implanted and tensile tested F82H-mod specimens. The results show significant data scatter, even within one specimen, with the highest measured values being in accordance with the implanted amount. In general, the H content is decreased after testing at higher temperatures, but even after testing at 350 °C, some specimens contain significant amounts.

Microstructural origins of radiation-induced changes in mechanical properties of 316 L and 304 L austenitic stainless steels irradiated with mixed spectra of high-energy protons and spallation neutrons

A number of candidate alloys were exposed to a particle flux and spectrum at Los Alamos Neutron Science Center (LANSCE) that closely match the mixed high-energy proton/neutron spectra expected in accelerator production of tritium (APT) window and blanket applications. Austenitic stainless steels 316 L and 304 L are two of these candidate alloys possessing attractive strength and corrosion resistance for APT applications. This paper describes the dose dependence of the irradiation-induced microstructural evolution of SS 316 L and 304 L in the temperature range 30–60°C and consequent changes in mechanical properties. It was observed that the microstructural evolution during irradiation was essentially identical in the two alloys, a behavior mirrored in their changes in mechanical properties. With one expection, it was possible to correlate all changes in mechanical properties with visible microstructural features. A late-term second abrupt decrease in uniform elongation was not associated with visible microstructure, but is postulated to be a consequence of large levels of retained hydrogen measured in the specimens. In spite of large amounts of both helium and hydrogen retained, approaching 1 at.% at the highest exposures, no visible cavities were formed, indicating that the gas atoms were either in solution or in subresolvable clusters.

Effect of Morphological Change of Carbide on Elongation of Boron-bearing Al-killed Steel Sheets.

It has been reported that boron has a beneficial effect on the mechanical properties of continuously annealed Al-killed steel sheets. However, the effect of boron on elongation of boron-bearing steels had not been fully investigated. In this study, the effect of boron on elongation was investigated for boron-bearing Al-killed steel sheet by analyzing the substantial effects of fine carbides precipitating in matrix and at the grain boundaries. Using scanning electron microscope and image analyzer, an investigation was made for the precipitation site and the size distribution of fine carbides caused by the variation of excess boron and over-aging temperature. The effects of fine carbides on the tensile elongation of the steels were examined. The carbide morphology and the precipitation site changed with excess boron since the excess boron accelerated carbide precipitation in matrix by segregating at the grain boundaries. In the case where uniform elongation and post-uniform elongation were evaluated by n-value and m-value respectively, n-value decreased with increasing amount of fine carbides in matrix. Furthermore, excess boron deteriorated n-value irrespective of carbide morphology. On contrast, m-value rose with a decrease in the diameter of carbide at the grain boundary irrespective of small amount of excess boron. It was indicated that the deterioration of elongation by the small amount of excess boron was caused by a decrease in uniform elongation, which might be attributed to fine carbides in matrix and boron in solution.

Influence of neutron irradiation on deformability and fracture micromechanisms of titanium α-alloys

The deformability and fracture appearance of neutron irradiated Ti–Zr alloy were investigated. It was shown that irradiation at 200–250°C resulted in a drastic decrease of uniform elongation. The material irradiated at 350°C and over was characterized by high deformability. The dimple ductile transgranular fracture was found to be typical for the examined material both in initial state and after the low temperature irradiation. Elements of ductile intergranular fracture were observed in the material after neutron irradiation at 400°C.

Effect of fast-neutron irradiation on tensile properties of precipitation-hardened Cu-Cr-Zr alloy

High-strength, high-conductivity copper alloys are suitable heat-sink materials for the divertor of magnetically confined fusion reactors subject to high thermal loads. Tensile specimens of a commercial high-strength Cu, Cr, Zr alloy were irradiated in the HFR reactor in Petten at 150 and 250°C. The total accumulated damage (10 dpa) and the damage production rate (3.3 × 10 −7 dpa/ s) correspond to the envisaged operating requirements of the ITER divertor. Tensile testing after irradiation was performed in the LMA hot laboratory at JRC Ispra, at a single constant strain rate of 2.9 × 10 −4/ s. At 155°C an increase of yield strength (YS) was measured and a decrease of uniform elongation (UE) from 5% to virtually zero. Persisting high total elongation (TE) values of about 9% indicate that the ductile fracture mode is not affected. Irradiation hardening at 255°C is less pronounced. In the light of this study and of published results, the temperature of 275°C is proposed as a conservative temperature limit for use of fully precipitation hardened Cu-Cr-Zr (Mg) alloys to exclude irradiation softening.

