Characterization of deformation localization in cold-rolled metastable β-Ti–Nb–Ta–Zr alloy

The deformation-induced microstructural variation in the metastable β-type biomedical Ti–35Nb–5Ta–7Zr (wt.%) alloy subjected to multi-pass cold-rolling to 90% reduction has been investigated by a combination of X-ray diffraction, optical microscopy, conventional transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) techniques. Multi-pass cold rolling for the Ti–35Nb–5Ta–7Zr alloy includes various localized deformation processes which can result in dislocation tangle, stress-induced ω-phase transformation and deformation-band formation. Deformation-induced amorphization caused by high-density defect accumulation in deformation bands has been identified. By means of TEM and HRTEM observations, distributional, morphological and structural features for deformation bands have been clearly revealed.     

Electron-microscopic examination of microstructural changes in structural martensitic steel after low-cyclic fatigue tests

Microstructural changes in a structural martensitic steel upon low-cyclic fatigue (LCF) deformation have been investigated. The micromechanics of plastic deformation and accompanying effects have been studied at the scale of martensite laths and packets with the aid of transmission electron microscopy (TEM). It has been shown that with increasing LCF deformation, changes in both the morphology and the internal structure of martensite occur. The changes are manifested in the form of a refinement of the structural units of martensite. It has been revealed that in the limits of a packet the fatigue deformation occurs inhomogeneously. However, the laths of the same orientation are deformed equally and almost simultaneously. The influence of the dimensions of former austenite grains and orientation of packets on the LCF process has been considered. The mechanics of the fatigue plastic deformation on the nano-, meso-, and microlevels and the processes that accompany this deformation have been studied.     

Effect of thermomechanical processing parameters on phase transformation and hardness of dual matrix ductile iron

The microstructural changes and hardness exhibited by ductile iron with dual matrix structure (DMS) are investigated. In particular, DMS microstructures are obtained by continuous cooling in the (ferrite+austenite) region followed by quenching to transform the austenite into martensite or by austempering at 375°C, to transform the austenite into ausferrite. Additionally, two deformation steps are applied in the austenite region. The structure was produced in a thermomechanical simulator equipped with a dilatometry system. The dilatometry is used to monitor the structure development throughout the thermomechanical processes. The structure was investigated using light optical microscopy and scanning electron microscopy. The influence of introducing ferrite to the microstructure and the deformation magnitude on the structure development and hardness properties are explored.     

14Cr1MoR钢控轧和直接淬火工艺研究

研究控制轧制和直接淬火相结合生产14Cr1MoR压力容器钢的工艺.结果表明奥氏体再结晶区变形时,单道次变形量越大则再结晶进行得越充分,再结晶后的晶粒就越细.奥氏体未再结晶区总变形量增大则奥氏体晶粒内形变区域增多,从而使马氏体形核点增多,淬火后得到细板条马氏体组织.该钢种控轧后有足够高的温度进行直接淬火,经高温回火得到细小回火索氏体,机械性能优良.     

Kinetics of deformation processes in high-alloyed cast transformation-induced plasticity/twinning-induced plasticity steels determined by acoustic emission and scanning electron microscopy: Influence of austenite stability on deformation mechanisms

The austenite stability and the stacking fault energy of high-alloyed metastable transformation-induced plasticity/twinning-induced plasticity (TRIP/TWIP) steels, both depending on the chemical composition, have a strong influence on the deformation processes and stress/strain-induced martensitic phase transformation. Aiming at a better understanding of the kinetics of TRIP/TWIP-assisted plastic deformation, acoustic emission (AE) measurements were performed during room temperature tensile deformation of high-alloyed cast model steels with different austenite stability. The real-time AE investigations were complemented by detailed scanning electron microscopy investigations of deformed microstructures using electron backscattered diffraction to determine the martensitic phase transformation and electron channelling contrast to visualize dislocations and their arrangements. The quantitative AE analysis revealed different AE patterns at different plastic strains, which were correlated with underlying deformation mechanisms and microstructural transformations.     

