V-5Cr-5Ti的常温动态压缩力学性能

采用分离式霍普金森压杆(SHPB)装置,在常温下对V-5Cr-5Ti合金分别进行了应变率效应试验、 限制应变试验和应变累积的动态压缩试验;分析了应变率、应变累积对V-5Cr-5Ti合金动态压缩性能的影响; 并对压缩试验后的试件进行了金相分析。结果表明,V-5Cr-5Ti合金具有明显的应变率和应变历史效应,而 应变历史对材料的动态压缩性能的影响更明显。在高应变率下,应变低于0.20时,出现孪晶组织;而应变高 于0.20时,孪晶组织急剧减少甚至消失。     

大应变率范围内AM80镁合金的变形行为及组织演变

采用INSTRON准静态压缩试验机和分离式霍普金森压杆装置，研究固溶态AM80镁合金在室温准静态和冲击载荷下的变形行为及组织演变。准静态载荷下，流变应力随应变率（3×10-5~4×10-1 s-1）的升高逐渐降低，表现为负应变率敏感性；冲击载荷下，流变应力随应变率（7.00×102~5.20×103 s-1）的升高而升高，呈现出明显的正应变率敏感性。冲击载荷下AM80镁合金的变形机制以基面滑移和孪生为主，大量细小致密的形变孪生以及适量非基面滑移的启动是AM80镁合金在冲击载荷下流变应力明显高于准静态载荷的重要原因。此外，随应变率的升高，AM80镁合金变形的均匀性明显增强，当应变速率升至3.65×103 s-1时，冲击变形所引起的局部绝热温升软化大于应变硬化与应变速率硬化的总和，部分晶粒产生了明显的动态回复，使得孪晶密度和变形均匀性反而降低。     

纯锆在压缩加载下微观变形断裂机制研究

利用材料试验机和SHPB装置研究了常态下纯锆的准静态和动态压缩力学性能。塑性变形阶段的流动应力及硬化率随应变率提高而增大,应变10％～20％段的应力-应变曲线表现出凹向上趋势。金相观测表明孪晶是纯锆的重要变形机制,孪晶密度随应变及应变率的增长而增大。大应变时,在动态压缩试样表面观测到与压缩轴呈45°的宏观裂纹。试样纵向剖开后,金相观测到发展的绝热剪切带。     

密排六方金属中的孪生及孪晶位错机制

密排六方晶体结构金属中可同时启动的滑移系少,孪生成为密排六方金属中重要的塑性变形形式．由于密排六方金属复杂的晶体结构,均匀切变不能保证所有晶格点都能与基体形成对称的晶体结构,因此密排六方金属的孪生通常为滑移和原子重组（shuffle）机制相结合．本文以密排六方金属中常见的{101 ̅2}、{101 ̅1}、{112 ̅2}及{102 ̅1}孪生为例,阐述不同类型孪生过程中的孪晶位错机制．分析表明,由于原子重组机制的参与,密排六方金属的孪生可以通过不同形式的孪晶位错实现．以上四种密排六方金属孪晶中,只有{112 ̅2}孪生中的一层孪晶位错是纯剪切机制,其余的孪生机制都需要原子重组的参与．孪生机制可以大致分为滑移主导、原子重组主导以及滑移-重组相结合的机制．当孪生类型确定时,即第一不畸变面（孪晶面）k_1（和孪晶剪切方向η_1）确定时,不同孪晶位错机制对应的孪晶剪切大小和方向均不同,第二不畸变面k_2和共轭剪切方向η_2也不相同,所导致孪晶的拉压性质也不同．不同剪切方向和大小的孪晶位错机制有可能在不同应力和温度条件下被激活,从而作为密排六方金属塑性的重要来源．     

