Hot rolling and annealing effects on the microstructure and mechanical properties of ODS austenitic steel fabricated by electron beam selective melting

The grain morphology, nano-oxide particles and mechanical properties of oxide dispersion strengthened (ODS)-316L austenitic steel synthesized by electron beam selective melting (EBSM) technique with different post-working processes, were explored in this study. The ODS-316L austenitic steel with superfine nano-sized oxide particles of 30–40 nm exhibits good tensile strength (412 MPa) and large total elongation (about 51%) due to the pinning effect of uniform distributed oxide particles on dislocations. After hot rolling, the specimen exhibits a higher tensile strength of 482 MPa, but the elongation decreases to 31.8% owing to the introduction of high-density dislocations. The subsequent heat treatment eliminates the grain defects induced by hot rolling and increases the randomly orientated grains, which further improves the strength and ductility of EBSM ODS-316L steel.     

Enhanced tensile properties of 316L steel via grain refinement and low-strain rolling

Excellent strength–ductility synergy of metallic materials is significant for their industrial applications. This study presents a fine-grained 316L stainless sheet (average grain size of ～5?µm) with a good combination of strength and ductility achieved via low-strain cold rolling (rolling strain of 30%). The fabricated steel sheet exhibits maximum yield strength and ultimate tensile strength values of 1045 and 1080?MPa, respectively, with a uniform elongation of 7%. Experimental results confirm that the high density of dislocations, strain-induced martensitic phase, and deformation twins together contribute to the high strength of the rolled stainless steel. Moreover, its good ductility is attributable to the strain-induced martensitic transformation and deformation twins.     

Effect of grain size on tensile behaviour of a submicron grained Al–3 wt-%Mg alloy produced by severe deformation

Abstract

Equal channel angular extrusion has been used to deform an Al–3 wt-%Mg alloy to an effective strain of 10, resulting in a 0.2 µm grain size. In the as deformed condition the yield strength was increased to ～500 MPa. During annealing the grain structure coarsened uniformly and the yield stress was found to follow the Hall–Petch relationship, even in the submicron range. There was an abrupt transition in elongation at a grain size of ～0.5 µm. Samples with smaller grain sizes showed no uniform elongation and limited ductility. For slightly greater grain sizes there was only a relatively small reduction in elongation, compared to a coarse grained material, while the yield stress was still increased by a factor of over four. Reducing the grain size to the submicron range led to far higher Lüders strains than are normally observed in Al–Mg alloys.     

Effect of annealing on tensile properties of electrodeposited nanocrystalline Ni with broad grain size distribution

Abstract

The microstructures and tensile properties of electrodeposited nanocrystalline Ni (nc-Ni) with a broad grain size distribution after annealing at 150, 200 and 300°C for 500 s were investigated. The as deposited broad grain size distribution nc-Ni sample exhibited a moderate strength σ_UTS of ～1107 MPa but a markedly enhanced ductility ?_TEF of ～10%, compared with electrodeposited nc-Ni with a narrow grain size distribution. Annealing below 200°C increased the strength but caused a considerably reduction in tensile elongation. This behaviour is attributed to the grain boundary relaxation and the increased order of grain boundaries after annealing, which can make the grain boundary activities, such as the grain boundary sliding and grain rotations, more difficult. Further annealing at 300°C decreased both the yield strength and tensile elongation significantly due to significant grain growth.     

Tensile ductility and deformation mechanisms of a nanotwinned 316L austenitic stainless steel

A nanotwinned 316 L austenitic stainless steel was prepared by means of surface mechanical grinding treatment.After recovery annealing,the density of dislocations decreases obviously while the average twin/matrix lamella thickness still keeps in the nanometer scale.The annealed nanotwinned sample exhibits a high tensile yield strength of 771 MPa and a considerate uniform elongation of 8%.TEM observations showed that accommodating more dislocations and secondary twinning inside the nanotwins contribute to the enhanced ductility and work hardening rate of the annealed nanotwinned sample.     

High Temperature Tensile Properties of 30 vol. pct ZrC_p/W Composite

In order to improve the high temperature strength of tungsten, 30 vol. pct ZrC particles were added to the tungsten matrix to form a 30ZrCp/W composite. The tensile properties from 20C to 1880C of the composite were examined. It was shown that with increasing testing temperature, the nonlinearity of the stress strain curve of 30ZrCp/W composite becames obvious over 1200C and the Young's modulus decreases and the elongation increases. The ultimate tensile strength increases at first and then decreases with increasing testing temperature. The maximum strength of 431 MPa was obtained at 1000C. The strengthening mechanism at high temperatures is the load transfer to ZrC particles and dislocation strengthening of the tungsten matrix with an effect of grain boundary strengthening.     