Microstructural analysis of thermally induced and deformation induced martensitic transformations in Fe–12.5 wt.% Mn–5.5 wt.% Si–9 wt.% Cr–3.5 wt.% Ni alloy

Martensitic transformations induced by thermally and compression deformation at room temperature in Fe–12.5 wt.% Mn–5.5 wt.% Si–9 wt.% Cr–3.5 wt.% Ni alloy were studied in detail by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From microstructural observations, it was seen that heat treated samples exhibited regular overlapping of stacking faults and ɛ martensite plates were formed parallel to each other. Also, TEM investigations showed that the orientation relationship between γ (fcc) and ɛ (hcp) phases corresponds to Shoji–Nishiyama type. With applied low plastic deformation rate, only ɛ martensite occurred in austenite grain. As a consequence, 4 and 25% plastic deformation at room temperature caused ɛ martensite formation in austenite phase and the new ɛ (hcp) and α′ (bcc) martensite formation in martensite phases, respectively. Orientation relationship between ɛ and α′ phases was found by the electron diffraction analysis.     

Different Behavior in Electron Beam Welding of 18 Ni Co-free Maraging Steels

Microstructures of two different 18 Ni Co-free maraging specimens A and B and their electron beam (EB) weld joints were investigated comparatively by optical microscopy and scanning electron microscopy (SEM). It is shown that both of the steels are typical lath martensite; however, grain size of specimen A is about three times as large as that of specimen B. X-ray diffraction (XRD) reveals that the amount of the reverted austenitic phase in A is obviously less than that in B. Most of the austenite distributes in plate form along grain and lath boundaries while some of it distributes as fine particles within the matrix. The microstructural differences between the two specimens led to diverse behaviors in EB welding. The specimen A is weldable but B shows obvious welding defects of pits and burn-through holes in weld face. The welding microstructure exhibits a typical dendritic morphology, and the grains in the heat-affected zone recrystallized and grew because of high temperatures of welding EB. The weldablity of the examined materials is related to the microstructure characteristics that markedly affected thermal conduction performance.     

Substructures of lenticular martensites with different martensite start temperatures in ferrous alloys

This study investigated the substructures of lenticular martensites with different martensite start temperatures (Ms) by transmission electron microscopy. Observation of Fe–33Ni revealed a substructural change from fine transformation twins in the midrib and twinned region to several sets of screw dislocations in the untwinned region during growth. Tangled and curved dislocations also appeared near the martensite–austenite interface of the untwinned region, as the martensite inherited the dislocations in the surrounding austenite. In contrast, curved and tangled dislocations appeared in the entire untwinned region in Fe–31Ni and in the whole martensite plate in Fe–20.5Ni–35Co, as the higher Ms temperatures facilitated the plastic deformation of the surrounding austenite. Thermally transformed thin plate martensite in Fe–31Ni–10Co–3Ti grew into a lenticular shape accompanied by a substructure with dislocations after deformation at temperatures above the Ms temperature. The change in the substructure of lenticular martensite presumably resulted from the local temperature rise in the martensite plate.     

Precise measurement of strain accommodation in austenite matrix surrounding martensite in ferrous alloys by electron backscatter diffraction analysis

Local strain distributions in austenite matrix that is deformed to accommodate shape strain associated with formation of martensite were investigated by means of electron backscatter diffraction (EBSD) analysis for various morphologies of lath, lenticular and thin plate martensite in ferrous alloys. By detecting small changes in EBSD patterns through image analysis of the patterns, components of both strain and rotation tensors in austenite matrix adjacent to martensite were measured quantitatively. In the austenite matrix surrounding thin plate martensite, the magnitude of components of strain tensor is nearly as large as those of rotation tensor, implying that shape strain of thin plate martensite is accommodated by elastic deformation of austenite. On the other hand, in the austenite matrices surrounding lenticular and lath martensite, components of strain tensor are found to be much smaller than those of rotation tensor even near the austenite/martensite interface. This indicates that most of the shape strain associated with the formation of lenticular and lath martensite is accommodated by plastic deformation in the austenite matrix. The misorientation axis of austenite adjacent to lenticular and lath martensite coincides well with that predicted from the phenomenological theory of martensite crystallography.     

