Induced Plasticity Effect Research Articles

Fatigue behavior and retained austenite transformation of Al-containing TRIP steels

In material selection for the design of advanced lightweight automotive steel components, fatigue performance is of particular significance. High strength TRIP steels offer very good cyclic behavior, especially under cyclic plastic strains, which is assisted by the Transformation Induced Plasticity effect (TRIP). In the present study the TRIP effect has been quantified under both elastic (HCF regime) and plastic (LCF regime) cyclic strains for two Al-containing TRIP steels with similar chemical composition and different initial retained austenite (RA) content. The results illustrate that transformation behavior differs for the two materials under elastic and plastic cyclic straining and fatigue behavior is in both cases linked to the amount of RA transformation. The latter is discussed in the paper considering relevant RA microstructural aspects like content and particle size. In the investigation the fatigue crack initiation resistance of the materials has been experimentally evaluated, resulting in different damage tolerance ability for the two steels. A numerical simulation is developed to determine the local strains at the notch tip under monotonic loading conditions and is used with the LCF material characteristics to discuss the differences obtained in crack initiation resistance.

Role of brain-derived neurotrophic factor in the plasticity of enteric nervous-smooth muscle and its effects on gastrointestinal motility in mice

Objective To observe the role of brain-derived neurotrophic factor (BDNF) in the plasticity of enteric nervous-smooth muscle system, and to investigate the effects of BDNF induced plasticity on gastrointestinal motility in mice. Methods Male hybrid BDNF knockout (BDNF+ /-) mice and wild type (BDNF+ /+ ) mice were selected, eight in each group. Gastrointestinal motility of BDNF+ /+ mice and BDNF+ /- mice were tested and compared. Longitudinal muscle strips of mice colon smooth muscle were prepared.The effects of carbachol (1×10-5 mol/L) and BDNF (1×10-7 mol/L) on contractile function of muscle strips were observed. And the effects of tetrodotoxin (TTX, 1×10-6 mol/L) on BDNF induced contractile function of muscle strips were also studied. The changes of the density of mice intestinal myenteric plexus and the expression of smooth muscle α-actin (α-SMA) in colon smooth muscle were detected by immunohistochemical techniqne. The ultrastructural alterations of myenteric plexus, neuromuscular junction (NMJ) and smooth muscle cells were detected by transmission electron microscope (TEM). T-test or Rank sum test was performed for comparison between groups. Results Number of feces particles and water content in feces of BDNF+ /- mice ((3.80±0.75) and (39.60±1.47)%) were both lower than those of BDNF+ /+ mice ((6.30±1.03) and (51.00±1.61)%), and the differences were statistically significant (t=4.792, 12.827; both P<0.05). Carbachol (1×10-5 mol/L) could significantly increase contraction activity of smooth muscle of BDNF+ /+ mice (R=3.26±0.43) and BDNF+ /- mice (R=2.15±0.36), and the difference was statistically significant (t=15.754, 9.632; both P<0.05). The effects on contraction exciting of smooth muscle strips of BDNF+ /+ mice were more significant than BDNF+ /- mice, and the difference was statistically significant (t=5.972, P<0.05). BDNF could significantly increase contraction of muscle strips of BDNF+ /+ mice and R value increased from 1 to 1.41±0.09, and the differences were statistically significant (t=13.674, P<0.05). TTX could obviously inhibit the excitatory effects of BDNF, R value decreased from 1.41±0.09 to 1.03±0.04 (t=11.692, P<0.05). The density of myenteric plexus of BDNF+ /- mice (median 5.8%, interquartile range 4.2%-7.0%) was significantly lower than that of BDNF+ /+ mice (median 9.0%, interquartile range 7.1%-10.8%), and the difference was statistically significant (Z=3.730, P<0.05). The expression of α-SMA of BDNF+ /- mice (median 33.4%, interquartile range 28.8%-38.5%) was significantly lower than that of BDNF+ /+ mice (median 44.6%, interquartile range 39.2%-48.8%), and the difference was statistically significant (Z=4.565, P<0.05). The results of TEM indicated ultrastructural alterations of myenteric plexus, NMJ and smooth muscle in BDNF+ /- mice. Conclusionss BDNF could induce the plasticity of morphology and function in enteric nervous-smooth muscle system, which may play an important role in mice gastrointestinal motility. Key words: Brain-derived neurotrophic factor; Enteric nervous system; Plasticity of smooth muscle; Gastrointestinal motility; Mice

