ZrTiNiCuBe块体非晶合金剪切带内温升与断裂温升

在室温环境下(25℃)对某ZrTiNiCuBe块体非晶合金材料进行不同应变率条件下的静态与动态压缩实验。并采用扫描电镜技术(SEM)对试样断口、侧面等进行表征,对比静、动态条件下的应力-应变曲线形貌的差异。结果表明:静态压缩时为剪切断裂,微观形貌上出现脉状花样与剪切带;剪切带诱发裂纹的形成,裂纹随着剪切带扩展。动态压缩时为脆性解理断裂,断面粗糙且发现大量熔滴;断口处出现解理台阶,塑性阶段出现明显的锯齿流变现象。从能量守恒定律出发,利用变形过程中弹性应变能的变化规律推测剪切变形区域内温升的变化规律,温升的变化规律揭示锯齿流变与试样的断裂机制。     

Ti40Zr25Ni8Cu9Be18块体非晶合金低温动态压缩性能研究

采用铜模喷铸法制备Ti40Zr25Ni8Cu9Be18块体非晶合金,通过分离式霍普金森压杆装置(SHPB)对Ti40Zr25Ni8Cu9Be18块体非晶合金进行室温(25℃)和液氮温度(–196℃)条件下的高应变率加载动态压缩测试,结合S-4800型扫描电镜(SEM)对压缩试样断口进行观察,对比在室温和液氮温度下Ti40Zr25Ni8Cu9Be18块体非晶合金动态压缩性能及其断口形貌特征的差异。结果表明:Ti40Zr25Ni8Cu9Be18块体非晶合金室温动态压缩时,随应变率提高抗压强度无明显变化,没有应变率硬化效应。在液氮温度动态压缩时,抗压强度随着应变率提高有较大幅度增加,存在应变率硬化效应。液氮温度时的动态抗压强度明显高于室温动态抗压强度。Ti40Zr25Ni8Cu9Be18块体非晶合金室温动态压缩为剪切断裂,微观形貌上出现脉状花样和剪切带,剪切带诱发了裂纹的形成,裂纹沿着剪切带扩展。液氮温度下断口微观形貌有解理台阶和河流花样。室温动态断裂过程中,局域应变集中产生塑性变形;液氮温度下压缩动能转化的热量大部分被抵消,削弱了绝热剪切作用。     

锆基块体非晶合金剪切带扩展过程的“应变硬化现象”

选择典型的锆基块体非晶合金Vit.105(Zr_(52.5)Cu_(17.9)Ni_(14.6)Al_(10)Ti_5),采用小高径比准静态压缩试验,对合金进行两种不同方式的加载。通过对不同变形模式试样自由体积及维氏硬度的测定,研究了非晶合金的应变诱发体积扩张特性;讨论了剪切带空间增殖的微观机制。结果表明,非晶合金中应变诱发体积扩张过程发生于剪切带外部仅经受弹性应变的区域;剪切带扩展是一个"软化-硬化"的动态过程,这为剪切带的空间增殖创造条件。     

块体非晶合金剪切带的原子力纳米压痕行为

通过原子力显微镜(AFM)纳米压痕实验研究传统纳米压痕实验形成的Fe基非晶合金剪切带区域及未变形区域的变形;结合压痕形貌实时原位观测,探讨块体非晶合金塑性变形局域化的原因.结果表明最大载荷44.6μN下,材料的剪切带、剪切带间和未变形区域的AFM纳米压痕残余面积分别为3274.7、2976.5和2879.2nm2,对应的硬度分别为13.62、14.98和15.49GPa,剪切带区域的硬度值比未变形区域的硬度值降低了约10%,说明塑性变形过程中,过剩自由体积的产生使剪切带结构发生软化.     

Zr55Cu30Al10Ni5块体非晶合金在过冷液态区的流变行为及本构关系

Zr55Cu30Al10Ni5块体非晶合金(BMG)在过冷液态区内的单向压缩实验表明:材料在过冷液态区内的形变行为强烈依赖于温度和变形速率.随着应变速率的增加,材料的流变特征由Newtonian流变转变为非Newtonian流变.利用扩展指数本构方程模型建立了非晶合金的流变应力,应变速率和温度的关系.     