大塑性变形纳米结构Al-Mg合金中的形变缺陷和电子辐照效应

为澄清大塑性变形纳米结构Al-Mg合金中形变缺陷形成的本质,采用高分辨透射电子显微镜（HRTEM）研究电子辐照对高压扭转合金中面缺陷形成的影响。结果表明：对已有高密度面缺陷的HRTEM图像,经电子束照射一段时间后,这些面缺陷会完全消失;而在没有缺陷的HRTEM图像区域进行电子辐照,即使电子束的照射提高到足以在该区域击出孔洞,整个过程均未观察到任何晶格缺陷。因此,高压扭转合金中的面缺陷主要来源于极度的塑性变形,而与HRTEM观察过程中的电子辐照效应无关。     

Ultragrain refinement of plain low carbon steel by cold-rolling and annealing of martensite

Simple cold-rolling and annealing of martensite starting structure can produce ultrafine grained structure in carbon steel. The microstructural evolution during the process was studied in a 0.13%C steel. The ultrafine lamellar dislocation cells (LDCs) with mean thickness of 60 nm were mainly observed in a 50% cold-rolled specimen as well as the irregularly bent lamellas (IBLs) and the kinked laths (KLs). The LDCs and the IBLs had large local misorientations. The specimens annealed at temperatures from 723 to 773 K showed the multiphased ultrafine structure composed of equiaxed ultrafine ferrite grains with the mean grain size of 180 nm, nano-carbides distributed uniformly and small blocks of tempered martensite. The formation of the ultrafine grained structure was discussed from the viewpoint of characteristics of the martensite starting structure. It was concluded that the fine grained structure of martensite play an important role for ultrafine grain subdivision during plastic deformation.     

异构片层结构304不锈钢的制备及其力学性能

采用形变诱导马氏体退火逆转变工艺制备了异构片层结构(HLS)的304奥氏体不锈钢。通过扫描电镜和X射线衍射仪分析了304奥氏体不锈钢的显微组织和物相组成,并采用室温拉伸试验研究了其力学性能。结果表明,通过变形量为34%的热轧、75%的冷轧以及700 ℃退火12 min后,试验钢中的马氏体相逆转变为奥氏体相,部分残留奥氏体发生再结晶,获得了由微米再结晶晶粒与超细晶/纳米晶晶粒以及残留奥氏体晶粒组成的异构片层结构,微米再结晶晶粒和残留奥氏体被超细晶/纳米晶晶粒所包围。异构片层结构304奥氏体不锈钢的屈服强度为940.1 MPa,断裂总延伸率为43.1%,获得了良好的强度-塑性匹配。     

Quantitative analysis of variant selection in ausformed lath martensite

Variant selection in lath martensite transformed from deformed austenite in a low-carbon low-alloy steel is examined quantitatively on the basis of electron backscatter diffraction analysis of martensite, in concert with a novel method for the reconstruction of austenite orientation. At a strain of 10%, variants whose (0 1 1)_α plane was nearly parallel to the primary slip plane in austenite ((1 1 1)_γ) were formed dominantly. At strains of 30% and 50%, variants whose (0 1 1)_α plane was nearly parallel to the secondary slip plane ((−1 1 1)_γ) as well as the primary slip plane were formed. Transmission electron microscopy observation of an austenite-stabilized alloy deformed under the same condition as the low-carbon low-alloy steel clarified that microband structures develop along the primary and secondary slip planes of austenite when its orientation is close to the main component of the deformation texture in austenite. A simple variant selection model is proposed in which martensite variants with habit planes nearly parallel to the primary and secondary slip planes nucleate preferentially on microband boundaries as a result of a smaller amount of activation energy and grow dominantly as a result of less inhibition from the microband boundaries.     