Effect of retained austenite on wear resistance of nanostructured dual phase steels

Nanostructured super bainitic and quenching–partitioning (Q&P) martensitic steels with a significant amount of retained austenite obtained by low temperature bainitic transformation and Q&P respectively were studied to explore the effect of retained austenite on stirring wear resistance. The results suggest that the Q&P martensitic steel significantly enhanced the hardness of the worn surface (from 674 to 762 HV1) and increased the thickness of the deformed layer (∼3.3 μm), compared to the nanostructured bainitic steel. The underlying reason is that the Q&P martensitic steel has a higher stability of retained austenite thereby providing a superior transformation induced plasticity effect to increase surface hardness and reduce wear rate during the wear process.

An in-situ neutron diffraction study of a multi-phase transformation and twinning-induced plasticity steel during cyclic loading

In-situ neutron diffraction during cyclic tension-compression loading (∼+3.5% to −2.8%) of a 17Mn-3Al-2Si-1Ni-0.06C steel that exhibits concurrent transformation and twinning -induced plasticity effects indicated a significant contribution of intragranular back stresses to the observed Bauschinger effect. Rietveld analysis revealed a higher rate of martensitic transformation during tension compared to compression. Throughout cycling, α′-martensite exhibited the highest phase strains such that it bears an increasing portion of the macroscopic load as its weight fraction evolves. On the other hand, the ε-martensite strain remained compressive as it accommodated most of the internal strains caused by the shape misfit associated with the γ→ε and/or ε→α′ transformations.

Fe-Mn-Al轻质高强钢组织和力学性能研究 *

对热轧态与固溶处理后Fe-Mn-A1轻质高强钢进行力学性能检测及组织形貌观察，分析950～100℃固溶处理工艺对其组织和力学性能的影响规律．根据真实应力一应变曲线和加工硬化曲线分析拉伸变形特征，对比拉伸变形前后微观组织形貌和XRD谱，研究其微观变形机理．研究结果表明，所设计的成分体系实验用钢，热轧后为奥氏体基体与少量带状铁素体的双相组织，密度为6．55g／cm3，达到了轻质高强的设计目标．固溶处理有利于奥氏体晶粒长大与带状铁素体的破碎分解，使钢板强度降低而塑性提高，但是过高的固溶温度会促进铁素体长大，使铁素体体积分数增大，钢的断后伸长率降低．1050℃固溶处理后Fe-Mn-A1钢抗拉强度为925．9mPa，断后伸长率为50．20％，强塑积为46．48GPa％．连续的应变强化行为使得Fe-Mn-A1钢获得高强度与塑性的良好匹配，稳定硬化阶段应变范围越宽，断后伸长率越大；较高的层错能使其变形机理区别于TRIP和TWlP效应，变形后仍为奥氏体＋铁素体双相组织，变形后奥氏体中可以观察到Taylor晶格、高密度位错墙以及微带结构，为明显的平面滑移特征．

Evolution of microstructure and mechanical properties during thermomechanical processing of a low-density multiphase steel for automotive application