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．     

Zr41.25Ti13.75Ni10Cu12.5Be22.5块体非晶合金的拉伸断裂行为

Zr41．25Ti13.75Ni10Cu12．5Be22．5块体非晶合金在过冷液相区内的拉伸断裂行为与拉伸温度及应变速率有关．在室温条件下拉伸表现为剪切带扩展形成脆性断裂,在断口表面层出现了大量放射状脉状纹络,断口表面层的绝热升温现象导致断裂层温度超过了液相线温度,从而在断面上出现熔融状液滴．在玻璃化转变温度点,断裂模式依然为脆性断裂,韧窝状形貌及脉状纹络代替了放射状的脉状纹络分布于断口表面,同时伴随有熔融状液滴．计算表明,断口表面层绝热升温温度依然高于液相线温度．在更高温度或较低应变速率条件下,断裂方式转变为颈缩断裂模式,在颈缩点顶端依然存在脉状纹络,而未能观察到熔融状液滴．分析表明,在较高温度或较低应变速率条件下,绝热升温对断裂层的影响非常微弱,断口表层的温度没有明显升高．     

一种新的具有室温大塑性的Zr基块体非晶合金

通过二元共晶比例法设计了一种新的具有室温大塑性的Zr51Al9.96Ni14.34Cu24.7块体非晶合金.该合金室温压缩最大强度达2356 MPa,塑性应变达14.6%.高分辨透射电镜(HRTEM)及热分析(DSC)结果表明,压缩过程的变形诱发了纳米晶化.纳米晶的形成一方面诱发新的剪切带的形成,增加了剪切带的密度;另一方面阻碍了剪切带的进一步扩展,使大量初始剪切带与二次剪切带的交割和分叉,因而导致所研究非晶合金优异的塑性变形能力.     

块状非晶态合金Zr65Al7.5Ni10Cu17.5的室温单轴压缩断裂行为

利用IUTM和SEM研究了Zr65Al7 5Ni10Cu17.5块状非晶合金的室温单轴压缩变形和断裂行为.该合金的室温压缩变形过程主要表现为弹性和塑性变形,并且塑性变形阶段没有加工硬化现象.宏观形貌观察发现样品形成与压缩方向呈约45°的变形带.塑性变形以剪切带粘性流层相对滑动的方式进行,在变形过程中形成脉络状断面形貌.裂纹于塑性变形积累到一定程度之后,在切变变形较大的区域表面形成.裂纹从萌生、扩展到断裂,时间极短.流变积累和应力集中综合作用的结果导致断裂.     

Zr61Cu25Al12Ti2和Zr52.5Cu17.9Ni14.6Al10Ti5块体非晶合金过冷液相区的塑性流变行为

利用Gleeble 3500热模拟试验机研究了块体非晶合金Zr₆₁Cu₂₅Al₁₂Ti₂（ZT1）和Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅（Vit105）过冷液相区的压缩变形性能及其流变行为。结果表明，温度和应变速率影响其在过冷液相区的变形模式，主要包括3种典型方式：低温或高应变速率时局部剪切导致的脆性断裂，较高温度或者低应变速率时的Newtonian流变，中温区间或者较高应变速率时的非Newtonian流变。给出了2种合金Newtonian流变向非Newtonian流变转变的边界条件及过冷液相区的变形图。当压缩应变速率小于1×10^-3s^-1时，ZT1和Vit105分别在678～703 K和703～738 K温度区间表现出Newtonian流变行为，适合进行热塑性加工。利用自由体积模型分析发现，与ZT1相比，Vit105合金的流变激活体...     