On the room-temperature tensile deformation behavior of a cast dual-phase high-entropy alloy CrFeCoNiAl0.7

The microstructure and room-temperature tensile deformation behavior of the cast CrFeCoNiAl0.7 high-entropy alloy (HEA) were studied in details.The cast HEA consisted of a dual-phase structure of 77.3 vol.％ face-centered-cubic (FCC) phase plus 22.7 vol.％ B2 phase,and exhibited excellent room-temperature tensile properties with a high yield strength of 876 MPa,ultimate tensile strength of 1198 MPa and a relatively large elongation to fracture of ～9 ％.Dislocations gliding in the FCC phase governed the plastic deformation at the early stage of room-temperature tensile,and disordered dislocations were to form dislocation walls as the deformation proceeded.With further increase in strain to a high level,the stacking faults were generated through the dissociation of the geometrically necessary dislocations,serving as the potential heterogeneous nucleation sites for the deformation twins.     

Deformation mechanism of bimodal microstructure in Ti-6Al-4V alloy: The effects of intercritical annealing temperature and constituent hardness

The so-called bimodal microstructure of Ti-6Al-4V alloy,composed of primary α grains (αp) and transformed β areas (βtrans),can be regarded as a "dual-phase" structure to some extent,the mechanical properties of which are closely related to the sizes,volume fractions,distributions as well as nano-hardness of the two constituents.In this study,the volume fractions of primary α grains (vol.％(αp)) were systematically modified in three series of bimodal microstructures with fixed primary α grain sizes (0.8 μm,2.4 μm and 5.0μm),by changing the intercritical annealing temperature (Tint).By evaluating the tensile properties at room temperature,it was found that with increasing Tint (decreasing vol.％(αp)),the yield strength of bimodal microstructures monotonically increased,while the uniform elongation firstly increased with Tint until 910 ℃ and then drastically decreased afterwards,thereby dividing the Tint into two regions,namely region Ⅰ (830-910 ℃) and region Ⅱ (910-970 ℃).The detailed deformation behaviors within the two regions were studied and compared,from the perspectives of strain distribution analysis,slip system analysis as well as dislocation analysis.For bimodal microstructures in region Ⅰ,due to the much lower nano-hardness of βtrans than αp,there was a clear strain partitioning between the two constituents as well as a strain gradient from the αp/βtrans interface to the grain interior of αp.This activated a large number of geometrically necessary dislocations (GNDs) near the interface,mostly with components,which contributed greatly to the extraordinary work-hardening abilities of bimodal microstructures in region Ⅰ.With increasing Tint,the αp/βtrans interface length density gradually increased and so was the density of GNDs with components,which explained the continuous increase of uniform elongation with Tint in this region.For bimodal microstructures in region Ⅱ,where the nano-hardness ofβtrans and αp were comparable,neither a clear strain-partitioning tendency nor a strain gradient across the αp/βtrans interface was observed.Consequently,only statistically stored dislocations (SSDs) with component were activated inside αp.The absence of dislocations together with a decreased volume fraction of αp resulted into a dramatic loss of uniform elongation for bimodal microstructures in region Ⅱ.     

Enhancing tensile properties of ultrafine-grained medium-carbon steel utilizing fine carbides

The aim of the present study is to evaluate the influence of nano-sized carbides upon tensile behavior in UFG medium-carbon steels and to develop a material with improved tensile properties. UFG medium-carbon steels with fine carbides were successfully fabricated by multi-pass caliber rolling at 773 K. Alloying chromium and molybdenum resulted in thinner pearlitic lamellae, which were transformed into finer particles after severe plastic deformation. The UFG steel containing the alloying elements exhibited superior tensile properties, which was attributed to the enhanced strain hardening rate by the imbedded finer particles. Subsequent annealing induced growth of grains and particles, which also recovered elongation at the expense of strength. All UFG steels investigated here showed a yield-point phenomenon due to the decreased hardening rate and lack of mobile dislocations and their sources. The deteriorating effect of particle growth overwhelmed the improving effect of grain growth after annealing of the UFG medium-carbon steel, leading to a reduced strain hardening rate. This resulted in a positive correlation between a grain size and Lüders elongation in the investigated UFG steels.     

