The effect of dynamic strain aging on fatigue properties of dual phase steels with different martensite morphology

Dual phase (DP) steels with network and fibrous martensite were produced by intercritical annealing heat treatment cycles. Some of these steels were deformed at dynamic strain aging temperatures. Room temperature tensile tests of specimens deformed at 300 °C showed that both yield and ultimate tensile strengths for both morphologies increased, while total elongation decreased. Fatigue test results before and after high temperature deformation showed that dynamic strain aging has a stronger effect on fatigue properties of dual phase steels with fibrous martensite. Cracks in DP steels with fibrous martensite propagate in a tortuous path in soft ferrite phase, while they pass of both hard and soft phases in DP steels with network martensite.     

应变速率对TWIP钢Fe-23Mn-2Al-O.2C力学性能的影响

开展了固溶处理后TWIP钢Fe-23Mn-2Al-0.2C的拉伸实验,研究了应变速率对其拉伸变形行为的影响.结果表明,当应变速率在2.97×10-4-1.49×10-1s-1范围内变化时,钢的屈服强度没有明显变化,随着应变速率增大,抗拉强度稍有降低,延伸率明显减小.当应变速率较低时,其加工硬化速率随着真应变呈现三个阶段...     

Cyclic strain hardening of polygonal and acicular ferrite/bainite microstructures in microalloyed steels in the temperature range − 150°C to 27°C

Cyclic strain hardening has been observed to be markedly sensitive to microstructural changes in microalloyed steels. Two significantly different microstructures - polygonal ferrite grains of average grain size 10–120 μm and acicular ferrite/upper bainite colonies of dimensions 200–625 μm - were examined in order to determine the influence of each on cyclic strain hardening and related properties. Tests were conducted at temperatures between ?150 and 27°C. The cyclic strain hardening exponent, β_c, was significantly more sensitive to changes in the size of the polygonal ferrite grains than to changes in the acicular ferrite/upper bainite colony size.     

Comparative study on microstructures and properties of Mn-containing and Mn-free bainitic steels

ABSTRACT

The effects of Mn on the microstructure and impact-abrasion wear resistance of bainitic steel were studied. Results showed that the Mn-containing steel possessed finer microstructure and higher volume fraction of retained austenite, in comparison with the Mn-free steel. This was caused by lower transformation temperature and higher strength of undercooled austenite. The weight loss of Mn-free steel varying with the impact load was larger than that of Mn-containing steel. High strength, hardness and toughness of Mn-containing steel were conducive to improving wear resistance. More retained austenite in Mn-containing steel played an active role in work hardening and hindering crack propagation. However, the portion of retained austenite that induced martensitic transformation was the same with increasing impact-wear load.     

Strain rate-dependent hardening with dislocation-twin interaction of Fe–Mn–Al–C steel using crystal plasticity

A crystal plasticity finite element model with dislocation-twin interaction was developed to study the strain rate-dependent hardening of Fe–Mn–Al–C twinning-induced plasticity steel. Microstructural state variables including twinning space and dislocation density were incorporated to describe the mechanical twins hindering gliding dislocations. In situ scanning electron microscope tension and electron backscatter diffraction tests were conducted as validation and supplement. Predicted stress and strain hardening rate at various strain rates agree well with the experimental results. The increasing strain hardening stage is attributed to the dynamic competition between deformation twinning and dynamic recovery of dislocations. The intergranular deformation heterogeneity associated with the competitive activities of deformation mechanisms was also studied. The results indicate a larger contribution of slip to overall hardening than twinning.     

Additive manufacturing of metals: a brief review of the characteristic microstructures and properties of steels,Ti-6Al-4V and high-entropy alloys

Abstract

We present a brief review of the microstructures and mechanical properties of selected metallic alloys processed by additive manufacturing (AM). Three different alloys, covering a large range of technology readiness levels, are selected to illustrate particular microstructural features developed by AM and clarify the engineering paradigm relating process–microstructure–property. With Ti-6Al-4V the emphasis is placed on the formation of metallurgical defects and microstructures induced by AM and their role on mechanical properties. The effects of the large in-built dislocation density, surface roughness and build atmosphere on mechanical and damage properties are discussed using steels. The impact of rapid solidification inherent to AM on phase selection is highlighted for high-entropy alloys. Using property maps, published mechanical properties of additive manufactured alloys are graphically summarized and compared to conventionally processed counterparts.     

