ECAP强变形过程中θ′相的碎化及溶解行为分析

试验研究了铝-铜二元合金中的θ′相在ECAP强变形中的碎化及溶解行为.X射线衍射分析结果表明,ECAP 变形至8道次时θ’相的衍射峰消失.透射电镜观察表明,在切应力作用下θ′相发生弯曲变形,内部形成位错墙、亚晶、剪切带及扭转带结构.θ’相首先在亚晶界面、剪切带及扭转带与基体的界而结合处发生溶解,继而断裂碎化及球化,并逐渐发生分解溶入基体.分析表明,θ′相在强变形中主要通过其亚晶界面、扭转带及剪切带界面等缺陷处的溶解效应而发生碎化,而其溶解过程并非由表面能主导引起的.     

单晶γ-TiAl合金纳米切削过程声发射响应的原子模拟

本工作采用分子动力学方法研究了单晶γ-TiAl合金纳米切削过程的声发射响应。从原子尺度阐述了单晶γ-TiAl合金切削过程中裂纹形成机理。研究发现：切削初期随着切削力持续增大,剪切区域产生周期性的剪切带;与此同时,在高压应力和弹性应力波共同作用下,类晶粒晶界的非晶原子带的产生阻碍了剪切带的持续发射,使主剪切区的应力无法及时通过剪切带释放,产生局部应力集中现象,导致裂纹萌生并扩展;通过对采集的声发射信号分析,压应力会导致切削过程中声发射功率下降。在时域上,通过对微观缺陷演化和声发射功率-频率对比分析,阐述了纳米切削过程中晶格振动、剪切带以及裂纹萌生与扩展的声发射响应特征,并通过聚类分析得到了损伤的功率和频率特性。     

ECAP强变形过程中θ'相的碎化及溶解行为分析

试验研究了铝-铜二元合金中的θ′相在ECAP强变形中的碎化及溶解行为。X射线衍射分析结果表明,ECAP变形至8道次时θ′相的衍射峰消失。透射电镜观察表明,在切应力作用下θ′相发生弯曲变形,内部形成位错墙、亚晶、剪切带及扭转带结构。θ′相首先在亚晶界面、剪切带及扭转带与基体的界面结合处发生溶解,继而断裂碎化及球化,并逐渐发生分解溶入基体。分析表明,θ′相在强变形中主要通过其亚晶界面、扭转带及剪切带界面等缺陷处的溶解效应而发生碎化,而其溶解过程并非由表面能主导引起的。     

等通道转角挤压Al-Cu合金的应力和应变疲劳行为比较

通过恒应力和恒塑性应变控制疲劳实验,比较了等通道转角挤压四道次的Al—0．7％Cu(质量分数)合金的疲劳寿命、表面变形形貌、疲劳开裂和疲劳断口．结果表明：样品发生明显的循环软化,致使应变和应力疲劳寿命在高、低应力范围存在差别．在应变控制疲劳样品中,塑性变形既可由剪切带来承担,也可以由剪切带和形变带共同承担,进而疲劳裂纹分别沿剪切带或形变带萌生．而应力控制疲劳的塑性变形只集中在剪切带中,并导致剪切疲劳开裂．疲劳断口上存在典型的疲劳裂纹萌生区、缓慢扩展区、快速扩展区和最后瞬断区．     

金属玻璃的剪切带变形与断裂机制研究进展

金属玻璃的室温塑性变形一般高度局域于剪切带中,因此剪切带行为主导着金属玻璃的变形与断裂机制,对力学性能(如强度、塑性、韧性及疲劳性能等)有重要影响.鉴于剪切带行为对理解和改善金属玻璃力学性能的极端重要性,长期以来关于剪切带的研究一直是金属玻璃领域的热点之一.本文基于作者近年来在剪切带变形与断裂机制上的研究结果,阐述金属...     

Monel合金高速塑性剪切变形与动态再结晶

采用Ｈｏｐｋｉｎｓｏｎ杆加载技术和帽状试样，研究Ｍｏｎｅｌ合金在εｓ≈４．３×１０５ｓ－１的高应变速率条件下的应力－应变关系及塑性变形行为．实验及计算结果表明，塑性区（简称剪切带）内温度随剪切应变增加呈线性升高．透射电镜证实剪切带组织发生了动态再结晶，再结晶晶粒约为１７０ｎｍ．分析了剪切带中组织结构的演变过程，讨论了动态再结晶的机制     

结构迟豫对Zr基大块非晶敏感因子的影响

采用Rockwell压痕测试和SEM、DSC技术研究了铸态和退火Zr41.2Ti13.8Cu12.5Ni10Be22.5大块非晶的剪切带和焓变特征.样品在玻璃转变温度以下等温退火,DSC放热峰随着退火温度的增加不断减少,这表明自由体积不断减少.对压痕周围剪切带形态的观察表明,退火样品中被激活的剪切带减少,剪切带之间的夹角2θ不断变化,铸态大约为88°,经过633K保温1h减小为79°.由Mohr-Coulomb准则可知,通过测量2θ可以确定压力敏感因子,随着退火温度的升高,原子间相互摩擦的作用加剧,导致压力敏感因子增加.     