微量Cu和形变热处理对Al-Fe合金性能的影响

观察Al-Fe合金的显微组织并测量其力学性能和导电性能,研究了Cu元素和形变热处理对其性能的影响。结果表明：在铸态Al-Fe-Cu合金组织中,Cu元素在基体内均匀分布,而Fe元素在晶界处偏析;挤压态的Al-0.7Fe-0.2Cu合金其性能最优：导电率为59.90%IACS,抗拉强度为108 MPa,硬度为31.2HV;随着退火温度的提高Al-0.7Fe-0.2Cu合金的抗拉强度急剧降低,在400℃退火时其抗拉强度最低(100 MPa),伸长率最高(31.3%);在250℃退火时导电率出现峰值(62.61%IACS)。在退火Al-0.7Cu-0.2Cu合金中有许多细小针状的θ(Al₂Cu)相析出,并与位错交互缠结。随着退火温度的提高合金中的位错密度降低,晶粒细化。     

The effect of aging process on the microstructure and mechanical properties of a Cu–Be–Co–Ni alloy

The paper investigated the effect of two aging processes (i.e. normal aging and interrupted aging) on the microstructure and mechanical properties of a Cu–Be–Co–Ni alloy. The results of tensile and Kahn tear tests showed that the interrupted aging (IA) process could significantly improve the uniform elongation and plane stress fracture toughness with tiny decrease in ultimate tensile strength, when compared with the results from normal aging (NA) process. Under the scanning electron microscope, the fracture surface of samples treated by NA followed the intergranular fracture, while that of the samples treated by IA followed the transgranular fracture. The transmission electron microscope study revealed the differences between the microstructure of the alloy treated by NA and IA processes. After the NA process, the slender strip of γ′ precipitates aggregated at grain boundaries with a length of approximately 10 to 45 nm; the disk-shaped γ″ precipitates in the alloy treated by IA distributed homogenously throughout whole grains with a length of about 3 to 10 nm. The discussion of strengthening mechanisms demonstrated that the mechanism of precipitate shearing by dislocations made a contribution to the strengthening of the alloy treated by IA, while the Orowan mechanism was the dominant strengthening mechanism in the alloy treated by NA.     

Grain size effect on high-speed deformation of Hadfield steel

The influence of the microstructure on the tensile properties and fracture behavior of Hadfield steel at high strain rate were studied. Hadfield steel samples with different mean grain sizes and carbon phases were prepared by rolling at medium temperatures and subsequent annealing. A sample with an average grain size larger than 10 μm, and a small number of carbides shows ductility with local elongation (post uniform elongation) at a high-speed tensile deformation rate of 10³ s⁻¹. In addition, the fracture surface changes from brittle to ductile with increasing strain rate. In contrast, a fine-grained sample with carbides undergoes brittle fracture at any strain rate. The grain size dependence is discussed by considering the dynamic strain aging as well as the emission of dislocation from cracks. The accelerated diffusion of carbon due to grain refinement is considered as one of the important reason for brittle fracture in the fine-grained Hadfield steel.     

Microstructure and tensile behaviour of pure titanium produced after high-energy shot peening

In this study, the microstructure and tensile behaviour of pure titanium processed by means of high-energy shot peening have been studied. The results show that a nanocrystalline surface layer is prepared on the surface of pure titanium. During severe deformation, the dislocations, twins and slip bands with single orientation are formed first, then the intersections of twins and slip bands are observed, and then the subgrains appear near or in the slip bands and finally the randomly oriented nano-grains are formed due to the further breakdown. After high-energy shot peening, the ultimate tensile strength and the yield strength increase by 27 and 40%, while the elongation decreases by 64% after treatment, due to the increasing dislocation density, micro-strain and the grain refinement.     

SiCp/AZ91D镁基纳米复合材料的室温拉伸行为及塑性变形机理

为得到高强度和高塑性的镁基复合材料,通过高能超声分散法和金属型重力铸造工艺制备了SiC纳米颗粒分散均匀的SiCp/AZ91D镁基纳米复合材料,并进行T4固溶热处理和室温拉伸。利用扫描电子显微镜(SEM)、透射电子显微镜(TEM)对试样拉伸后的显微组织和塑性变形机理进行观察与研究。结果表明:T4态SiCp/AZ91D镁基纳米复合材料室温下抗拉强度达到296 MPa,伸长率达到17.3%。经室温拉伸变形后复合材料基体微观组织中出现了大量的孪晶和滑移,孪生和滑移是复合材料塑形变形的主要机制。在室温拉伸过程中,α-Mg基体中SiC纳米颗粒周围形成高应变场,高应变场内形成大量位错和堆垛层错,这些位错和堆垛层错在拉伸应变的作用下演变成大量的滑移带和孪晶,这是SiCp/AZ91D镁基纳米复合材料在室温下具有高塑性的微观塑性变形机理。     

Observation on Emitting Dislocation from Grain Boundary by TEM

The observation on emitting dislocations from grain boundaries by TEM during Cu elongation has been pefformed. It is shown that there exists the "ledge" at the grain boundaries in fcc pure Cu, which is able to emit dislocations into grain under action of stress.     