9%Cr-1%Mo耐热钢焊缝金属连续冷却组织转变

利用Ｆｏｒｍａｓｔｏｒ－Ｄ全自动热膨胀记录仪测定了三种改进型９％Ｃｒ－１％Ｍｏ耐耐热钢焊缝金属的连续冷却组织转变相图,并用光学显微镜和透射是分析了各种冷却条件下的组织转变特点。结果表明,在很大的冷却范围内,三种焊缝金属的工体均只发生马氏体转变,只有当冷速足够慢时才发生先共析铁素体转变和共析转变。其中,含合金元素较少的２号焊缝金属具有最快的先共析铁素体形成冷速。研究发现,三种焊缝金属的奥氏体均形成板     

Control of Austenite Characteristics and Ferrite Formation Mechanism by Multiple-Cyclic Annealing in Quenching and Partitioning Steel

Quenching and partitioning (Q&P) treatment is a novel method to produce advanced high strength steel with excellent mechanical properties. In this study, combination of multiple-cyclic annealing and Q&P process was compared with traditional cold-rolled Q&P steel to investigate the microstructural characteristics and austenite retention. The results showed that retained austenite in traditional Q&P sample was principally located in the exterior of austenite transformation products, while those in multiple-cyclic annealing samples were mainly distributed inside the transformation products. With the increase in cyclic annealing number, both of austenite fraction and austenite carbon content increased, attributing to higher initial austenite carbon content and larger number of austenite/neighbored phase interface to act as carbon partitioning channel. In traditional Q&P sample, the deformed ferrite was recrystallized by sub-grain coalescence, while the austenite was newly nucleated and grew up during annealing process. As a comparison, the ferrite in multiple-cycle annealing samples was formed by means of three routes: tempered martensite that completely recovered with retention of interior martensite variant, epitaxial ferrite that formed on basis of tempered martensite, ferrite that newly nucleated and grew up during the final annealing process. Both of lath martensite and twin martensite were formed as initial martensite and then tempered during partitioning process to precipitate ε carbide with C enrichment, Mn enrichment and homogeneous Si distribution. Compared with the traditional cold-rolled Q&P steel, the Q&P specimens after multiple-cyclic annealing show smaller strength and much larger elongation, ascribing to the coarser microstructure and more efficient transformation induced plasticity (TRIP) effect deriving from retained austenite with high carbon content and larger volume fraction. The application of double annealing treatment can optimize the mechanical properties of Q&P steel to show a striking product of strength and elongation as about 29 GPa%, which efficiently exploit the potential of mechanical performance in low carbon steel.     

变形温度对形变强化相变完成时临界应变量的影响

利用“形变强化相变”机制研究了低碳钢过冷奥氏体在740℃和780℃,10s-1变形时的变形温度对相变完成时临界应变量εc的影响。结果表明,变形温度对εc和组织演变的影响很大。在740℃和780℃变形时,εc分别为0.96和1.39,变形温度降低明显促进了相变。变形温度对εc的影响在组织演变上主要表现为铁素体形核地点的不同。740℃变形时,铁素体由奥氏体晶界形核过渡到以形变带形核为主,形核速率极高;780℃变形时,铁素体由奥氏体晶界形核过渡到在铁素体/奥氏体相界面前沿高畸变区快速形核。     

Grain refinement and mechanical properties of asymmetrically rolled low carbon steel

