Investigation of time-dependent characteristics of materials and micromechanisms of plastic deformation on a submicron scale by a new pulse indentation technique

A new pulse technique is proposed to investigate dynamics and micromechanisms of plastic deformations of materials underneath the indenter during microindentation. It is established that the process of indenter penetration under pulse loading conditions can be described by several distinct stages (as many as four in some cases) differing in kinetics and activation parameters. In each investigated material, the first stage was characterized by a high strain rate (10³ s^–1) and the high contact stresses (dynamic hardness), which exceeded static hardness by factor of 5–10. Typical values of activation volumes during the first stage were of the order of 10^–30 m³, i.e. close to the volume occupied by an atom (ion) in a lattice. During the second and the subsequent stages, the activation volume in ionic crystals increased up to about 10^–28 m³,( i.e. 10b ³, where b is Burger's vector of glide dislocations). Dynamics of initial stages of the indenter penetration was, therefore, determined by elastic and subsequently by plastic deformation, which is carried out by non-equilibrium point defects (most likely by interstitials or crowdions). A relative role of point defects in the process of mass transfer underneath the indenter and its contribution to microhardness is estimated. In soft materials (NaCl, LiF, Pb) during long indentation times (1 s) contribution of point defects can be estimated as more than 10%, and in the case of hard materials (Si, amorphous alloys) became closer to 100%. Dynamics of the final stages of the indenter penetration in soft crystals were governed by the dislocation creep. In all investigated materials for short indentation times (1 s) plastic deformation underneath the indenter occurred predominantly via generation and motion of interstitials and their clusters containing a few atoms.     

Structural transformations in the HP-HT formation of monophase superhard materials based on carbon and boron nitride dense phases

The known data on the mechanisms of structural transformations in the formation of bulk monophase materials based on diamond and cubic boron nitride at high pressures and temperatures have been briefly summarized. Two groups of materials have been defined: those based on the initial micron powders of diamond, cBN, and graphite-like BN as powder and pyrolytic deposits. The structure formation of the first group materials has been shown to be governed by the development of the lattice plastic deformations (by translational slip and twinning) and rotational deformation, while the structure formation of the cBN-based materials is governed by the primary recrystallization occurring at T > 1900°C and p = 7.7 GPa. The rotational deformation governs a plastic fragmentation of grains in materials. The formation of materials based on the initial graphite-like BN has been found to depend on the crystal-oriented phase transformation into BN dense (wurtzitic and sphaleritic) phases and recrystallization of the forming cBN phase. The mechanisms of the cBN primary recrystallization have been discussed.     

Introduction of Planar High Pressure Torsion (P‐HPT) for Fabrication of Nanostructured Sheets

A new severe plastic deformation process based on conventional high pressure torsion is introduced. The process, called planar high pressure torsion (P‐HPT), is capable of inducing large shear strains into materials with planar geometries, such as sheets or strips and can basically be implemented on every standard HPT machine. The principles of this technique will be presented and accompanied by a case‐study, where P‐HPT will be applied on a sheet of pure copper with dimensions of 220 × 110 mm² and a thickness of 0.75 mm. For comparison, the material is deformed by conventional high pressure torsion using standard specimens with a diameter of 8 mm as well. It will be shown that the mechanical properties and microstructure obtained by P‐HPT correspond well to conventional high pressure torsion results.

     

Chemical/mechanical polishing of diamond films assisted by molten mixture of LiNO3 and KNO3