Ti-Cu基非晶合金玻璃形成能力与力学性能

基于"二元共晶混合"法设计Ti-Cu-Ni-Zr合金成分,通过水冷铜模铸造法制备出不同直径Ti-Cu-Ni-Zr合金棒。利用X射线衍射仪(XRD)、差示扫描量热仪(DSC)、万能材料试验机和扫描电镜(SEM)研究了Ti-Cu-Ni-Zr合金玻璃形成能力和力学性能。结果表明,Ti-Cu-Ni-Zr合金具有较高玻璃形成能力,其临界直径可达4 mm;Ti-Cu-Ni-Zr合金玻璃形成能力近似相等,而表征玻璃形成能力的热力学参数过冷液相区ΔTx,参数γ,约化玻璃转变温度Trg也近似相等。Ti_(32.3)Cu_(47.6)Ni_(7.9)Zr_(12.2)和Ti_(31.6)Cu_(48.2)Ni_(7.7)Zr_(12.5)大块非晶合金分别具有0.7%和0.2%的塑性,而Ti_(30)Cu_(49.5)Ni_(7.2)Zr_(13.3)和Ti_(28.55)Cu_(50.7)Ni_(6.75)Zr_(14)大块非晶合金断裂机制近似为脆性断裂。Ti-Cu-Ni-Zr大块非晶合金塑性越大,其剪切带数量越多且扩展深度越大,反之亦然。另外,对于塑性材料,锯齿流变振幅越大,对应样品表面剪切带扩展深度越明显;锯齿流变振幅越小,对应样品表面剪切带扩展深度越浅;近似脆性材料的锯齿流变对应次剪切带萌生,而对于完全脆性大块非晶合金,在应力-应变曲线上并未发现锯齿流变现象,相应的在样品外表面也并未发现次剪切带。     

铝合金在两种应力状态下损伤机理研究

铝合金汽车构件在撞击的过程中,构件的应力状态各点均不相同,而且在撞击的过程中各点的应力状态随着时间变化而变化.为了研究铝合金构件在不同应力状态下损伤机理,采取了两种代表性的三轴应力状态,即缺口拉伸与纯剪切所产生的应力状态.研究结果表明缺口拉伸试验中,缺口根部产生相对较高的三轴应力,而且缺口根部明显产生微孔洞,随着应力的不断升高,微孔洞的体积分数不断增大.当达到材料的临界孔洞体积分数时,试样断裂;纯剪切试验中,三轴应力几乎等于0,在材料内部几乎没有产生微孔洞而产生了剪切带.剪切带在切应力的作用下变形不断增大,当到达材料的等效塑性断裂应变时,试样发生断裂;用Gurson损伤模型和剪切失效分别模拟缺口拉伸和剪切试验,试验的工程应力-应变曲线与模拟的工程应力-应变曲线拟合的很好.     

铝合金在两种应力状态下损伤机理研究

铝合金汽车构件在撞击的过程中,构件的应力状态各点均不相同,而且在撞击的过程中各点的应力状态随着时间变化而变化.为了研究铝合金构件在不同应力状态下损伤机理,采取了两种代表性的三轴应力状态,即缺口拉伸与纯剪切所产生的应力状态.研究结果表明:缺口拉伸试验中,缺口根部产生相对较高的三轴应力,而且缺口根部明显产生微孔洞,随着应力的不断升高,微孔洞的体积分数不断增大.当达到材料的临界孔洞体积分数时,试样断裂;纯剪切试验中,三轴应力几乎等于0,在材料内部几乎没有产生微孔洞而产生了剪切带.剪切带在切应力的作用下变形不断增大,当到达材料的等效塑性断裂应变时,试样发生断裂;用Gurson损伤模型和剪切失效分别模拟缺口拉伸和剪切试验,试验的工程应力-应变曲线与模拟的工程应力-应变曲线拟合的很好.     