The formation of fine ferrite grains by the asymmetric rolling of low carbon steel and their mechanical properties were studied. Super-cooled low carbon austenite was deformed by asymmetric rolling at 750 °C with a roll size ratio of 1.5 and immediately cooled at various cooling rates ranging from 3 °C/s to 15 °C/s. Fine ferrite grains (∼2 μm) were formed after asymmetric rolling, preferentially at the prior austenite grain boundaries. The volume fraction of the fine ferrite grains increased with increasing rolling reduction. A ferrite plus pearlite microstructure was obtained at smaller strains and slower cooling rates. However, after heavy deformation, a fine ferrite grain structure with carbide particles dispersed at the ferrite grain boundaries was obtained and the pearlite structure was not observed even after very slow cooling, which implies that most of the ferrite grains were formed dynamically, i.e. during deformation. The yield strength of the asymmetrically rolled steel plates increased with increasing deformation; however, the yield ratio also increased with increasing rolling reduction. The best combination of strength and yield ratio was obtained by using a low level of deformation and a high cooling rate, in which case a portion of the untransformed austenite transformed to martensite.     

Characterization of deformation-induced metallurgical change in austenite stainless steel sheet with acoustic microscopy

This article reports on the non-destructive evaluation (NDE) of metallurgical changes induced by the deformation processes. In particular, the martensite and reversed austenite states of a meta-stable austenitic stainless steel sheet were evaluated. The sheet was elongated up to 40% at room temperature, and was then brought below room temperature to produce martensite. This was then followed by annealing for reversion. First, martensite content was measured with a Feritscope. Second, the surface acoustic wave (SAW) velocity was measured with a high numerical aperture spherically focused ultrasonic beam with a frequency equal to 600 MHz. The measured SAW velocity values were found to be dependent on the elongation, the ambient temperature during elongation, and the annealing temperature. A useful trend was found in the correlation between the measured SAW velocity and the martensite content as measured with the Feritscope. Second, using a high frequency (i.e., 800 MHz) acoustic imaging technique, the deformed and annealed grain structures were mapped. In comparing the acoustic images with the optical images, the deformed grains shown in the acoustic images were found to be significantly clearer than those shown in the optical images. These studies reveal the importance of NDE, in connection with the forming of stainless steel structures, to map the deformation induced metallurgical changes. Here, unique NDE techniques are seen to be effective in mapping the changes with the use of hypersonic SAW velocity measurements and imaging.     

镍钛形状记忆合金在局部包套压缩下的塑性屈服

作为一种崭新的尝试,局部包套压缩被应用于实现镍钛形状记忆合金在室温下的大塑性变形。基于主应力法和塑性屈服准则,分析了镍钛形状记忆合金局部包套的压缩塑性力学。采用透射电镜、高分辨透射电镜和扫描电镜研究镍钛形状记忆合金在局部包套压缩下的显微组织演变和变形行为。静水压力随着包套外径的增加而增加,有效地抑制了显微裂纹的萌生和扩展,有助于提高镍钛形状记忆合金的塑性,避免了脆性断裂的发生。在0．15～0．50的真实应变范围内,镍钛形状记忆合金在三向压应力状态下的塑性变形满足密席斯塑性屈服准则。在更大的塑性应变下,由于非晶相的出现,镍钛合金不能满足密席斯塑性屈服准则。     

Mechanically-induced martensite transformation of NiTiFe shape memory alloy subjected to plane strain compression

Based on the channel die compression, NiTiFe shape memory alloy (SMA) was subjected to plane strain compression. Mechanically-induced martensite transformation, nanocrystalline and amorphous phase can be observed in the case of large plastic strain. Mechanically-induced martensite transformation is obviously different from the conventional stress-induced martensite transformation. The former generally occurs after dislocation slip, whereas the latter arises prior to dislocation slip. The occurrence of B19’ martensite phase contributes to accommodating subsequent plastic deformation of NiTiFe SMA. Mechanically-induced B19’ martensite is partially stabilized due to the existence of local high stress field and consequently it is unable to be reverted to B2 austenite phase during unloading.