Microstructure and tensile properties of high-strength high-ductility Ti-based amorphous matrix composites containing ductile dendrites

In the present study, two Ti-based amorphous matrix composites containing ductile dendrites dispersed in an amorphous matrix were fabricated by a vacuum arc melting method, and deformation mechanisms related to the improvement of strength and ductility were investigated by focusing on how ductile dendrites affected the initiation and propagation of deformation bands, shear bands or twins. Ti-based amorphous matrix composites contained 70–73 vol.% coarse dendrites of size 90–180 μm, and had excellent tensile properties of the yield strength (1.2–1.3 GPa) and elongation (8–9%). The Ta-containing composite showed strain hardening after yielding, and reached fracture without showing necking, whereas necking occurred straight after yielding without strain hardening in the Nb-containing composite. The improved tensile elongation and strain hardening behavior was explained by the homogeneous distribution of dendrites large enough to form deformation bands or twins, the role of β phases surrounding α phases to prevent the formation of twins, and deformation mechanisms such as strain-induced β to α transformation.     

Shear Localization and Damage in AA5754 Aluminum Alloy Sheets

In this article, we study the Portevin-Le Chatelier (PLC) bands and their influences on strain localization and fracture in continuous cast (CC) AA5754 aluminum sheets. Three types of tensile tests are conducted: (1) tensile samples are pulled directly to fracture at 223 K, (2) tensile samples are pulled at 223 K to initiate diffuse necking followed by unloading and reloading to fracture at room temperature, and (3) tensile samples are pulled at 223 K to localized necking and unloaded followed by reloading to fracture at room temperature. Furthermore, in situ V-bending test coupled with deformation mapping using digital image correlation is used to study damage at large strains. The results show that PLC bands detect favorable geometrical sites for shear band initiation. The formation of shear bands precedes damage and damage is a consequence of shear band formation. This article was presented at Materials Science & Technology 2007, Automotive and Ground Vehicles symposium held September 16-20, 2007, in Detroit, MI.     

Cr15Mn9Cu2Ni1N不锈钢连铸坯的高温拉伸变形特性

对于Cr15Mn9Cu2Ni1N不锈钢连铸坯,热变形过程中变形局部化的发生会影响其表面质量。从连铸坯的表层及芯部制取小型试样,利用热/力模拟试验机,进行温度950℃～1150℃范围内的拉伸试验。结果发现,随变形温度升高,该钢强度降低而延伸率提高;试样在发生颈缩,即变形局部化之前,要经历均匀变形和扩散颈缩变形,两种变形均使试样变形区获得均匀的宏观变形形貌;而高温拉伸的延伸率主要由扩散颈缩阶段的变形量决定。分析表明,均匀变形阶段主要靠应变强化抑制变形局部化的发生,而扩散颈缩变形阶段应变速率强化起主导作用。随变形温度升高,尤其在温度高于1100℃时,该钢的应变速率强化效应增强,可推迟最终变形局部化的发生,从而获得较大的延伸率。     

On the effect of hydrogen on plastic instabilities in metals

Experimental observations and theoretical calculations have demonstrated that hydrogen solute atoms increase the dislocation mobility in metals and alloys, thus promoting highly localized plastic processes which eventually lead to localized ductile rupture. While the underlying mechanism for hydrogen-enhanced dislocation mobility is well understood, little is known on how this mechanism acting at the microscale can lead to macroscopic plastic instability. In this paper, a theoretical investigation is carried out in a specimen under plane-strain tension in an effort to understand how hydrogen-induced softening and lattice dilatation at the microscale can lead to macroscopic i) shear localization (shear banding bifurcation) or ii) necking bifurcation.     