Impact and tensile properties of ferrite–martensite dual‐phase steels

The effect of martensite morphology on the impact and tensile properties of dual phase steels with a 0.25 volume fraction of martensite (V_m) under different heat treatments was investigated. These treatments are direct quenching (DQ) and step quenching (SQ) that result in different microstructures and mechanical properties. To process dual phase steels, a low carbon manganese steel was used. At first the banding present in the initial steel was eliminated, then the two different heat treatments were applied. To reach a 0.25 volume fraction of martensite a variation of intercritical annealing temperatures was adopted for both treatments that allowed the evolution of different volume fraction of martensite. Phase analysis showed that an intercritical temperature of 725 °C (between A₃, A₁) gives the desired 0.25 V_m of martensite. A comparison of impact, tensile and ductile–brittle transition temperature (DBTT) indicates that the microstructure of the direct treatment has a better toughness. The DBTT for the DQ and SQ treatment is ?49 and ?6 °C, respectively.     

Effects of samarium on microstructures and tensile properties of Mg-5Al-0.3Mn alloy

Microstructures and tensile properties of Mg-5Al-0.3Mn-xSm (x = 0, 1, 2 and 3 wt.%) alloys prepared by metal mould casting method were investigated. It was demonstrated that Mg-5Al-0.3Mn alloy was mainly composed of α-Mg and β-Mg₁₇Al₁₂ phases. However, the other two precipitates (Al₁₁Sm₃ and Al₂Sm) were observed along grain boundaries in the alloys containing Sm. The amount of Al₁₁Sm₃ and Al₂Sm precipitates was increased with the increment of Sm content. Meanwhile, volume fraction of β-Mg₁₇Al₁₂ phase was decreased. Moreover, the morphology of β-Mg₁₇Al₁₂ was altered from bulk bone-like shape to spherical one. Tensile results showed that Mg-5Al-0.3Mn-2Sm alloy exhibited the highest tensile properties both at room temperature and 150 °C. Compared with ultimate tensile strength (UTS), yield strength (YS) and elongation (?) of Mg-5Al-0.3Mn alloy, UTS, YS and ? of Mg-5Al-0.3Mn-2Sm alloy were enhanced by 30%, 45% and 35% at room temperature, and by 17%, 48% and 96% at 150 °C, respectively. The improvement of tensile properties was attributed to the decreased amount of β-Mg₁₇Al₁₂ and its refined morphology, and high thermal stable Al₁₁Sm₃ and Al₂Sm precipitates which effectively prohibited dislocation movement and grain boundary sliding during deformation process.     

Finite element formulations of strain gradient theory for microstructures and the C0–1 patch test

Based on finite element formulations for the strain gradient theory of microstructures, a convergence criterion for the C^0–1 patch test is introduced, and a new approach to devise strain gradient finite elements that can pass the C^0–1 patch test is proposed. The displacement functions of several plane triangular elements, which satisfy the C⁰ continuity and weak C¹ continuity conditions are evaluated by the C^0–1 patch test. The difference between the proposed C^0–1 patch test and the C⁰ constant stress and C¹ constant curvature patch tests is elucidated. An 18-DOF plane strain gradient triangular element (RCT9+RT9), which passes the C^0–1 patch test and has no spurious zero energy modes, is proposed. Numerical examples are employed to examine the performance of the proposed element by carrying out the C^0–1 patch test and eigenvalue test. The proposed element is found to be without spurious zero energy modes, and it possesses higher accuracy compared with other strain gradient elements. Copyright © 2004 John Wiley & Sons, Ltd.     

Effect of microstructure and overaging on the tensile behavior at room and elevated temperature of C355-T6 cast aluminum alloy

The present study was focused on the microstructural and mechanical characterization of the Al–Si–Cu–Mg C355 alloy, at room and elevated temperature. In order to evaluate the influence of microstructural coarseness on mechanical behavior, samples with different Secondary Dendrite Arm Spacing (SDAS) (20–25 μm for fine microstructure and 50–70 μm for coarse microstructure), were produced through controlled casting conditions. The tensile behavior of the alloy was evaluated at T6 condition and at T6 with subsequent high temperature exposure (41 h at 210 °C, i.e. overaging), both at room and elevated temperature (200 °C). Microstructural investigations were performed through optical and electron microscopy.The results confirmed the important role of microstructure on the tensile behavior of C355 alloy. Ultimate tensile strength and elongation to failure strongly increased with the decrease of SDAS. Larger SDAS, related to lower solidification rates, modify microstructural features, such as eutectic Si morphology and size of the intermetallic phases, which in turn influence elongation to failure. Overaging before tensile testing induced coarsening of the strengthening precipitates, as observed by STEM analyses, with consequent reduction of the tensile strength of the alloy, regardless of SDAS. A more sensible decrease of tensile properties was registered at 200 °C testing temperature.     