The evolution of the microstructure and mechanical properties in a low-density, low-alloy steel containing 6.57Al–3.34Mn–0.18C (wt.%) has been investigated as a function of processing. The steel was designed to have a duplex microstructure with ferrite as the major phase and austenite as the minor phase within the temperature range 800–1350°C. The steel was processed to sheet form, which was thermomechanically treated and characterised by a variety of techniques such as optical metallography, scanning electron microscopy, electron backscatter diffraction, electron probe microanalysis, X-ray diffraction, transmission electron microscopy, tensile testing and density and elastic modulus measurements. The amount of austenite was found to decrease with an increase in the annealing temperature. While C and Mn partitioned into the austenite phase, Al partitioned into the ferrite. The tensile elongation increased with an increase in the amount of austenite in the microstructure. However, the austenite showed only a small transformation induced plasticity effect during tensile deformation due to its high stability. After annealing, mostly κ-carbide precipitates with a (Fe+Mn)/Al ratio of 3.6 appeared in the ferrite matrix, along with some cementite and complex carbides. These precipitates became coarser (330nm) during a short time overageing treatment at 400°C, causing a significant increase in elongation. The κ-carbides were found to have a close to Nishiyama–Wasserman type orientation relationship with the ferrite matrix. Although the Young’s modulus of the steel dropped due to the large amount of Al added, the reduced density was found to be beneficial for automotive applications, overriding the effect of a drop in Young’s modulus.

Microstructure evolution in TRIP-aided seamless steel tube during T-shape hydroforming process

Transformation-induced plasticity aided seamless steel tube comprising of ferrite, bainite, and metastable austenite was processed through forging, piercing, cold-drawing, and two-stage heat treatment. T-shape hydroforming is a classic forming method for experimental research and practical production. The current work studied austenite-to-martensite transformation and microcrack initiation and propagation of the tube during T-shape hydroforming using electron backscattering diffraction, scanning electron microscopy, and transmission electron microscopy. The strain distribution in the bcc-phase and fcc-phase was studied by evaluating changes in the average local misorientation. Compared to the compressive stress, metastable austenite with similar strain surrounding or inside the grains transformed easier under tensile loading conditions. The inclusions were responsible for microcrack initiation. The propagation of the cracks is hindered by martensite/austenite constituent due to transformation induced plasticity effect. The volume fraction of untransformed retained austenite decreased with increase in strain implying transformation-induced plasticity effect.

Interface Phenomena Responsible for Bonding between TRIP Steel and Partially Stabilised Zirconia as Revealed by TEM

In this study, we demonstrate results of the transmission electron microscopy investigations of phenomena at the interface between partially stabilized zirconia (PSZ) and TRIP steel (steel with the Transformation Induced Plasticity effect). The interface phenomena were studied in thin film samples, which were prepared as a model system by the deposition of PSZ on TRIP steel using magnetron sputtering, and in a bulk composite containing TRIP steel and PSZ powder. The bulk composite was produced by Spark Plasma Sintering. The phase composition and the orientation relationship of several crystallites of PSZ and TRIP steel in the vicinity of their mutual interface were analysed using high resolution transmission electron microscopy (HRTEM). The phase identification was facilitated by using energy dispersive X-ray spectroscopy and electron energy loss spectroscopy (EELS) in scanning transmission electron microscope. In particular, the analysis of the zirconium N2,3 edge in the electron energy loss spectra allowed the cubic, tetragonal and monoclinic ZrO2 phases to be distinguished from each other. Thus mainly EELS complemented the HRTEM analysis of the orientation relationships. The same experimental methods were utilized to follow the local phase transformations near the PSZ/TRIP steel interfaces after a mechanical deformation of the samples.

Effect of annealing temperature on transformation induced plasticity effect of a lean duplex stainless steel

Abstract Effects of annealing temperature on the mechanical property and martensite transformation of a new lean duplex stainless steel have been investigated. It was found that the elongation increased with the annealing temperatures and reached the maximum of 61.2% after annealing at 1150 °C. A sequential γ → e → α ' martensite transformation was generated during tensile and brought the transformation induced plasticity effect. More than 30% α ′ martensite was induced after tensile in the specimens annealed at 800 ~ 1000 °C, whereas only 8.8% α ′ was induced at 1150 °C. The M d (30/50) temperatures of austenite phase reduced from 97 °C to 46 °C with the annealing temperatures varying from 800 °C to 1150 °C. Therefore the austenite phase became more stable and transformed to e and α ′ martensite phase gradually in a wide region and thus caused better plasticity. This also indicated that γ → e transformation was important for transformation induced plasticity effect.