Chemical/mechanical polishing can be used to polish the rough surface of diamond films prepared by chemical vapor deposition (CVD). In this paper, a mixture of oxidizing agents (LiNO₃ + KNO₃) has been introduced to improve the material removal rate and the surface roughness in chemical/mechanical polishing because of its lower melting point. It had been shown that by using this mixture the surface roughness R_a (arithmetic average roughness) could be reduced from 8-17 to 0.4 μm in 3 h of polishing, and the material removal rate can reach 1.7-2.3 mg/cm²/h at the temperature of 623 K. Pure aluminium is compared with cast iron as the contact disk material in the polishing. Although the material removal rate of aluminiumdisk is lower than that of cast iron, it can eliminate the carbon contamination from the contact disk to the surface of diamond films, and facilitate the analysis of the status of diamond in the chemical/mechanical polishing. The surface character and material removal rate of diamond films under different polishing pressure and rotating speed have also been studied. Graphite and amorphous carbon were detected on the surface of polished diamond films by Raman spectroscopy. It has been found that the oxidization and graphitization combined with mechanical cracking account for the high material removal rate in chemical/mechanical polishing of diamond films.     

Characteristics of diamond regrowth in a synthetic diamond compact

A transmission electron microscopic study of a commercial sintered diamond compact is reported that identifies and characterizes the diamond that has regrown between the grains of the original diamond powder during the high-pressure, high-temperature manufacturing process of the compact. The majority of the original grains are strongly deformed whereas the regrown diamond shows little or no plastic deformation. The dislocations in diamond regrown between the original grains occur in low-angle boundaries and other configurations typical of grown-in dislocations in crystals. The manufacturing process involves infiltrating the diamond aggregate by molten cobalt, and the regrown diamond is characterized by the presence of cobalt inclusions in sizes ranging from a few tenths of a micrometre down to a few nanometres, possessing the same orientation and lattice parameter as the diamond host. Graphite inclusions also occur in regrown diamond, few in comparison with cobalt inclusions and in random orientation. The graphite crystals exhibit axial ratios, (c/a), lowered by several per cent due to the containment pressure exerted by the diamond host.     

A micro‐mechanical damage model based on gradient plasticity: algorithms and applications

As soon as material failure dominates a deformation process, the material increasingly displays strain softening and the finite element computation is significantly affected by the element size. Without remedying this effect in the constitutive model one cannot hope for a reliable prediction of the ductile material failure process. In the present paper, a micro‐mechanical damage model coupled to gradient‐dependent plasticity theory is presented and its finite element algorithm is discussed. By incorporating the Laplacian of plastic strain into the damage constitutive relationship, the known mesh‐dependence is overcome and computational results are uniquely correlated with the given material parameters. The implicit C¹ shape function is used and can be transformed to arbitrary quadrilateral elements. The introduced intrinsic material length parameter is able to predict size effects in material failure. Copyright © 2002 John Wiley & Sons, Ltd.     

Deformation transitions

All solids with given mechanical properties will fracture brittly when of large enough size; vice versa it is difficult to comminute solids below certain sizes. Both effects are caused by the fracture stress changing with size (according to cube/square scaling principles) whereas the flow stress is essentially independent of size. Again, a fixed size of body, made of different materials, can respond in quite different ways: simple elasticity, elastic fracture, elastoplastic flow, elastoplastic fracture, plastic flow, plastic fracture or plastic collapse are all possible, depending upon the different mechanical properties of the different materials from which it may be made. This review shows that such deformation transitions are controlled by the relative values of size and a material parameter given byER/ _Y ² whereE is Young's modulus,R the specific work of fracture and _Y the flow stress. At fixed size of body, made of given material, transitions occur when one or more of the mechanical property terms are altered by rate, temperature, environment, superimposed hydrostatic stress and so on. A wide range of examples is used to illustrate these effects, and their role in load-bounding methods in elastoplastic design of structures is considered.     

The formation mechanism of intermetallic compounds in Al/Mg friction-stir weld joint

The mechanical properties of dissimilar metal joints are strongly related to the brittle intermetallic compounds (IMCs) that form at the weld interfaces. In this study, the effects of temperature, local composition and plastic strain on the formation mechanism of IMCs in a welded joint between 6061 Al and AZ31 Mg were investigated. The results demonstrate that Al₃Mg₂ is formed when the strain rate reaches 10?s^?1, even though the temperature is lower than the Al–Mg eutectic temperature, which suggests that plastic deformation may accelerate the formation of IMCs in the deformation zone. Meanwhile, the formation of Al–Mg IMCs can be suppressed by Mg–Zn and Al–Mg–Zn compounds when Zn filler is added at the Al/Mg interfaces.     