New regime of homogeneous flow in the deformation map of metallic glasses: elevated temperature nanoindentation experiments and mechanistic modeling

The character of plastic deformation in metallic glasses is investigated through instrumented nanoindentation experiments on amorphous Pd₄₀Ni₄₀P₂₀ and Mg₆₅Cu₂₅Gd₁₀. Using a customized experimental apparatus, nanoindentation experiments have been conducted over four decades of indentation strain rate and from ambient temperature up to the glass transition, allowing rapid evaluation of an extensive deformation map with only small volumes of experimental material. At low rates and temperatures, inhomogeneous or serrated flow is observed, owing to the discrete operation of individual shear bands. Two distinct regimes of homogeneous flow can be identified. The first, expected, regime of homogeneous flow corresponds to the onset of viscous deformation at high temperatures and low rates, and is well described by existing mechanistic models. The second homogeneous regime occurs at high deformation rates even well below the glass transition, and arises when deformation rates exceed the characteristic rate for shear band nucleation, kinetically forcing strain distribution. By extending an existing model for glass deformation to explore shear band nucleation kinetics, this second regime is quantitatively rationalized and the natural frequency for shear band nucleation is extracted from the data. From this analysis the critical radius of a shear band as it transitions from nucleation to propagation is estimated to be in the submicron range.     

Surface and Subsurface Deformation of Wear-Resistant Steels Exposed to Impact Wear

The impact wear resistance of four different wear-resistant steel grades was investigated using different impact bodies. Post-test evaluation of the impact tested samples was performed by different techniques including 3D surface profilometry, microhardness indentation, optical and scanning electron microscopy, and energy dispersive x-ray spectroscopy. The tribological response of the steel plates during the impact is strongly dependent on the properties of the impacting body. The subsurface deformation was found to increase with increasing impact energy and/or impact velocity and decreasing steel hardness. On a microscopic scale, a number of interesting mechanisms were revealed within the deformed impact sites. Besides an overall plastic deformation, localized deformation resulting in narrow adiabatic shear bands with an ultra-fine microstructure was observed. Within these shear bands, showing intense shearing strain, nucleation of microvoids was frequently observed. Growth and linkage of these voids lead in crack formation along the shear bands and eventually flake-like wear fragments are detached when these cracks reach the surface.     

Atomic rearrangements in metallic glass: Their nucleation and self-organization

The atomic rearrangement in metallic glass (MG) is a dynamics process. However, this problem has always been investigated within the quasi-static limit of the material. As such, the physical mechanisms of how the local rearrangements nucleate and coalesce into shear bands have not been fully understood. This study is to clarify the issue with the aid of a systematic molecular dynamics analysis. The present study unveils that the underlying mechanism of plastic deformation is through the rearrangement of atoms, characterized by the sudden surge in kinetic energy or strain rate of a local region, and that the shear banding is a stress-driven dynamic coalescence of these rearranging clusters, propagating in the speed of a shear wave. The ratio of the internal strain rate in forming a cluster to the applied strain rate is a measure of the severity of the local atomic rearrangement. The larger the severity, the easier the shear banding forms. The additional kinetic energy associated with the atom rearrangement in clustering is due to the descending of the potential energy after crossing the energy barrier. As temperature increases, the thermal vibration energy becomes larger than the barrier height, leading to thermal activations in MG and hence giving rise to a homogeneous deformation.     

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．     

Ti—55合金中的热塑剪切带

研究了三种不同处理的Ｔｉ－５５合金在Ｈｏｐｋｉｎｓｏｎ压杆上高速冲击变形时产生的热塑剪切带。结果表明,三种不同处理的合金试样在不同应变率下出现两类型剪切带：形变剪切带和“白色”剪切带,它们是在不同应变阶段下形成的,对应“白色”剪切带有一应变突变。ＴＥＭ观察未发现“白色”剪切的内发生相变。孪生是该合金动态冲击时变形的重要方式。     

Effect of equal-channel angular pressing routes on high-strain-rate deformation behavior of ultra-fine-grained aluminum alloy

The effect of equal-channel angular pressing (ECAP) route on the high-strain-rate deformation behavior of ultra-fine-grained aluminum alloy was investigated. The 8-pass ECAPed specimens deformed via three different routes consisted of ultra-fine grains 0.5 μm in size, and contained a considerable amount of second-phase particles, which were fragmented and distributed in the matrix. In the torsion tests, the maximum shear stress significantly increased with increasing number of ECAP passes, while the maximum shear stress and fracture shear strain were lowest in the specimen deformed via route A among the three 8-pass ECAPed specimens. Observation of the deformed area beneath the fractured surface revealed the adiabatic shear bands of 100 μm in width in the specimen deformed via route A, which minimized the maximum shear stress and fracture shear strain, whereas they were hardly formed in the specimens deformed via route B or C. The formation of adiabatic shear bands was explained in terms of critical shear strain, deformation energy required for void initiation, and microstructural homogeneity related to ECAP routes.     