A Three-Parameter Model for Predicting Fatigue Life of Ductile Metals Under Constant Amplitude Multiaxial Loading

In this article, a fatigue damage parameter is proposed to assess the multiaxial fatigue lives of ductile metals based on the critical plane concept: Fatigue crack initiation is controlled by the maximum shear strain, and the other important effect in the fatigue damage process is the normal strain and stress. This fatigue damage parameter introduces a stress-correlated factor, which describes the degree of the non-proportional cyclic hardening. Besides, a three-parameter multiaxial fatigue criterion is used to correlate the fatigue lifetime of metallic materials with the proposed damage parameter. Under the uniaxial loading, this three-parameter model reduces to the recently developed Zhang’s model for predicting the uniaxial fatigue crack initiation life. The accuracy and reliability of this three-parameter model are checked against the experimental data found in literature through testing six different ductile metals under various strain paths with zero/non-zero mean stress.     

Deformation texture transition in brass: critical role of micro-scale shear bands

The transition in deformation textures between low stacking fault energy f.c.c. metals (e.g. brass textures) and the medium to high stacking fault energy f.c.c. metals (e.g. copper textures) is addressed. A detailed microscopy investigation was conducted in parallel with texture measurements on deformed samples of copper and 70/30 brass to different strain levels in three different deformation paths, namely, plane strain compression, simple compression, and simple shear. The objective of the study was to identify the specific trends in the transition between the brass textures and the copper textures that correlated with the onset of deformation twinning and those that correlated with the onset of micro-scale shear banding. It was found that several important transitions in the evolution of the deformation textures, especially in the rolled samples, correlated not with the onset of deformation twinning but with the onset of micro-scale shear banding. These results strongly suggest that the critical feature in texture transition is not twinning directly, but the shear banding promoted by the high strain hardening rates of low stacking fault energy f.c.c. metal.     

Analysis of plastic strain and deformation mode of a Zr-based two-phase bulk metallic glass in compression

A ductile phase-separated Zr-based bulk metallic glass (BMG) deformed to different strains at room temperature and low strain rate was characterized. The BMG samples were compressed to nominal strains of 3%, 7%, and 10%, after which the samples were unloaded for morphological observation using scanning electron microscopy. The morphological observation was subsequently used for the interpretation of the measured load–displacement curves. It was found that the BMG exhibited apparent uniform deformation initially (at plastic strain <1%) and, then, visible local shear bands began to developed. Afterwards, a principal shear band was soon developed and dominated the deformation process until fracture. In this study, we also found that the local shear strain varies along the principal shear plane and decreases monotonically from the shear band initiation site.     

Failure of high strength steel sheets: Experiments and modelling

Failure in sheet metal structures of ductile material is usually caused by one of, or a combination of, ductile fracture, shear fracture or localised instability. In this paper the failure of the high strength steel Docol 600DP and the ultra high strength steel Docol 1200M is explored. The constitutive model used in this study includes plastic anisotropy and mixed isotropic-kinematic hardening. For modelling of the ductile and shear fracture the models presented by Cockroft–Latham and Bressan–Williams have been used. The instability phenomenon is described by the constitutive law and the finite element (FE) models. For calibration of the failure models and validation of the results, an extensive experimental series has been conducted including shear tests, plane strain tests and Nakajima tests. The geometries of the Nakajima tests have been chosen so that the first quadrant of the forming limit diagram (FLD) were covered. The results are presented both in an FLD and using prediction of force–displacement response of the Nakajima test employing element erosion during the FE simulations. The classical approach for failure prediction is to compare the principal plastic strains obtained from FE simulations with experimental determined forming limit curves (FLCs). It is well known that the experimental FLC requires proportional strains to be useful. In this work failure criteria, both of the instability and fracture, are proposed which can be used also for non-proportional strain paths.     

