【作者】 黄宝旭；

【导师】 戎咏华；

【作者基本信息】 上海交通大学 ， 材料物理与化学， 2007， 博士

【摘要】 获得高强塑积性能一直是结构材料设计的目标。近年来新开发的含15-25mass%Mn、2-4mass%Si和2-4mass%Al的高Mn钢显示出极高的延伸率(60-95%)和高的强度(600-1100MPa),其优良的力学性能来自于形变过程中的孪生诱发塑性效应(Twinning Induced Plasticity,TWIP效应)或马氏体相变诱发塑性(Transformation Induced Plasticity,TRIP效应)。尽管Mn、Si、Al对TWIP钢的显微组织及其性能的影响已有较多的研究,但对含N和Nb的TWIP钢组织与性能的研究尚未报道,而且对TWIP效应和TRIP效应的机制尚未充分揭示。本文设计了几种不同N、Al含量的TWIP钢和含Nb新型TWIP钢,通过力学性能测试,结合光学显微镜(OM)、X射线衍射(XRD)、扫描电镜(SEM)和透射电镜(TEM)等表征技术,探讨N、Al、Nb等合金元素对TWIP钢层错几率(与层错能成反比)、力学性能的影响,并对TWIP钢中的TWIP效应机制、马氏体相变方式及εhcp马氏体的形成机制进行了较为系统的研究,获得以下主要研究结果。对不同N、Al含量TWIP钢层错几率进行了测定,并在不同温度下进行了力学性能测试。研究结果表明合金元素N、Al均可提高TWIP钢的层错能,抑制马氏体相变,促进TWIP效应。不同化学成分TWIP钢XRD图谱的分析结果显示,含N新型TWIP钢在形变过程中能够强烈抑制α马氏体相变,这是由于合金在由FCC结构的奥氏体或HCP结构的ε马氏体向BCC结构α马氏体相变时,其最大间隙由0.1044nm降低至0.0733nm,存在于间隙位置的N显著增大α马氏体切变阻力,抑制了α马氏体相变。不同的温度下含Nb新型TWIP钢力学性能测试、层错几率测定结果表明,TWIP钢中加入铌以后可显著降低层错几率,提高钢的层错能,有利于形变孪晶和TWIP效应的产生,因而具有优良的塑性。对含Nb新型TWIP钢的微观结构表征显示,铌的加入显著降低奥氏体晶粒的尺寸,通过细晶强化增加了钢的强度。合金元素Nb的细晶强化作用以及提高层错能改善塑性的优点导致含Nb新型TWIP钢具有优异的综合力学性能,其强塑积最高达到70000MPa%比无Nb的传统TWIP钢的强塑积高出20000 MPa%;在0至-30℃低温使用范围内平均强塑积高达56150.5MPa%,明显高于化学成分相近的无Nb TWIP钢(44729MPa%)。在层错的TEM观测过程中,发现在局部高密度层错区的某些电子衍射斑点存在定向的位移现象,并建立了FCC晶体中衍射斑点位移与层错几率的关系和层错几率的测定方法。基于电子衍射法测定局部区域层错几率的实验结果,提出了应变诱发ε马氏体相变的机制,即形变为全位错分解提供了能量,因而形变过程中产生了大量的不全位错和层错,而不全位错附近的应力场将更加有利于其他全位错的产生及其分解,从而促进局域层错密度的进一步增加,随后层错由无规到有规的排列,导致γfcc→εhcp马氏体相变的发生。当层错几率α=1时,形成无缺陷的ε马氏体,当α→1时,形成有层错缺陷的ε马氏体。ε、α马氏体的形核和长大均需要消耗大量的层错,致使ε、α马氏体附近的层错数量急剧下降,即层错几率显著下降,从而使TWIP钢相变后由XRD测定的层错几率与层错能不呈现反比关系。不同温度下的力学性能测定和XRD衍射图谱表明,层错能与温度呈正比关系,即温度越低,层错能越低。层错能与温度的密切联系导致材料在不同的温度下表现为不一样的力学性能。在较高温度,材料的层错能也较高,形变过程中有利于孪晶转变,发生TWIP效应;而在较低的温度,形变过程中倾向于发生TRIP效应而提高塑性。层错能不同的钢,其发生TWIP效应和TRIP效应的形变温度不同,随钢的层错能增加,由TWIP效应和TRIP效应所导致的高延伸率峰逐渐向较低的温度偏移。应用晶体学分析方法对FCC结构TWIP钢中的层错、孪晶形态进行了预测,并采用OM、SEM和TEM等微观结构表征方法对层错和孪晶进行实验验证。通过不同形变条件孪晶组态的观察和马氏体相变基本原理,提出了TWIP钢诱发塑性的机制。形成形变孪晶的最高温度(Tf)和热诱发马氏体开始温度(Ms)决定了其主导形变机制是滑移诱发塑性或孪生诱发塑性,还是相变诱发塑性。在Tf温度以上,只有通过滑移实现TWIP钢的塑性;在Tf温度和Ms温度之间,诱发形变孪晶为该温度区间的主导机制,导致显著的TWIP效应;当温度低于Ms,则呈现出显著的TRIP效应。在不同形变速率下的力学性能测试结果显示,TWIP钢的力学性能受形变速率的影响。屈服强度随形变速率的增加,呈缓慢上升的态势;而断裂强度则呈缓慢下降的趋势。断裂延伸率和均匀延伸率则随形变速率的增加先迅速降低然后缓慢地上升。总体上,形变速率对屈服强度、断裂强度的影响远弱于对延伸率的影响。TWIP钢在不同温度下的力学行为揭示,在较高温度时,钢中的马氏体相变方式仅有两种:γfcc→εhcp,γfcc→εhcp→αbcc;而在较低温度,当钢中的主要形变机制为TRIP效应时,除上面两种马氏体相变方式外,还存在另外一种情况,即γfcc→αbcc,奥氏体可直接转变为α马氏体。TWIP钢由于具有高的延伸率,在形变过程中造成母相FCC结构的γ奥氏体内部形成大量位错并相互缠结,位错的缠结不利于ε马氏体的形成,而直接导致位错型αbcc马氏体。更多还原

【Abstract】 The combination of high strength and good ductility is a seeking objective of the structural material design. Recently, the new developed high manganese steels containing 15 to 25 mass% Mn and additions of silicon and aluminum of about 2 to 4 mass% exhibit the large elongation （60-95%） and high tensile strength （600-1100MPa） due to twinning induced plasticity （TWIP） effect or transformation induced plasticity （TRIP） effect via multiple martensitic transformations. Although the effects of Mn, Si and Al on the microstructure and mechanical properties of TWIP steel had been investigated, the effect of N, Nb on those of TWIP steel are not reported, and the mechanism of TWIP effect, TRIP effect are not revealed sufficiently. Several TWIP steels with different content of N, Al or with addition of Nb were designed, and the effects of N, Al and Nb on stacking fault probability （SFP, reverse proportional to stacking fault energy） and mechanical properties were studied by means of tensile tests and characterization through optical microscope （OM）, X-ray diffraction （XRD）, scanning electron microscope （SEM） and transmission electron microscope （TEM）. And the mechanisms of TWIP effect andεhcp martensite formation as well as the approaches of martensitic transformation were systematically investigated, and the main conclusions are described as follows.The SFP of TWIP steels with different addition of N, Al were determined, and tensile tests were performed at different temperature. The results indicate that both N and Al increase stacking fault energy （SFE） of TWIP steel, suppress martensitic transformation and favour the TWIP effect. The XRD spectra of TWIP steels with different composition were analyzed and the results show thatαmartensitic transformation is intensively suppressed in the TWIP steel with addition of N. When FCC austensite or HCPεmartensite transforms to BCC structureαmartensite in these steels with addition of N, the biggest interstice decreased from 0.1044nm in FCC or HCP structure to 0.0733nm in BCC, and the lattice distortion energy of BCC martensite were greatly enlarged by N situated in the interstices, leading to the suppression of theαmartensitic transformation .The tensile tests were carried out and the SFP of TWIP steels with different addition of Nb were determined at different temperature. The results show that the addition of Nb remarkably decreases SFP, or increases SFE, and lead to superior ductility by favouring the formation of deformation twinning and TWIP effect. The microstructure characterization of TWIP steel containing Nb show that the addtion of Nb greatly reduces the size of austensite grain, and thus improves the strength of steel. The comprehensive effects of Nb on grain refinement and SFE give rise to excellent mechanical properties, the product of tensile strength and elongation（PSE） can be as high as 70000MPa% and is 20000MPa% higher than the TWIP steel without addition of Nb; the average PSE is as high as 56150.5MPa% at low temperature range from 0 to -30℃and is also higher than the 44729 MPa% of the later.During the observation of stacking faults by TEM, the directional shifts of some diffraction spots were found in the local area with high density of parallel stacking faults, the relationship between the shift of diffraction spot and SFP in FCC structure is deduced and the method of determining SFP was established. Based on the results of local SFP measured by electron diffraction, the mechanism ofγfcc→εhcp martensitic transformation was proposed, i.e. deformation provides the energy for the dissociation of perfect dislocation, and thus a large number of partial dislocations accompanying stacking faults are formed during deformation, and the stress field of partial dislocations can induce the formation of other perfect dislocations and their dissociation, resulting in the localization of stacking faults, then the high density of local stacking faults evolute from stacking disorder to stacking order and formedεmartensite eventually. When the stacking fault probabilityα=1, the perfectεmartensite is