Mg-Al-Ca-Mn-Zn变形镁合金的组织和力学性能

将Mg-1Al-0.4Ca-0.5Mn-0.2Zn(质量分数,%)合金在不同温度挤压,研究其微观组织和力学性能。结果表明：在260℃和290℃挤压的合金均发生不完全动态再结晶,再结晶晶粒尺寸分别为0.75 μm和1.2 μm。二者均具有高密度的G.P.区和球状纳米析出相,能抑制位错运动并为动态再结晶提供丰富的形核位点。沿晶界析出的纳米相能抑制晶界的运动和限制再结晶晶粒的生长,从而生成尺寸为0.75 μm的超细晶粒。随着挤压温度从260℃提高到290℃,合金的屈服强度从322 MPa提高到343 MPa,伸长率分别为13.4%和13%,没有明显的变化。挤压温度的提高促进了动态析出和动态回复,使合金中积累了高密度纳米盘状相和球状相,大量位错通过动态回复转变成小角度晶界,将未再结晶区域细分成密集的层状亚晶粒,二者均能抑制新位错的运动。这些因素,是在290℃挤压后的合金仍具有较高屈服强度和塑性没有明显变化的主要原因。纳米相对位错的钉扎在一定程度上限制了动态回复的发生,使合金中仍存在较高数量的残余位错,也有利于提高其屈服强度。     

预变形对X90管线钢显微组织和力学性能的影响

采用拉伸试验、冲击试验、光学显微镜(OM)、扫描电镜(SEM)、显微硬度测试仪等研究了0.5%~6%预拉伸变形对X90管线钢显微组织及力学性能的影响。结果表明:随着预拉伸变形量的增加,X90管线钢晶粒增大,位错塞积导致强度增加,均匀延伸率下降,呈现典型的加工硬化特点,抗拉强度的增幅要小于屈服强度,屈强比增大;随着变形量的增加冲击吸收功逐渐由291J减小至235J,冲击试样断口的韧窝减小,伴随第二相粒子析出;显微硬度中间层较边缘区增加少,预拉伸在6%时边缘显微硬度为325HV。X90管线钢的预拉伸在4%以内能保证管线钢的正常服役。     

Tensile deformation behavior of high manganese austenitic steel: The role of grain size

The tensile deformation behavior and microstructural evolutions of twinning induced plasticity (TWIP) steel with the chemical composition of Fe–31Mn–3Al–3Si and average grain sizes in the range of 2.1–72.6 μm have been analyzed. For each grain size, the Hollomon analysis and also the Crussard–Jaoul (C–J) analysis as an alternative method to describe the work hardening behavior were investigated. The results indicated that the optimum mechanical properties as a function of work hardening capacity can be obtained by changing the grain size. The microstructural observations showed that the pile-ups of planar dislocations are necessary for triggering the mechanical twinning and grain refinement suppresses the mechanical twinning in TWIP steel. Furthermore, the mechanical twinning increases with increasing applied strain. As a result, a high instantaneous work hardening due to the mechanical twin boundaries enhances the uniform elongation. The contribution from the strain of twinning and hardening due to an increase in the hardness of the twinned regions (i.e., the Basinski mechanism) may be also useful in achieving the high strength–ductility in TWIP steels.     

High temperature tensile properties of oxide dispersion strengthened T91 and their correlation with microstructural evolution

Abstract

In the present work, high temperature deformation behavior of oxide dispersion strengthened T91 was investigated and linked to the corresponding microstructure. First, tensile properties are presented and discussed in terms of yield strength, tensile stress and total elongation as a function of temperature. The results are compared to the matrix material and other ODS alloys. Second, transmission electron microscopy was applied to as received and deformed tensile test specimens. It is shown that the Y₂O₃ particle diameter increases slightly upon deformation at elevated temperatures. Additionally, distinctive coarsening of M₂₃C₆ carbides occurs at prior austenite grain boundaries. At temperatures above 500°C, dislocations are straight and pile up at grain boundaries due to thermally activated climbing. Oxide dispersion strengthened T91 provides high strength due to strong particle/dislocation interactions and good toughness properties.     

TC18钛合金的超塑性行为与变形机制

通过高温拉伸实验研究TC18钛合金在温度为720~950℃,初始应变速率为6.7×10~(-5)~3.3×10~(-1)s~(-1)时的超塑性拉伸行为和变形机制。结果表明:TC18钛合金在最佳超塑性变形条件下(890℃,3.3×10~(-4)s~(-1)),最大伸长率为470%,峰值应力为17.93MPa,晶粒大小均匀。在相变点Tβ(872℃)以下拉伸,伸长率先升高后下降,在温度为830℃,初始应变速率为3.3×10~(-4)s~(-1)时取得极大值373%,峰值应力为31.45MPa。TC18钛合金在两相区的超塑性变形机制为晶粒转动与晶界滑移,变形协调机制为晶内位错滑移与攀移;在单相区的超塑性变形机制为晶内位错运动,变形协调机制为动态回复和动态再结晶。