Effects of strain rate on plastic flow and fracture in pure tantalum

This paper describes the strain rate effects on flow and failure properties of pure tantalum. The strain rate dependent plastic behavior was determined from both quasistatic and split Hopkinson bar (SHB) tests. The competing effect of strain rate and inertia on the onset of necking, post-elongation, and strain to failure (ductility) was investigated. The quasistatic tests were performed at strain rates 0.001, 1, and 10/sec. The strain rates in the SHB tests ranged between 700 and 1600/sec. Under quasistatic loading, the strain hardening in tantalum was found to be rate sensitive. The strain hardening coefficient (n) decreased continually from a maximum value of about 0.28 to almost 0 in the quasistatic loading regime. In the SHB tests, it was not possible to determine the shape of the stress-strain curves for strains less than 5% due to spurious wave reflections. Nonzero positive values forn at high strain rates were obtained through viscoplastic, constitutive modeling. The flow stress at SHB strain rate levels was almost twice the quasistatic value at a strain rate of 0.001/sec, indicating significant strain rate sensitivity in tantalum. Both the ductility and ultimate strain decreased with increasing strain rate under the quasistatic loading regime reaching a minimum of 0.36 and about 0.02, respectively, at 10/sec. While the ductility remained at this level in the SHB (dynamic) loading regime, the ultimate strain (at the onset of necking) increased to values greater than 0.1, indicating deformation stability due to inertia.     

Introducing a strain-hardening capability to improve the ductility of bulk metallic glasses via severe plastic deformation

The great technological potential for bulk metallic glasses (BMGs) arises primarily because of their superior mechanical properties. To realize this potential, it is essential to overcome the severe ductility limitations of BMGs which are generally attributed to shear localization and strain softening. Despite much international effort, progress in improving the ductility of BMGs has been limited to certain alloys with specific compositions. Here, we report that severe plastic deformation of a quasi-constrained volume, which prevents brittle materials from fracture during the plastic deformation, can be used to induce strain hardening and to reduce shear localization in BMGs, thereby giving a significant enhancement in their ductility. Structural characterizations reveal the increased free volume and nanoscale heterogeneity induced by severe plastic deformation are responsible for the improved ductility. This finding opens a new and important pathway towards enhanced ductility of BMGs.     

Structural bulk metallic glasses with different length-scale of constituent phases

Bulk metallic glass composites containing constituent phases with different length-scales are prepared via an in situ method by copper mold casting homogeneous Zr–Ti–Nb–Cu–Ni–Al melts. The phase formation and the microstructure of the composite materials are investigated by X-ray diffraction, optical, scanning and transmission electron microscopy, and microprobe analysis. The composition of the melt as well as the cooling conditions realized during casting determine the type and the morphology of the phases present in the composite. The mechanical properties of composite materials with quasicrystalline or ductile bcc phase reinforcements are tested in uniaxial compression at room temperature, showing that the deformation is controlled by the type of the constituent phases and their morphology. Ductile phase-containing metallic glass composites demonstrate improved work hardening and ductility compared to monolithic metallic glasses. Similar results are obtained for composites with ductile bcc phase dendrites embedded in a nanocrystalline matrix. The improved ductility of the composites is due to the presence of the ductile second phase, which counteracts catastrophic failure by shear localization.     

Microstructural Aspects of Damage and Fracture in AZ31 Sheet Materials

There are considerable data in the literature dealing with deformation mechanisms in AZ31 sheets. However, there is little information on the damage and fracture processes in this material. In this contribution, digital image correlation is used to follow deformation patterns occurring during tensile and v-bending tests at room temperature. A variety of surface analysis techniques and three-dimensional x-ray tomography have been used to examine the relationship between deformation, damage initiation, and the final fracture processes. The results show that premature diffuse necking occurs in the tensile tests without transit into localized necking. Deformation twins cluster by an autocatalytic process to form shear bands serving as preferential sites for strain localization and crack initiation. Damage appears in the form of microcracks within the shear bands at a late stage of necking and lead to the final fracture. The presence and the distribution of second-phase particles and their distributions help accelerate the final fracture processes.     