TA4纯钛带材各向异性屈服行为表征与研究

目的 基于复杂加载状态试验和先进屈服准则,实现考虑塑性演化的TA4纯钛在复杂加载状态下塑性各向异性行为的精确表征。方法 通过0°、45°、90°方向的单拉试验和复杂加载比例的十字形试件双向拉伸试验,获得TA4纯钛的基本力学性能参数和拉伸屈服轨迹,采用不同的屈服准则对试验屈服轨迹进行预测,并通过变r值的屈服准则预测其屈服轨迹的塑性演变规律。结果 在小变形范围内,Yld2000?2d屈服准则对TA4屈服轨迹的预测精度最高;塑性变形过程中,呈线性增大趋势的r值与TA4纯钛的屈服轨迹演变现象直接相关。结论 试验与理论屈服轨迹的对比表明,Yld2000?2d屈服准则可以实现TA4纯钛初始屈服行为的精确表征。TA4纯钛带材的r值随塑性变形呈线性增大趋势,考虑塑性演化的Barlat89屈服准则预测的TA4屈服轨迹外凸性更显著。在TA4纯钛带材冲压成形过程的有限元分析、模具设计和工艺优化中,仅考虑初始屈服轨迹时,可采用Yld2000?2d屈服准则;当各向异性特征存在较强的塑性演化相关性时,可采用形式相对简单的Barlat89屈服准则。     

Multi‐scale modeling of plastic deformations in nano‐scale materials; Transition to plastic limit

A large amount of research in computational mechanics has biased toward atomistic simulations. This trend, on one hand, is due to the increased demand to perform computations in nanoscale and, on the other hand, is due to the rather simple applications of pairwise potentials in modeling the interactions between atoms of a given crystal. The Cauchy–Born (CB) hypothesis has been used effectively to model the behavior of crystals under different loading conditions, in which the comparison with molecular dynamics simulations presents desirable coincidence between the results. A number of research works have been devoted to the validity of CB hypothesis and its application in post‐elastic limit. However, the range of application of CB hypothesis is limited, and it remains questionable whether it is still applicable beyond the validity limit. In this paper, a multi‐scale technique is developed for modeling of plastic deformations in nanoscale materials. The deformation gradient is decomposed into the plastic and elastic parts, i.e., F  =  F ^p F ^e. This decomposition is in contrast to the conventional decomposition, F  =  F ^e F ^p, generally encountered in continuum and crystal plasticity. It is shown that the former decomposition is more appropriate for the problem dealt within this work. Inspired by crystal plasticity, the plastic part is determined from the slip on potential slip systems. Based on the assumption that the CB hypothesis remains valid in the homogeneous deformation, the elastic deformation gradient resulting from the aforementioned decomposition is employed in conjunction with the CB hypothesis to update the state variables for face‐centered cubic crystals. The assumption of homogeneity of elastic deformation gradient is justified by the fact that elastic deformations are considerably smaller than the plastic deformations. The computational algorithms are derived in details, and numerical simulations are presented through several examples to demonstrate the capability of the proposed computational algorithm in the modeling of golden crystals under different loading conditions. Copyright © 2016 John Wiley & Sons, Ltd.     

Analysis of the Results of Mechanical Testing of Metals by Constructing the Diagrams of Structural States

For polycrystalline metals and alloys with bcc (Mo, Fe, and Fe–Si) and fcc (Ni and Cu) crystal lattices, it is shown that the application of the method of reconstruction of parabolic stress–strain diagrams on S –eⁿcoordinates, where S and e are the actual stress and strain, respectively, and n is the strain-hardening index, makes it possible to represent the results of mechanical testing in the form of a diagram of structural states plotted in the form of the dependences of actual stresses and strains on temperature. This diagram practically completely describes the changes in the strength and plastic characteristics of the materials and their structural states in the process of plastic deformation. We study specific features of the construction of these diagrams depending on the composition of the alloy, its structural state, the energy of stacking faults, etc. On the basis of the analysis of the character of correlations between the critical values of strains and stresses and the mechanisms of plastic deformation within broad ranges of temperature and structural states, we discuss the possibilities of scientific and practical application of the diagrams of actual stresses and strains vs temperature.     