An electron microscopy appraisal of tensile fracture in metallic glasses

Three glass-forming alloy compositions were chosen for ribbon production and subsequent electron microscopy studies. In situ tensile testing with transmission electron microscopy (TEM), followed by ex situ TEM and ex situ scanning electron microscopy (SEM), allowed the deformation processes in tensile fracture of metallic glasses to be analysed. In situ shear band propagation was found to be jump-like, with the jump sites correlating with the formation of secondary shear bands. The effect of structural relaxation by in situ heating is also discussed. Nanocrystallization near the fracture surface was observed; however, no crystallization was also reported in the same sample and the reasons for this are discussed. Both the TEM and the SEM observations confirmed the presence of a liquid-like layer on or near the fracture surface of the ribbons. The formation of a liquid-like layer was characterized by the vein geometries and vein densities on the fracture surfaces and its dependence on shear displacement, δ, is discussed. A simple model is adapted to relate the temperature rise during shear banding to the glass transition and melting temperatures and this is used to explain the variety of fracture surfaces which are developed for macroscopically identical tensile testing of metallic glasses together with features which exhibit local melting.     

块状非晶合金纳米压痕微区形变的研究

对Zr55A110Ni5Cu30块状非晶合金进行了室温纳米压痕实验，利用扫描电子显微镜、透射电子显微镜揭示了非晶合金的微区形变特征。结果表明：在压痕周围形成离散的剪切带，形变引起了块状非晶合金局部自由体积增加。利用自由体积模型对非晶合金的微区形变机制进行了分析。     

样品尺寸对Pd基块体非晶合金内部剪切带的影响

以Pd79Cu4Au2Si10P5块体非晶合金为对象,研究了不同尺寸合金的塑性变形行为及其内部剪切带特征。结果表明,直径为1,2和3 mm的该非晶合金都具有良好的塑性变形能力,其塑性变形量随着试样尺寸的增大而降低,分别约为13%,10%,8%。不同尺寸试样的应力-应变曲线有明显不同的锯齿流变,2 mm试样锯齿振幅最大,3 mm试样锯齿振幅最小,而且1和3 mm试样的应力-应变曲线的锯齿间隔都比2 mm试样的小。金相显微镜的观察结果表明,随着试样尺寸的增加,其内部主剪切带密度降低,次生剪切带密度增加。在扫描电镜下能明显看到内部主剪切带厚度随着试样尺寸的增加而增加。     

Size-independent strength and deformation mode in compression of a Pd-based metallic glass

We present quasi-static, room temperature compression data for Pd₄₀Ni₄₀P₂₀ metallic glasses, with specimen sizes ranging from the submicron to several millimeters in diameter. We observe no change in deformation mode over this range. At all sizes, plastic flow is localized in shear bands, which are accompanied by sudden strain bursts. This metallic glass shows only a modest increase in strength in going from bulk to micrometer-sized specimens. We show that stress gradients in tapered specimens can complicate measurement of the yield strength of metallic glasses in microcompression. Estimates of yield strength based on the minimum cross-sectional area implicitly assume that yielding is controlled by a maximum effective shear stress criterion. An alternative is the shear plane yield criterion, in which the minimum shear stress on the shear band trajectory determines yield. Application of this criterion in tapered microspecimens reinforces the notion that metallic glasses possess relatively size-independent mechanical properties.     

Microstructural characteristics of post-shear localization in cold-rolled 316L stainless steel

The microstructural evolution in a cold-rolled 316L stainless steel during shear localization was comprehensively studied using transmission electron microscopy (TEM). The TEM results indicate that the main substructure inside a shear band consists of elongated lath, fine rectangular and equiaxed subgrains. The substructure at an early stage of shear banding was found to strongly depend upon existing defects, especially deformation twin patterns. These twin structures determine the initial dimensions of the elongated subgrains inside the shear bands. The coexistence of both rectangular and equiaxed subgrains suggests that no melting occurred though the predicted temperature was much higher than the bulk melting temperature. Dynamic/static recovery and continuous dynamic recrystallization are thought to be the main mechanisms by which these substructures form inside the shear bands. A new mechanism for nanostructure deformation of subgrains within shear bands is proposed to explain the temperature divergence between the experimental and calculated results.     