Exploring Carbide-Free Bainitic Structures for Hot Dip Galvanizing Products

Recently, advanced high strength steels for automotive applications were designed to achieve a carbide-free bainitic microstructure after conventional thermo-mechanical processing and a continuous annealing treatment. The microstructure obtained consists of ferrite laths interwoven with thin films of untransformed retained austenite. The sufficiently tough matrix and the control of the heterogeneity in the microstructure allowed an optimum combination of strength, ductility, and formability to be achieved. However, this work probed that even using a devoted theoretical alloying, carbide-free bainitic steels are hardly compatible with a conventional hot dip galvanizing annealing process, without any consideration about Zn coatability and adherence. The hardenability of the designed alloys was insufficient in some cases and the amount of austenite retained in the microstructure was too low or/and mechanically too unstable for high ductility. As a consequence, the obtained mechanical properties were comparable to those in high-Si dual phase steels without a beneficial transformation induced plasticity effect.

Fatigue Behavior of Dual‐Phase and TWIP Steels for Lightweight Automotive Structures

AbstractCar bodies are increasingly made with high‐strength steels, for both lightweighting and safety purposes. Steel sheets, made by continuous casting, hot rolling, cold rolling, and continuous heat treating, are used to deep draw the car body parts, which are then joined by resistance spot welding (RSW). Two high‐strength automotive steels, with similar tensile strength, are studied here. The low alloy, dual‐phase steel consist of ferrite and martensite, obtained by an intercritical heat treatment, followed by fast cooling. The innovative, high‐Mn TWIP steel exhibits a promising combination of strength and toughness, arising from the austenitic structure, strengthened by C, and from the twinning induced plasticity effect. Tensile specimens are fatigue tested at room temperature with zero load ratio, both in the as‐fabricated (unnotched) condition and after the RSW of an homologous sheet square. Moreover, pre‐cracked compact tension specimens are tested with load ratio 0.1 to determine the fatigue crack growth behavior. These results are completed with crystallographic, microstructural, tensile, and fractographic examinations, and the influence of the microstructure and of the welding process is discussed.

Microhardness characterisation in developing high strength, high toughness and superior ballistic resistance low carbon Ni steel

To develop new steels that exceed the ballistic resistance, strength and toughness of current naval ship steel plates, we designed optimally quenching–lamellarising–tempering treated low carbon 10%Ni steel plates. These plates displayed exceptional properties in all aspects; when compared to the widely used HSLA-100 steel plates, they showed superior toughness and an improvement of over 15% in strength and 20 mm fragment simulation projectile ballistic limit V50. A preliminary vibrating sample magnetometer study on the effect of quenching–lamellarising–tempering process and dynamic deformation on the microstructure evolution of the 10%Ni steel during ballistic impact was valuable to our understanding of the steel. The data led us to conclude that dynamic deformation during ballistic perforation induced an austenite to martensite phase transformation, which improves the global dynamic plasticity and ballistic resistance of the steel target. This mechanism, similar to the transformation induced plasticity effect, can be appropriately named ballistic induced plasticity. Characteristics of the microhardness maps of the sectioned craters created by ballistics are consistent with ballistic induced plasticity.

Mechanical and wear behavior of alloyed gray cast iron in the quenched and tempered and austempered conditions

Three groups of hypereutectic cast irons alloyed with Cu, Ni and microalloying additive like Ti and Nb were examined for its hardness and wear resistance in the austempered (360 °C/3 h) and quenched and tempered conditions at varying tempering temperatures. It is observed that the cast irons in the quenched and tempered condition showed good wear resistance and moderate hardness at 400 °C. This was comparable with the wear resistance in austempered condition. The study also showed that in quenched and tempered condition, increasing Cu content in cast irons improved its wear resistance moderately while increasing Ni content has decreased its wear resistance. The presence of strong carbide formers (Nb, Ti) did not give significant improvement in wear resistance in quenched and tempered condition. Even in austempered alloys, higher Cu content increases its wear resistance and higher Ni content decreases their wear resistance. The austempered alloys showed ausferritic microstructure with 20% austenite phase which enhances wear resistance through transformation induced plasticity effect. On the other hand, the quenched and tempered alloys showed good wear resistance at 400 °C due to fine tempered carbides in the matrix.