Deformation mechanisms in oriented high-density polyethylene

The deformation of samples of oriented high-density polyethylene has been analysed in terms of three principal deformation mechanisms,fibrillar slip, lamella slip andchain slip. From a study of small- and wide-angle X-ray diffraction patterns it is possible to deduce which mechanism or mechanisms are operating in particular cases. Material prepared in three different ways has been examined and it appears that in all three cases the primary mechanism for plastic deformation is [001] chain slip.In oriented and annealed material with a well-defined lamella crystal structure it has been possible to show that the recoverable elastic deformation is primarily due to reversible lamella slip. In this material plastic deformation by chain slip starts at a well-defined critical resolved shear stress of about 15 MNm^–2.Deformation of oriented unannealed material, in which the crystal structure is not so well-defined, appears to be more complicated. In material prepared by cold drawing some of the plastic strain may be accounted for by permanent lamella slip. Fibrillar slip does not appear to be a major deformation mechanism in any of the three materials.     

Structure and hardness of octahedral natural diamond single crystals depending on the HPHT treatment conditions

The structure evolution of octahedral natural diamond single crystals has been studied depending on the HPHT treatment using Raman scattering and infrared spectroscopy. It has been found that the formation of a polycrystalline diamond capsule around a single crystal at p = 8 GPa and T = 1500°C gives rise to a combined structural-stressed state in the single crystal due to its plastic strain. This state has been manifested by a significantly (more than double) broadening of the characteristic line of diamond (1332 cm^?1) in the Raman spectrum and the increase of a single crystal hardness from 105 to 120 GPa.     

Deformation behaviour of Fe-3Si steel

Abstract

The deformation behaviour of an Fe-3Si (wt-%) steel was studied in the temperature range 400-900°C over six orders of magnitude of strain rate. It was found that the Fe-3Si steel exhibits a threshold behaviour. A correlation between the deformation behaviour and the temperature dependence of the threshold stress was etermined. An analysis in terms of the threshold stress showed that two modes of deformation behaviour exist in the power law creep regime. At normalised strain rate ?kT/(D ₁ Eb) ranging from 2 × 10^-6 to 10^-3, the value of the true stress exponent n is equal to 7, and at lower values of ?kT/(D ₁ Eb) the value of n is ~ 5. The true activation energy for plastic deformation Q _c increases from 250 ± 15 to 290 ± 30 kJ mol^-1 with increasing temperature from 550 to 700°C, and remains virtually unchanged at high temperatures. The relationship between rate controlling mechanisms of plastic deformation and mechanisms of interaction between lattice dislocations and dispersoids is discussed.     

Plastic deformation behavior in dual-phase Ni–31Al intermetallics at elevated temperature

Plastic deformation behavior of dual-phase Ni–31Al intermetallics at elevated temperature was examined. It was found that the alloy exhibited good plasticity under an initial strain rate of 1.25 × 10⁻⁴ s⁻¹ to 8 × 10⁻³ s⁻¹ in a temperature range of 950–1075 °C. A maximum elongation of 281.3% was obtained under an initial strain rate of 5 × 10⁻⁴ s⁻¹ at 1000 °C. The strain rate sensitivity, m value was correlated with temperature and initial strain rate, being in the range of 0.241–0.346. During plastic deformation, both the two phases Ni₃Al and NiAl in dual-phase Ni–31Al could co-deform without any void formation or debonding, the initial coarse microstructure became much finer after plastic deformation. Dislocation played an important role during the plastic deformation in dual-phase Ni–31Al alloy, the deformation mechanism in dual-phase Ni–31Al could be explained by continuous dynamic recovery and recrystallization.     