Microstructural evolution and plastic flow behavior of As-equal channel angular pressed 5083 Al alloy in low temperature superplasticity regime

In this study, the plastic flow behavior of ultrafine grained 5083 Al alloy fabricated by severe plastic deformation was examined in conjunction with microstructural evolution during deformation in the low temperature superplasticity regime. The present investigation was aimed at providing a better understanding of the nature of the low temperature superplasticity of ultrafine grained metallic materials. For this purpose, an ultrafine grained structure was introduced into the commercial 5083 Al alloy by equal channel angular pressing. A series of tensile tests was performed on the as-equal channel angular pressed samples at the initial strain rates of 10^?5–10^?2 sec^?1 and temperatures of 498–548 K, belonging to the low temperature superplasticity regime. The relationship between the true stress and true strain rate showed a sigmoidal behavior in a double logarithmic plot. The superplastic elongation was obtained within the limited intermediate strain range of 10^?4–10^?3 sec^?1 at 523 and 548 K. The microstructural examination and analysis of plastic flow curves revealed that low temperature superplasticity of the present alloy was attributed to dynamic recrystallization. In addition, necking instability during low temperature superplastic deformation of the alloy was discussed by applying Harts necking instability criterion.     

Formability limits by fracture in sheet metal forming

The aim of this paper is twofold: first, to revisit the forming limit diagram (FLD) in the light of fundamental concepts of plasticity, damage and ductile fracture mechanics and, second, to propose a new experimental methodology to determine the formability limits by fracture in sheet metal forming. The first objective makes use of the theory of plasticity applied to proportional strain loading paths, under plane stress conditions, to analyze the fracture forming limit line (FFL) and to introduce the shear fracture forming limit line (SFFL). The second objective makes use of single point incremental forming (SPIF), torsion and plane shear tests to determine the experimental values of the in-plane strains at the onset of fracture. Results show that the proposed methodology provides an easy and efficient procedure to characterize the formability limits by fracture in sheet metal forming. In particular, the paper shows that the FFL determined by means of tensile and conventional sheet formability tests is identical to that determined from SPIF tests on conical and pyramidal truncated specimens. The new proposed approach is expected to have impact in the established methodologies to outline the formability limits on the basis of the forming limit curves (FLC's) at the onset of necking.     

Experimental investigation of void coalescence in metallic sheets containing laser drilled holes

Although important, ductility remains difficult to predict and there is a tremendous need for more precise modelling. Progress in this field is hampered by a lack of quantitative experimental results to assess the validity of these models due to the stochastic nature of ductile fracture. In this paper, tensile tests have been carried out in a scanning electron microscope on model materials made of thin metallic sheets containing laser drilled holes. Depending on the material and hole configuration, different failure modes and strains are observed. The results show the importance of void spacing and orientation, constraining effects, materials yield stress and work hardening rate, and the competition between ductile fracture and shear localization. Finally, it is shown that the Thomason model for void coalescence is not appropriate for predicting fracture of the model material. However, the McClintock model for void growth, and the Brown and Embury and the McClintock models for void coalescence provide relatively good predictions.     

Strain rate-dependent deformation in bulk metallic glasses

Metallic glasses have metastable structures. As a result, their plastic deformation is dependent upon structural dynamics. In the present paper, we present data obtained from Zr-base and La-base metallic glasses and discuss the kinetic aspects of plastic deformation, including both homogeneous and heterogeneous deformation. In the case of homogeneous deformation (typically occurring in the supercooled liquid region), Newtonian behavior is not universally observed and usually takes place only at low strain rates. At high strain rates, non-Newtonian behavior is usually observed. It is demonstrated that this non-Newtonian behavior is associated with in situ crystallization of the amorphous structure. In the case of heterogeneous deformation (occurring at room temperature), deformation is controlled by localized shear banding. The plastic deformation of a La-base metallic glass is also investigated using instrumented nanoindentation experiments over a broad range of indentation strain rates. At low rates, the load-displacement curves during indentation exhibit numerous serrations or pop-ins, but these serrations become less prominent as the indentation rate is increased. Using the tip velocity during pop-in as a gauge of serration activity, we find that serrated flow is only significant at indentation strain rates below a certain critical value.     