Effect of Pd on microstructures and tensile properties of as-cast Mg-6Al-1Zn alloys

The effects of Pd on the microstructure and mechanical properties of Mg-6Al-1Zn alloys were investigated. Mg-6Al-1Zn-xPd (x = 0-6 wt.%) alloys were prepared using a permanent mould casting method. The microstructure of the as-cast alloys was characterized by the presence of Mg₁₇Al₁₂ and Al₄Pd phases. The volume fraction of the Al₄Pd phase was increased by the addition of 1-6 wt.%Pd but the volume fraction of the Mg₁₇Al₁₂ phases decreased. At room temperature, the tensile strength increased with increasing Pd addition up to 2 wt.%Pd, and the elongation to fracture decreased with a concomitant increase in the aggregation of the coarse Al₄Pd phase. At 150 °C, the tensile strength increased with the addition of Pd. Therefore, the room and elevated temperature tensile properties of as-cast Mg-6Al-1Zn alloys can be improved by Pd addition.     

Structure and properties of ductile CuAlMn shape memory alloy synthesized by mechanical alloying and powder metallurgy

A ductile Cu–Al–Mn–Ti–B shape memory alloy with high fatigue strength has been prepared via mechanical alloying and powder metallurgy. With increasing milling time, the size of the crystallite grains decreases. Cu diffraction pattern appeared only after milling at a speed of 300 rpm for 25 h. The single phase CuAlMnTiB solid solution powder after 35 h milling was hot-pressed and extruded to form the final alloy. The quenched alloy had a single β phase at room temperature and its yield strength, maximum strength and strain were measured to be 390 MPa, 1015 MPa and 14.4%, respectively. The aged alloy showed a martensite structure at room temperature and had a shape memory recovery of 92% after 120 cycles.     

The hot ductility of Nb/V containing high Al,TWIP steels

Abstract

The hot ductility of Nb/V containing high Al, twin induced plasticity (TWIP) steels has been examined over the temperature range 650–1150°C after melting and after ‘solution treatment’. Previous work had shown that the hot ductility is poor for the 1·5 mass-%Al, TWIP steel due to precipitation of AlN at the austenite grain boundaries, the depth of the trough being similar to that for an X65 grade pipeline steel but with the trough covering a much wider temperature range. Adding Nb and V made the ductility even worse due to the additional precipitation of NbCN and VN. Very low reduction of area values, 10–20% were obtained in the temperature range 700–900°C. Increasing the cooling rate to the test temperature resulted in even worse ductility. The ductility of these steels after ‘solution treatment’ is similar to that obtained after melting but when the cast was hot rolled followed by ‘solution treatment’ and cooling to the test temperature ductility improved due to grain refinement.     

Assessment of creep rupture properties for dissimilar steels welded joints between T92 and HR3C

Dissimilar steels welded joints, between ferritic steel and austenitic stainless steel, are always encountered in high‐temperature components in power plants. As two new grade ferritic steel and austenitic stainless steel, T92 (9Cr0.5Mo2WVNb) and HR3C (TP310HCbN), exhibit superior heat strength at elevated temperatures and are increasingly applied in ultra‐supercritical (USC) plants around the world, a complete assessment of the properties for T92/HR3C dissimilar steels welded joints is urgently required. In this paper, metallographic microstructures across the joint were inspected by optical microscope. Particularly, the creep rupture test was conducted on joints under different load stresses at 625 °C to analyse creep strength and predict their service lives, while their fractograph were observed under scanning electron microscope. Additionally, finite element method was employed to investigate residual stress distribution of joints. Results showed that the joints were qualified under USC conditions, and T92 base material was commonly the weakest part of them.     