基于微结构演化的V5Cr5Ti合金塑性本构模型

为合理描述V5Cr5Ti合金的塑性变形行为,本文建立了基于微结构演化的塑性本构模型。首先,采用小尺寸试样开展了V5Cr5Ti合金单轴拉伸试验,并对其在不同应变程度下的微结构演化特征进行了分析。研究发现,影响V5Cr5Ti合金塑性变形行为的主要因素是位错密度演化以及团簇状和弥散析出相。据此建立了位错密度演化方程、组分相含量体积分量演化方程,并考虑团簇状和弥散状第二相对V5Cr5Ti合金流动应力的影响,进一步建立了包括非热应力、热激活应力和弥散相强化应力的流动应力关系式。最后,根据隐式应力更新算法对新模型进行了有限元实现,并与实验结果进行比较,验证了新模型的合理性和预测精度。     

基于微结构演化的V5Cr5Ti合金塑性本构模型

为合理描述V5Cr5Ti合金的塑性变形行为，本文建立了基于微结构演化的塑性本构模型。首先，采用小尺寸试样开展了V5Cr5Ti合金单轴拉伸试验，并对其在不同应变程度下的微结构演化特征进行了分析。研究发现，影响V5Cr5Ti合金塑性变形行为的主要因素是位错密度演化以及团簇状和弥散析出相。据此建立了位错密度演化方程、组分相含量体积分量演化方程，并考虑团簇状和弥散状第二相对V5Cr5Ti合金流动应力的影响，进一步建立了包括非热应力、热激活应力和弥散相强化应力的流动应力关系式。最后，根据隐式应力更新算法对新模型进行了有限元实现，并与实验结果进行比较，验证了新模型的合理性和预测精度。     

准一维应变下Al2O3陶瓷动态压缩失效的实验研究

为了考察Al2O3陶瓷准一维应变下的力学性能,采用LY12铝套筒和45钢套筒对陶瓷试样进行了环向约束,在准静态和动态下分别进行了压缩实验,得到了材料的应力-应变曲线。实验结果表明,Al2O3陶瓷的轴向压缩强度基本上随围压的增大而增大,动态压缩加载下的轴向压缩强度随应变率增大而增大。Al2O3陶瓷在准一维应变下的破坏形式为裂纹破坏,准静态的脆性失效行为可以用Mohr-Coulomb失效准则来描述。     

SnAgCu焊料的动态力学性能

对SnAgCu焊锡材料在应变率0.001、600、1 200、1 800 s-1下的拉伸和压缩力学性能进行了测试,得到了不同应变率下的应力应变曲线。结果表明,该材料不仅具有明显的应变率效应,而且其动、静态的塑性硬化模量差异很大。金相分析显示:准静态压缩时,塑性变形主要由晶粒的转动、变形和晶界的滑移控制;而动态压缩时,可观察到材料内部的枝状晶粒被折断为大量次级晶枝,呈现出明显不同于准静态情况下的变形机制。     

FeCrNi合金静动态物理本构模型研究

以金属材料塑性变形的位错动力学为基础,将FeCrNi合金的流动应力分解为非热应力和热激活应力两部分.通过对该合金屈服应力随温度变化特性、屈服应力的应变速率特性、孪晶组织的温度特性及位错组态的应变速率特性进行分析,认为非热应力不只是应变的函数,还与温度和应变速率相关,因此对Johnson-Cook模型方程形式进行修正以描述非热应力. 同时认为影响热激活应力的微结构参数主要为位错阻碍间距\Deltal, 定义并推导出表征\Delta l演化的g函数的表达式,将其引入Kocks的热激活方程,从而建立FeCrNi合金的物理型本构模型.该模型初步实现了对FeCrNi合金从室温到高温、从准静态到动态塑性变形行为的描述.      