Superplasticity and microstructure of TC21 titanium alloy during thermomechanical treatment

Abstract

As rolled TC21 titanium alloy was subjected to isothermal constant strain rate tensile tests using an electronic tensile testing machine. After tensile deformation, the alloys were subjected to double annealing. Superplastic behaviour and microstructure evolution were systematically investigated. Experimental results show that as rolled TC21 alloy exhibits good superplasticity at temperatures ranging from 870 to 930°C and strain rates ranging from 3×10^?4 to 3×10^?2 s^?1. A maximum elongation of 373·3% was obtained at 910°C and 3×10^?4 s^?1. In addition, the alloy microstructure comprises α and β phases during plastic deformation. The primary α-grains aggregate and merge to form new crystal grains with irregular grain boundaries because of dynamic recrystallisation. Furthermore, the primary α phase content gradually decreases with increasing temperature. The resulting microstructure after deformation and double annealing is a duplex microstructure comprising a primary equiaxed α phase and a β-transformed lamellar structure. The acicular α phase transformed from the β phase is mutually interlaced as a basketweave structure after deformation at 930°C and double annealing.     

Internal friction in copper and α-brasses during plastic deformation

The internal friction Q ^–1 of annealed copper and -brass wires containing 10, 20, 30 and 35 at. % of zinc was studied by a torsional oscillation method during plastic deformation. The results are interpreted in terms of two theoretical models ascribing the amplitude-dependent internal friction, observed in the pre-yield stage, to coupling of the cyclic stress with the creep component of the deformation, and the amplitude-independent internal friction at larger, plastic, strains to losses arising from contributions of the torsional stress to the plastic deformation. Up to the maximum tensile strain of 1 % used in the experiments, the influence of zinc content on Q ^–1 is not pronounced.     

Effect of grain size on cyclic microplasticity of ECAP processed commercial pure magnesium

Equal channel angular pressing (ECAP) was performed on the extruded commercial pure magnesium at 250 °C for 4 passes. Heat treatments were carried out to modify the microstructures. The cyclic plastic deformation behavior of pure Mg with different grain sizes in microstrain region was studied by tensile loading and unloading experiments. The microplastic deformation process of pure Mg can be divided into two stages. In the first stage, pronounced plastic deformation associated with dislocation motion on basal plane is initiated at several MPa. The materials are softened and characterized by low friction stresses and hardening exponents. The microplastic deformation enters into region II above the strain of about 8 × 10^?4. Annihilation and tangle of dislocations lead to the increase of hardening exponents and friction stresses. Pure Mg shows a very pronounced anelastic behavior during cyclic microplastic deformation, which results in a rapid increase of modulus defect, effectively decreasing the elastic modulus by up to 60 %. Grain size has a marked effect on microplastic deformation behavior of pure Mg. With increasing the grain size, the specimen shows a more pronounced microstrain and anelastic behavior.     

Plastic deformation and “work-hardening” of diamond

Extensive plastic deformation of diamond crystals can be accomplished by squeezing diamond embedded in diamond powder at high pressures and temperatures. By inhibiting brittle fracture, deformation takes place at temperatures as low as 900°C at 60kb. The {111} deformation lamellae have a higher abrasion resistance than even the {111} plane of diamond.     

10CrNi8MoV钢的拉伸塑性应变物理本构模型

研究了10CrNi8MoV钢不同温度和应变速率下的拉伸应力-应变曲线。根据位错动力学将流变应力分解为热激活应力和非热激活应力,忽略粘拽阻力的影响。通过对塑性变形过程的分析,在Kocks热激活方程中引入位错间距演化函数,并用线性强化模型描述非热激活应力的变化,建立了10CrNi8MoV钢的物理本构模型。该模型对10CrNi8MoV钢在低应变速率和较宽的温度范围内的塑性变形行为有较好的描述结果。