动态载荷下Al—Li合金剪切变形局部化

采用Ｈｏｐｋｉｎｓｏｎ压杆装置,对４种时效处理的Ａｌ－Ｌｉ合金在冲击压缩试验时变形局部化产生的力学条件和微结构进行了研究,结果指出,４种处理的合金均产生两种类型的剪切带即形变剪切带和白剪切带,它们是在局部化过程中不同形变阶段下形的。两者无本质上的差别。白色剪切带内未发生相变。大应变和高应变率是产生力剪切带的必要条件,即材料在一定的高应变率下形变到一定程度时才能形成剪切带,电镜研究表明,剪切带内无畸     

Microstructural evolution in adiabatic shear localization in stainless steel

Shear bands were generated under prescribed and controlled conditions in an AISI 304L stainless steel (Fe–18%Cr–8%Ni). Hat-shaped specimens were deformed in a Hopkinson bar at strain rates of ca 10⁴ s⁻¹ and shear strains that could be varied between 1 and 100. Microstructural characterization was performed by electron backscattered diffraction (EBSD) with orientation imaging microscopy (OIM), and transmission electron microscopy (TEM). The shear-band thickness was ca 1–8 μm. This alloy with low-stacking fault energy deforms, at the imposed strain rates (outside of the shear band), by planar dislocations and stacking fault packets, twinning, and occasional martensitic phase transformations at twin-band intersections and regions of high plastic deformation. EBSD reveals gradual lattice rotations of the grains approaching the core of the band. A [110] fiber texture (with the [110] direction perpendicular to both shear direction and shear plane normal) develops both within the shear band and in the adjacent grains. The formation of this texture, under an imposed global simple shear, suggests that rotations take place concurrently with the shearing deformation. This can be explained by compatibility requirements between neighboring deforming regions. EBSD could not reveal the deformation features at large strains because their scale was below the resolution of this technique. TEM reveals a number of features that are interpreted in terms of the mechanisms of deformation and recovery/recrystallization postulated. They include the observation of grains with sizes in the nanocrystalline domain. The microstructural changes are described by an evolutionary model, leading from the initial grain size of 15 μm to the final submicronic (sub) grain size. Calculations are performed on the rotations of grain boundaries by grain-boundary diffusion, which is three orders of magnitude higher than bulk diffusion at the deformation temperatures. They indicate that the microstructural reorganization can take place within the deformation times of a few milliseconds. There is evidence that the unique microstructure is formed by rotational dynamic recrystallization. An amorphous region within the shear band is also observed and it is proposed that it is formed by a solid-state amorphization process; both the heating and cooling times within the band are extremely low and propitiate the retention of non-equilibrium structures.     

The microstructures of polycrystalline Al–0.1Mg after hot plane strain compression

The microstructures of Al–0.1Mg polycrystals deformed in plane strain compression at temperatures of 20–400 °C have been investigated by electron backscatter diffraction. At all temperatures, the microstructures are characterized by two types of banded substructure, primary bands of aligned low angle boundaries whose alignment to the rolling plane is a function of strain and temperature, and secondary bands which develop into grain-scale shear bands with increasing strain. Several aspects of the deformation microstructures, including the orientation dependence of subgrain parameters and the inhomogeneity of the microstructure, differ from those of single crystals of similar orientation, and this it attributed to local grain interactions.     

工业纯钛动态压缩特性及破坏的实验研究

用分离式霍普金森压杆对TA2工业纯钛进行了动态压缩及绝热剪切破坏实验研究,得到了不同应变率和不同温度下的宏观应力-应变曲线。通过对压缩本构特性及微观金相破坏的比较分析,讨论了应变率、表观压缩本构特性对绝热剪切形成的影响。结果显示:TA2试样动态压缩呈现绝热剪切破坏特征,绝热剪切带在空间呈对称的双锥形状;应变率越高,形成绝热剪切带的临界应变越小;分析表明,动态压缩实验无法得到关于绝热剪切起始、发展过程的本构软化信息。