formed, otherwise, whereasα→1, theεmartensite with stacking fault is formed. Based on the mechanism ofεmartensite formation it is clear that both of the nucleation and growth ofε,αmartensites will consume a large number of stacking faults and make the number of stacking faults decrease sharply, which caused that the SFP measured by XRD is not reverse proportional to SFE after martentic transformations in TWIP steels.Tensile test and XRD analysis of several TWIP steels at different temperature indicate that stacking fault energy is proportional to temperature, i.e. stacking fault energy decreases with the lowering temperature. The close relationship between stacking fault energy and temperature result in different mechanical properties of the alloy at different temperatures. At higher temperature, the SFE is higher and is favoured to twinning during deformation, and TWIP effect is induced; at lower temperature, TRIP effect is favoured. The deformation temperature range in which TWIP or TRIP effect takes place is different for steels with different SFE. With the increasing of SFE, the peaks in an elongation-temprature curve caused by TWIP or TRIP effect move to lower temperature.The morphologies of stacking faults and twins in FCC TWIP steels were predicted by crystallographic analysis and were verified by means of OM, SEM and TEM. The mechanisms of inducing plasticity （sliding induced plasticity, twinning induced plasticity or transformation induced plasticity） in TWIP steels was suggested based on both the observation of morphology of twins at different deformation conditions and fundamental principle of martensitic transformation, namely, the start temperature （Tf） of deformation twinning and the start temperature （Ms） of therma-induced martensite determine the deformation mechanisms. Above the Tf, the ductility of TWIP steel is realized only by sliding; between Tf and Ms, the predominant mechanism is deformation twinning accompanying TWIP effect; and below Ms, it is TRIP effect.The results of tensile tests at different deformation rate show that, the mechanical properties of TWIP steel is affected by deformation rate. The yield strength is slowly enhanced with increasing deformation rate; while the ultimate strength lowly decreases with the increase of deformation rate. Both of the total elongation and uniform elongation decrease firstly and then increase with increasing deformation rate. Generally, the effect of deformation rate on elongation is more powerful than on yield strength and ultimate strength.The mechanical behavior of TWIP steel at different temperature show that, at higher temperature, martensitic transformation experiences in two ways:γ_fcc→ε_hcp,γ_fcc→ε_hcp→α_bcc; at lower temperature, there is the other way of martensitic transformation:γ_fcc→α_bcc, i.e. austensite can be directly transformed toαmartensite when TRIP effect became a predominant mechanism. The high elongation of TWIP steel result in the formation and tangle of a large number of dislocations inγaustensite during deformation, and such tangle dislocations are not favourable for the formation ofεmartensite, and cause the transformation of dislocation-type martensite from austensite directly.更多还原