The Portevin–Le Chatelier (PLC) effect and shear band formation in an AA5754 alloy

A non-contact strain measurement technique, based on digital image correlation (DIC) analysis, has been utilized in order to observe Portevin–Le Chatelier (PLC) band behaviour during tensile deformation of AA5754 sheet and subsequently to measure the level of incremental plastic strain carried within the bands. In addition, PLC banding was studied as a function of prior deformation under positive strain rate sensitivity conditions (i.e., at low temperature). Small prestrains (of the order of the Lüders strain) do not affect subsequent room-temperature deformation. However, prestrains of 0.1 or higher change the nature of the PLC bands that form at room temperature. Finally, careful experiments were performed involving low-temperature prestrains sufficient to cause immediate necking when deformation was continued at room temperature. These suggest that shear localization is independent of existing PLC bands.     

镁合金温变形过程中的孪生及孪晶交叉

对AZ31镁合金在3种温度(523,573和673 K)下进行了单向压缩变形.在523 K,当真应变ε达到0.22时真应力-真应变曲线出现尖锐的应力峰值,在应力峰值之前先后经历了缓慢加工硬化(0.02≤ε<0.06)和急剧加工硬化(0.06≤ε<0.22)2个阶段.利用SEM/EBSD技术分析了这2个阶段对应的显微组织.结果表明,在缓慢加工硬化阶段(ε=0.03),仅有少量孪晶出现;在急剧加工硬化阶段(ε=0.06),产生了大量{101~-2}孪晶,孪晶间的相互交叉导致材料产生急剧加工硬化.AZ31镁合金{101~-2}孪晶间交叉有5种可能存在的形式,孪晶的形成和交叉与压缩应力方向有密切关系.在基体应力方向分别为近似<112~-0>和<101~-0>方向时确认了(101~-2)-(011~-2)和(101~-2)-(01~-12)2种交叉形式.     

铜基非晶合金板弯曲塑性及剪切带间距的研究

制备了一种压缩断裂塑性应变接近2％的铜基块状非晶板材,并利用三点弯曲实验进行了其弯曲塑性、剪切带间距与试样厚度关系的研究．结果表明：非晶合金的弯曲断裂塑性应变明显依赖于试样厚度,即弯曲断裂塑性应变随试样厚度增加呈指数衰减关系减少;弯曲时剪切带间距随试样厚度增加而线性增加;剪切带间距与试样厚度的比值对于同一非晶合金为恒定值,但随非晶合金种类的不同而变化,与非晶合金的塑性变形能力有关．     

Three strategies to achieve uniform tensile deformation in a nanostructured metal

In nanostructured metals with grain sizes of the order of 100 nm, dislocation mechanisms remain dominant in controlling plastic deformation. These materials, similar to their coarse-grained counterparts that have been subjected to heavy cold work, can no longer go through the several strain hardening stages of normal metals and are hence susceptible to plastic instabilities such as necking in tension. For processing and applications, it is obviously important and often necessary to control such inhomogeneous plastic deformation. Here we demonstrate three strategies to achieve relatively large stable tensile deformation in nanostructured metals, using the pure Cu processed by equal channel angular pressing as a model. The first approach uses an in situ formed composite-like microstructure, such as a bimodal grain size distribution, to impart strain hardening to the material and attain large uniform tensile strains while maintaining the majority of the strengthening brought forth by nanostructuring. In the second route, deformation is conducted at low temperatures, such as 77 K. The material regains the ability to work harden due to suppressed dynamic recovery. Uniform elongation is achieved as a result, together with an elevated strength at the cryogenic temperature. The third method takes advantage of the elevated strain rate sensitivity of the flow stress of the nanostructured Cu, especially at slow strain rates. Using the stabilizing effects of strain rate hardening on tensile deformation, nearly uniform strains can be acquired in absence of strain hardening. We also discuss the deformation mechanisms involved in these approaches to assess their applicability to nanocrystalline metals with grain sizes well below 100 nm, where normal dislocation activities become severely suppressed.