The effects of the processing variables on the microstructure and tensile properties of naturally aged AA6022 wrought alloys

Abstract

The 6xxx series (Al‐Mg‐Si) alloys have properties of medium to high strength, excellent formability, good corrosion resistance, and weldability. In the present paper, we discuss the tensile properties of T4 heat‐treated AA6022 alloys produced by different fabrication processes, including differences in the homogenization temperature, the hot rolling temperature, the cold rolling reduction ratio, and the solid solution temperature. The analysis was done by differential scanning calorimetry, tensile strength testing, by measuring the electrical resistivity, and optical microscopy. The results indicate that the yield strength and the ultimate tensile strength of naturally aged AA6022 alloys increases with increases in the homogenization treatment temperature, hot rolling temperature, cold rolling ratio and solution treatment temperature.     

热处理和热暴露后Ti-46Al-8Ta合金的显微组织和拉伸性能

铸造Ti-46Al-8Ta合金经过专门设计的组合热处理,实现了晶粒细化.对晶粒细化的Ti-46Al-8Ta合金开展了在700℃大气环境中的热暴露5000h的热稳定性评定.采用光学显微镜、扫描电镜、透射电镜、X射线衍射等技术研究晶粒细化工艺以及热暴露对合金组织的影响,并进行了室温拉伸性能测试.研究发现,铸造Ti-46Al-8Ta合金经热等静压和α单相区固溶+空冷,以及随后的双相区退火能获取晶粒细小的"旋绕态"全片层组织.在700℃长期大气热暴露后,该细化组织发生明显的晶团融合、粗化,并生成出相当数量的B2(ω)和γ新相,导致屈服强度和塑性下降.     

Al-5Ti-0.25C合金制备工艺及其细化效果研究

目的 对机械加工过程中的Al屑和Ti屑进行回收再利用.方法 以Al屑和Ti屑为制备Al-5Ti-0.25C合金的原材料,分析Al屑和Ti屑的加料温度、反应温度和反应时间对Al-5Ti-0.25C微观组织的影响,确定最佳制备工艺.在此基础上研究Al-5Ti-0.25C的添加量、细化温度、细化时间和搅拌处理对纯铝的细化效果...     

The effects of compressive and tensile prestrain on ductile fracture initiation in steels

It has been recognized that ductility of prestrained steel is inferior to that without prestrain, and the critical equivalent plastic strain of ductile fracture initiation is inversely related to stress triaxiality. In this paper, the effects of compressive and tensile prestrain on ductile fracture initiation in steels are investigated quantitatively by adopting the relationship between stress triaxiality and critical equivalent plastic strain. It is found that compressive prestrain leads to cleavage cracking and reduces ductility. In the case of the TMCP steel, compressive prestrain up to 30% does not decrease the ductility, accompanied by no evidence of cleavage cracks. However, in the case of SM490B steel, 30% compressive prestrain leads to cleavage cracking and reduces ductility significantly.     

Enhancement of tensile properties by room-temperature quenching and partitioning of 0.2C–10Mn–2Al steel

ABSTRACT

Processing conditions better than those of conventional quenching and partitioning process are suggested for 0.2C–10Mn–2Al steel. The steel can retain 24% of austenite on quenching to room temperature and effectively partition carbon from martensite to austenite at 200°C. The resulting tensile properties were comparable to those produced by conventional quenching and partitioning. Moreover, the suggested processing condition resolves an issue of intercritically annealed medium Mn steels by improving the yield strength and eliminating yield point phenomenon as well as serrated flow.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

SiC-Al界面Al_4C_3的生成及其控制

系统研究了在Ａｌ中添加Ｓｉ对控制ＳｉＣ－Ａｌ之间生成Ａｌ４Ｃ３的化学反应的作用和对ＳｉＣ－Ａｌ系统化学反应动力学的影响，以及温度对化学反应程度的影响，探讨了Ａｌ４Ｃ３生成的机理。研究表明，添加Ｓｉ于Ａｌ中使ＳｉＣ和Ａｌ之间生成Ａｌ４Ｃ３的化学反应得到有效的控制并明显影响化学反应的速度；ＳｉＣ－Ａｌ系统化学反应的程度随温度升高而增大；Ａｌ４Ｃ３的生成通过两个步骤进行，即ＳｉＣ溶解于熔融金属Ａｌ中，然后和Ａｌ发生化学反应。研究结果为用熔融金属加工技术合成无Ａｌ４Ｃ３生成的ＳｉＣ／Ａｌ复合材料提供了可靠的途径。