The enhancement of transformation induced plasticity effect on austenitic stainless steels by cyclic tensile loading and unloading

The effect of loading modes of tensile deformation on the mechanical properties of a metastable austenite stainless steel has been investigated. The stress–strain curves, microstructures and fraction of the martensite are measured and analyzed separately. The results of tensile test indicate that a special loading mode referred as cyclic tensile loading and unloading can improve the strength and the formability of the specimens effectively. It is noted that the elongation and the ultimate strength are increased by 24.3% and 9.2% respectively at room temperature. Such enhancement of the transformation induced plasticity effect is mainly related to remarkable increase of the fraction of strain-induced martensite by the cyclic tensile loading and unloading.

MARTENSITIC TRANSFORMATION INDUCED BY DEFORMATION AND WORK--HARDENING BEHAVIOR OF HIGH MANGANESE TRIP STEELS

High manganese steels containing 15%—30%(mass fractions) Mn and additions of(2%—4%)Si and(2%—4%)A1 exhibit superior ductility and extraordinary strengthening behavior during plastic deformation,due to extensive twin formation under mechanical load(the so-called TWIP effect-twinning induced plasticity effect) or thee-martensite andα'-martensite transformation(the so- called TRIP effect-transformation induced plasticity effect).In this paper,different heat treatments were carried out for high-manganese TRIP steels of Fe-15Mn-4Si-2Al and Fe-20Mn-4Si-2Al,and the uniaxial tensile tests were performed.The results show that increase of holding time at 1373 K can increase elongation and strength.Through the analysis of stress-strain curves,the relationship between mechanical properties and deformation mechanism was established.The dependence of phase compositions in high manganese TRIP steels on the heat treatment parameters has been studied by metallographic characterization and XRD pattern.The transformation routes ofγ→e→α' andγ→α' were observed by TEM,which dominates the TRIP effects.Microstructural characterization ofe-martensite andα'-martensite indicated thatα'-martensite held the K-S orientation relationship with austenite,and the S-N(Shoji-Nishiyama) orientation relationship was observed between austenite ande-martensite,wherease-martensite andα'-martensite held the Burgers orientation relationship. Deformation behavior of the two kinds of high Mn steels has been clarified through the relationship between work hardening rate of dσ/deand true strain based on the true stress-true stain curves.It has been found that continuous transformation from austenite ore-martensite toα'-martensite can take place during deformation,which increases the work-hardening rate to improve the TRIP effect.

Relationship between relaxation mechanisms and strain aging in an austenitic FeMnC steel

Basic relaxation experiments confirm that dislocation gliding in a V-microalloyed Fe-22 wt.% Mn-0.6 wt.% C steel is thermally activated at room temperature. This impediment to gliding is believed to be a necessary condition for the occurrence of mechanical twinning inducing the twinning induced plasticity effect in the considered family of high manganese austenitic steels. The relaxation experiments also induce a static strain aging phenomenon enhanced by applied stress and systematically trigger the dynamic strain aging mechanism and observable serrations on further tensile loading.

Effects of Twin-Dislocation and Twin-Twin Interactions on the Strain Hardening Behavior of TWIP Steels

Abstract Tensile tests of Fe-30Mn-5Si-2Al steel were carried out for different strains of 0.05, 0.14, 0.26, and up to the strain-to-failure in order to observe the evolution of microstructure during deformation and investigating the strain hardening behavior. Three-stage strain hardening behavior was observed in this steel during tensile test. In stage I, planar dislocation structure was observed by TEM and regarded as main deformation mechanism, and low strain hardening rate exponent was exhibited. Primary deformation twinning occurred in stage II, and the strain hardening rate exponent increased due to the blockage of dislocations' motion by twin boundaries. In stage III, the strain hardening rate exponent had increased to a value higher than 0.5. The obstacle effect of twin boundaries and twin-twin interaction had been observed by TEM, and the interactions between primary and secondary twins were found to cause the additional hardening in addition to the obstacle effect on dislocations' motion, which led to the twinning induced plasticity effect in the later stage of deformation.