Tempering characteristics of a vanadium containing dual phase steel

Dual phase steels are characterized by a microstructure consisting of ferrite, martensite, retained austenite, and/or lower bainite. This microstructure can be altered by tempering with accompanying changes in mechanical properties. This paper examines such changes produced in a vanadium bearing dual phase steel upon tempering below 500 °C. The steel mechanical properties were minimally affected on tempering below 200 °C; however, a simultaneous reduction in uniform elongation and tensile strength occurred upon tempering above 400 °C. The large amount of retained austenite (≅10 vol pct) observed in the as-received steel was found to be essentially stable to tempering below 300 °C. On tempering above 400 °C, most of the retained austenite decomposed to either upper bainite (at 400 °C) or a mixture of upper bainite and ferrite-carbide aggregate formed by an interphase precipitation mechanism (at 500 °C). In addition, tempering at 400 °C led to fine precipitation in the retained ferrite. The observed mechanical properties were correlated with these microstructural changes. It was concluded that the observed decrease in uniform elongation upon tempering above 400 °C is primarily the consequence of the decomposition of retained austenite and the resulting loss of transformation induced plasticity (TRIP) as a contributing mechanism to the strain hardening of the steel.

The effects of oxygen on deformation and fracture of polycrystalline hafnium

The effects of O in solution on the mechanical properties of crystal-bar Hf were investigated. It was found that oxygen produces a significant increase in yield strength, a modest increase in strain hardening rate, and a drastic decrease in uniform elongation of hafnium at both 298 and 77/sup 0/K. The fracture mode of unalloyed hafnium is primarily dimpled rupture to temperatures as low as 4.2/sup 0/K, but the addition of as little as 0.485 at. percent oxygen causes a shift to cleavage at 77/sup 0/K. Cleavage also was observed at 298/sup 0/K with 1.45 at. percent oxygen. It is concluded that the suppression of cross slip by oxygen favors cleavage in hafnium. The cleavage plane was identified as (2130), but the possibility exists of more than one cleavage plane. (JRD)

Dynamic strain aging as a strengthening mechanism in iron and iron-phosphorus alloys

A study has been conducted to determine the effects of strain aging in tension at elevated temperature (dynamic strain aging) on the room-temperature mechanical properties of impure iron containing up to 0.1% phosphorus. The results show that the increase in yield strength after prestraining at 100°C (dynamic strain aging) is about twice as great as that resulting from prestraining the same amount at room temperature and then aging at 100°C (static strain aging). This greater increase in yield strength is accompanied by only a slight decrease in uniform elongation. The ultimate tensile strength is similar after either strain-aging treatment.

軟鋼の圧縮変形抵抗におよぼす歪速度の影響

Compressive deformation resistance of a killed carbon steel with 0.14%C was investigated at −78°C and room temperature as a function of the strain rate. The nominal strain rates employed in this experiment were 2×10−4, 5×10−3, 3×10−1, 4.3 and 3.7×102 1/sec. The main results are summarized as follows:Yield stresses and deformation resistances at three strains of 10, 20 and 30% had two strain rate sensitivities, respectively, i.e., the smaller sensitivity in the range of lower strain rates and the larger one in the range of higher strain rates. The rate sensitivity became smaller with increasing temperature and strain.Although the sensitivity for the compressive yield stress was quite similar to that for the tensile yield stress, the sensitivity for the compressive flow stress at a given strain was considerably smaller than that for the tensile stress. This difference in the rate sensitivities for both flow stresses resulted from the remarkable decrease in the coefficient of friction at the ends of the compression test piece with the deformation speed.The strain hardening exponent n in the equation σa=Cεαn described as a flow curve decreased with the strain rate, but it was somewhat larger, owing to the action of friction, than the exponent obtained from the tension test. The decrease in n corresponded well to that of total elongation in the tension specimen. Therefore, the decrease in n is also considered to suggest the decrease in uniform elongation with the strain rate.