Effect of Al Addition on the Mechanical Properties and Microstructure of Zr35Ti30Cu7.5Be27.5 Bulk Metallic Glass

利用单轴压缩试验研究了用部分Al替代Zr对Zr35-xTi30Cu7.5Be27.5Alx(x=0,1,1.5,2,2.5,5,at%)块体金属玻璃力学性能的影响。研究表明,当添加Al含量为1.5at%和2at%时,所得到的块体金属玻璃的压缩塑性从0.95%(x=0)分别提高至15.10%(x=1.5)和3.45%(x=2)。利用扫描电子显微镜(SEM)对金属玻璃样品的断裂形貌进行了表面分析。利用透射电子显微镜(TEM)对不同Al含量的块体金属玻璃试样进行了微观结构表征,结果显示,Al含量为1.5at%和2at%的金属玻璃样品的微观结构呈现出了纳米级别的"微观不均匀性"。最后,结合临界剪切应力(CSS)讨论了微观结构与塑性变形行为之间的关联性。     

Formation and evolution of spherocrystals in a NiTiZrAlCuSi amorphous alloy

The microstructure of a NiTiZrAlCuSi bulk metallic glass was investigated by transmission electron microscopy (TEM) attached with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). TEM results showed that some spherocrystals were embedded in the amorphous matrix, but XRD patterns from the sample did not reveal distinct diffraction peaks from the minor amount of spherocrystals. EDS analyses indicated that the composition of these spherocrystals was similar to that of the metallic glasses matrix. To further study the evolution of these spherocrystals, isothermal crystallization experiment was carried out using differential scanning calorimetry (DSC). DSC curves and XRD patterns reveal the microstructure difference of the sample at different heating times. TEM images record the evolution processes of the metastable spherocrystal and amorphous matrix.     

Zr_(65)Al_(7.5)Ni_(10)Cu_(12.5)Ag_5非晶合金压痕下方剪切带形貌的研究

以Zr65Al7.5Ni10Cu12.5Ag5块体非晶合金作为研究对象,采用界面压痕技术和扫描电子显微镜,分别对铸态、轧制态以及轧制-退火态试样压痕下方的剪切带形貌进行了研究。实验结果表明,铸态试样以半圆形剪切带为主,同时存在少量的射线状剪切带,并且形貌比较规则;轧制态试样的剪切带形貌变得很不规则,很难分辨半圆形和射线状剪切带,并且随着变形量的增加,剪切带形貌的不规则程度增加;轧制-退火态试样中重新出现了规则的半圆形和射线状剪切带,变形量对剪切带形貌几乎没有影响。这些结果说明,轧制态试样压痕下方剪切带形貌不规则的主要原因是轧制变形使剪切带中自由体积含量显著增加。     

Prevalence of shear banding in compression of Zr41Ti14Cu12.5Ni10Be22.5 pillars as small as 150 nm in diameter

Recently, the size dependence of mechanical behaviors, particularly the yield strength and plastic deformation mode, of bulk metallic glasses (BMG) has created a great deal of interest. Contradicting conclusions have been drawn by different research groups, based on various experiments on different BMG systems. Based on in situ compression transmission electron microscopy (TEM) experiments on Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 (Vit 1) nanopillars, this paper provides strong evidence that shear banding still prevails at specimen length scales as small as 150 nm in diameter. This is supported by in situ and ex situ images of shear bands, and by the carefully recorded displacement bursts under load control as well as load drops under displacement control. Finite element modeling of the stress state within the pillar shows that the unavoidable geometry constraints accompanying such experiments impart a strong effect on the experimental results, including non-uniform stress distributions and high level hydrostatic pressures. The seemingly improved compressive ductility is believed to be due to such geometry constraints. Observations underscore the notion that the mechanical behavior of metallic glasses, including strength and plastic deformation mode, is size independent at least in Vit 1.     

Interactions between mechanically generated defects and corrosion phenomena of Zr-based bulk metallic glasses

Using the example of a Zr-Ti-Al-Cu-Ni bulk metallic glass with an ex situ approach, two aspects were studied: (i) the effects of pre-formed corrosion pits on the room temperature deformation behaviour, and (ii) the effect of mechanically generated defects on the local corrosion susceptibility. For the latter, Vickers micro-indents generated with different loads were also employed as model defects. It is revealed that pre-formed corrosion pits act as favourable surface sites to which shear bands are linked and most likely nucleate, which leads to a slight reduction in the maximum compression stress. Further, mechanically generated defects have to be considered as critical weak points at a glassy sample surface which can act as preferential sites for local corrosion initiation and propagation. Local chemical and structural changes due to the formation of shear bands as a response to mechanical load are discussed as possible reasons for this. For potential high-load applications of bulk metallic glasses this interaction must be considered as an important lifetime limiting aspect.