The influence of martensitic transformation on mechanical properties of cast high alloyed CrMnNi-steel under various strain rates and temperatures

Metastable austenitic steels show excellent mechanical properties, such as high strength combined with excellent ductility and toughness due to martensitic transformation under mechanical loading (transformation induced plasticity effect). A good energy consumption, and, in the case of high-alloyed metastable austenitic steels, a high corrosion resistance, increase the potential of these materials for diverse applications, also in regard of safety requirements. Up to now, numerous wrought alloys were investigated concerning mechanical behaviour, TRIP-effect, martensitic transformation behaviour and modelling of transformation kinetics or stress-strain behaviour. New high alloyed cast CrMnNi-steels, developed at Technical University Bergakademie Freiberg, provide the chance to reduce processing steps, production time and costs. In order to understand the influence of temperature on the martensitic phase transformation behaviour and therefore on mechanical properties and failure, the mechanical response under tensile loading in a temperature range between -70°C and 200°C was investigated. The mechanical behaviour under compressive loading was also examined in a wide range of strain rates between 10−4 s−1 and 103 s−1 to obtain information about the strain rate effect on stress-strain behaviour and microstructural changes.

EFFECTS OF N CONTENT ON MICROSTRUCTURE AND PROPERTIES OF 29Cr CASTING SUPER DUPLEX STAINLESS STEEL

High manganese steels containing 15%—30%(mass fractions) Mn and additions of(2%—4%)Si and(2%—4%)A1 exhibit superior ductility and extraordinary strengthening behavior during plastic deformation,due to extensive twin formation under mechanical load(the so-called TWIP effect-twinning induced plasticity effect) or thee-martensite andα'-martensite transformation(the so- called TRIP effect-transformation induced plasticity effect).In this paper,different heat treatments were carried out for high-manganese TRIP steels of Fe-15Mn-4Si-2Al and Fe-20Mn-4Si-2Al,and the uniaxial tensile tests were performed.The results show that increase of holding time at 1373 K can increase elongation and strength.Through the analysis of stress-strain curves,the relationship between mechanical properties and deformation mechanism was established.The dependence of phase compositions in high manganese TRIP steels on the heat treatment parameters has been studied by metallographic characterization and XRD pattern.The transformation routes ofγ→e→α' andγ→α' were observed by TEM,which dominates the TRIP effects.Microstructural characterization ofe-martensite andα'-martensite indicated thatα'-martensite held the K-S orientation relationship with austenite,and the S-N(Shoji-Nishiyama) orientation relationship was observed between austenite ande-martensite,wherease-martensite andα'-martensite held the Burgers orientation relationship. Deformation behavior of the two kinds of high Mn steels has been clarified through the relationship between work hardening rate of dσ/deand true strain based on the true stress-true stain curves.It has been found that continuous transformation from austenite ore-martensite toα'-martensite can take place during deformation,which increases the work-hardening rate to improve the TRIP effect.

Fatigue Behavior of Four High-Mn Twinning Induced Plasticity Effect Steels

Fatigue behavior of four high-Mn (18 to 22 wt pct Mn) steels, after cold rolling and soft annealing, were investigated in reversed plane bending in a high-cycle regime. The surfaces of fatigued specimens were examined using an optical microscope, a scanning electron microscope (SEM), and an atom force microscope (AFM). It was discovered that the chemical composition of the steel had only a minor influence on fatigue behavior, and the fatigue limit (FL), i.e., the stress amplitude leading to more than 2·106 cycles to failure, of all steels is about 400 MPa. This is about 42 to 48 pct of their tensile strength whose value is comparable to that of austenitic stainless steel and 780 MPa transformation induced plasticity (TRIP) steel. Mechanical twinning does not occur in the course of cycling; however, intense slip bands are formed. Fatigue cracks tend to nucleate at quite an early stage of fatigue life and most commonly on grain and annealing twin boundaries due to intersections of slip bands and boundaries.