重组竹顺纹冲击力学性能研究

重组竹是一种新型竹基复合材料,其力学性能优于落叶松等木材。为评价重组竹在动态加载下的顺纹抗冲击力学性能,以密度1.06 g/cm3、含水率8.52%、龄期3～5年的毛竹基重组竹为研究对象,通过准静态单轴压缩和循环加卸载以及动态加载实验,研究了重组竹加载变形过程、各项力学性能指标以及对应变率的敏感性。结果表明:重组竹顺纹压缩过程可以分为弹性变形和弹塑性变形阶段,破坏类型为延性破坏,其各项强度指标随应变率的提高而提高,动态增长因子与应变率之间呈现线性关系,斜率为0.0024;重组竹压缩过程中的应变比能与应变之间呈线性关系,且随应变率的增长而增大,证明其吸能能力随着应变率的增大而提高。实验结果证明,重组竹顺纹具有良好的抗冲击力学性能和显著的应变率效应。     

Influence of deformation path on the strain hardening behavior and microstructure evolution in low SFE FCC metals

Following our recent studies of the influence of mechanical twinning on the strain hardening of low SFE FCC metals deformed by simple compression, the investigation was extended to two different deformation modes. These were plane strain compression and simple shear carried out on 70/30 brass, which exhibits only strain hardening, and on MP35N, a Co–Ni based alloy that also shows secondary hardening by deformation promoted precipitation. It was found that the magnitude of the primary strain hardening in both alloys, and the secondary hardening in MP35N, was dramatically reduced under simple shear compared to the other deformation paths. This reduced hardening in simple shear appears to be a consequence of the bulk of the deformation twins, and also the secondary hardening precipitates, forming on planes that were parallel to the primary {111} slip planes in this deformation path. These hypotheses are supported by deformation path change tests in which the shear samples that show low flow stress under continued shear, when subjected to simple compression showed a significant increase (jump) in the flow stress, reaching values that are similar to those of the alloy continuously compressed to the same equivalent strain. That is, the reduced strain hardening in shear deformation is due not to reduced twinning, but to the twins produced by shear providing only limited barriers to continued strain by simple shear. Shear banding was found to be more marked in plane strain compression than in simple compression after cold working, and particularly after the additional secondary hardening in MP35N.     

On the strain hardening and texture evolution in high manganese steels: Experiments and numerical investigation

We present a systematic investigation on the strain hardening and texture evolution in high manganese steels where twinning induced plasticity (TWIP) plays a significant role for the materials' plastic deformation. Motivated by the stress–strain behavior of typical TWIP steels with compositions of Fe, Mn, and C, we develop a mechanistic model to explain the strain-hardening in crystals where deformation twinning dominates the plastic deformation. The classical single crystal plasticity model accounting for both dislocation slip and deformation twinning are then employed to simulate the plastic deformation in polycrystalline TWIP steels. While only deformation twinning is activated for plasticity, the simulations with samples composed of voronoi grains cannot fully capture the texture evolution of the TWIP steel. By including both twinning deformation and dislocation slip, the model is able to capture both the stress–strain behaviors and the texture evolution in Fe–Mn–C TWIP steel in different boundary-value problems. Further analysis on the strain contributions by both mechanisms suggests that deformation twinning plays the dominant role at the initial stage of plasticity in TWIP steels, and dislocation slip becomes increasingly important at large strains.     

A crystal plasticity theory for latent hardening by glide twinning through dislocation transmutation and twin accommodation effects

Imagine a residual glide twin interface advancing in a grain under the action of a monotonic stress. Close to the grain boundary, the shape change caused by the twin is partly accommodated by kinks and partly by slip emissions in the parent; the process is known as accommodation effects. When reached by the twin interface, slip dislocations in the parent undergo twinning shear. The twinning shear extracts from the parent dislocation a twinning disconnection, and thereby releases a transmuted dislocation in the twin. Transmutation populates the twin with dislocations of diverse modes. If the twin deforms by double twinning, double-transmutation occurs even if the twin retwins by the same mode or detwins by a stress reversal. If the twin deforms only by slip, transmutation is single. Whether single or double, dislocation transmutation is irreversible. The multiplicity of dislocation modes increases upon strain, since the twin finds more dislocations to transmute upon further slip of the parent and further growth of the twin. Thus, the process induces an increasing latent hardening rate in the twin. Under profuse twinning conditions, typical of double-lattice structures, this rate-increasing latent hardening combined with crystal rotation to hard orientations by twinning is consistent with a regime of increasing hardening rate, known as Regime II or Regime B. In this paper, we formulate governing equation of the above transmutation and accommodation effects in a crystal plasticity framework. We use the dislocation density based model originally proposed by Beyerlein and Tomé (2008) to derive the effect of latent hardening in a transmuting twin. The theory is expected to contribute to surmounting the difficulty that current models have to simultaneously predict under profuse twinning, the stress-strain curves, intermediate deformation textures, and intermediate twin volume fractions.     

An efficient constitutive model for room-temperature,low-rate plasticity of annealed Mg AZ31B sheet

Metals and alloys with hexagonal close packed (HCP) crystal structures can undergo twinning in addition to dislocation slip when loaded mechanically. The complexity of the plastic response and the limited extent of twinning are impediments to their room-temperature formability and thus their widespread adoption. In order to exploit the unusual deformation characteristics of twinning sheet materials in designing novel forming operations, a practical plane stress material model for finite element implementation was sought. Such a model, TWINLAW, has been constructed based on three phenomenological deformation modes for Mg AZ31B: S (slip), T (twinning), and U (untwinning). The modes correspond to three testing regimes: initial in-plane tension (from the annealed state), initial in-plane compression, and in-plane tension following compression, respectively. A von Mises yield surface with initial non-zero back stress was employed to account for plastic yielding asymmetry, with evolution according to a novel isotropic and nonlinear kinematic hardening model. Texture and its evolution were represented throughout deformation using a weighted discrete probability density function of c-axis orientations. The orientation of c-axes evolves with twinning or untwinning using explicit rules incorporated in the model.     

Hardening evolution of AZ31B Mg sheet

The monotonic and cyclic mechanical behavior of O-temper AZ31B Mg sheet was measured in large-strain tension/compression and simple shear. Metallography, acoustic emission (AE), and texture measurements revealed twinning during in-plane compression and untwinning upon subsequent tension, producing asymmetric yield and hardening evolution. A working model of deformation mechanisms consistent with the results and with the literature was constructed on the basis of predominantly basal slip for initial tension, twinning for initial compression, and untwinning for tension following compression. The activation stress for twinning is larger than that for untwinning, presumably because of the need for nucleation. Increased accumulated hardening increases the twin nucleation stress, but has little effect on the untwinning stress. Multiple-cycle deformation tends to saturate, with larger strain cycles saturating more slowly. A novel analysis based on saturated cycling was used to estimate the relative magnitude of hardening effects related to twinning. For a 4% strain range, the obstacle strength of twins to slip is 3 MPa, approximately 1/3 the magnitude of textural hardening caused by twin formation (10 MPa). The difference in activation stress of twinning versus untwinning (11 MPa) is of the same magnitude as textural hardening.     

An experimental study of cyclic deformation of extruded AZ61A magnesium alloy

Thin-walled tubular specimens were employed to study the cyclic deformation of extruded AZ61A magnesium alloy. Experiments were conducted under fully reversed strain-controlled tension-compression, torsion, and combined axial-torsion in ambient air. Mechanical twinning was found to significantly influence the inelastic deformation of the material. Cyclic hardening was observed at all the strain amplitudes under investigation. For tension-compression at strain amplitudes higher than 0.5%, the stress-strain hysteresis loop was asymmetric with a positive mean stress. This was associated with mechanical twinning in the compression phase and detwinning in the subsequent tension phase. Under cyclic torsion, the stress-strain hysteresis loops were symmetric although mechanical twinning was observed at high shear strain amplitudes. When the material was subjected to combined axial-torsion loading, the alternative occurrence of twinning and detwinning processes under axial stress resulted in asymmetric shear stress-shear strain hysteresis loops. Nonproportional hardening was not observed due to limited number of slip systems and the formation of mechanical twins. Microstructures after the stabilization of cyclic deformation were observed and the dominant mechanisms governing cyclic deformation were discussed. Existing cyclic plasticity models were discussed in light of their capabilities for modeling the observed cyclic deformation of the magnesium alloy.