Effect of long-range order on the cyclic deformation behaviour of Ni3Fe single crystals

Effect of ordering on cyclic deformation in disordered and ordered Ni₃Fe single crystals was investigated focusing on stress–strain response and deformation substructure. The cyclic hardening depended strongly on the long range order. The maximum stress in the disordered crystals increased gradually with increasing number of cycles and then reached a saturation, while ordered ones exhibited cyclic softening after an initial strong cyclic hardening. The cyclic hardening at an early stage of fatigue in ordered crystals may be due to APB tubes and debris which were produced by the intersection between primary and secondary slips. Coarse slip bands were observed in fatigued ordered Ni₃Fe single crystals. In the bands, three-dimensional dislocation structure was formed accompanied by a decrease in the degree of order, which was responsible for the cyclic softening.     

Cyclic deformation and fatigue behaviour of 7Mo–0.5N superaustenitic stainless steel—slip characteristics and development of dislocation structures

The present work concerns the development of dislocation structures and surface slip markings during cyclic straining of a superaustenitic stainless steel. The composition of the tested material was Fe–25Cr–22Ni–7.6Mo–3Mn–0.46N (wt%). Two total strain amplitudes, 2.7×10⁻³ and 1.0×10⁻², were employed and specimens were investigated at specific numbers of cycles corresponding to certain stages on the cyclic hardening/softening curve. For both strain amplitudes, the developed dislocation structures are strongly planar and with increasing strain amplitude, the slip mode gradually changes from single slip to multiple slip. The short range ordering between Mo and N, as indicated by an atom probe investigation, is broken down during strain cycling leading to increased slip planarity. Early stages of cycling show dislocation multiplication. With increasing number of cycles, the dislocations are gradually grouped together in planar bands with high dislocation density, surrounded by dislocation-poor areas. The evolution of such bands is associated with decreasing effective stresses, while the internal stresses are only slightly reduced. Macroscopic slip bands, similar to PSBs, are formed upon prolonged cycling at the high amplitude. The slip markings created on the specimen surface show strong similarities with the bands of localised slip observed in the dislocation structures of the bulk.     

Cyclic saturation behavior of tungsten monofilament-reinforced monocrystalline copper matrix composites

Studies on saturation behavior produced by cyclic deformation have been conducted on tungsten monofilament-reinforced monocrystalline copper composites. The effect of the fiber on strain localization has been investigated using interferometry. For a given applied strain amplitude, local strain and volume fraction of the persistent slip bands (PSBs) in the composite appeared no different from those observed in monolithic copper single crystals. However, the distribution of the PSBs was observed to be more uniform, and the total number of PSBs is substantially higher than that in monolithic crystals. The PSBs appeared mostly in the form of micro-PSBs or macro-PSBs with very limited width. Instead of expanding existing PSBs, new PSBs were more likely to nucleate at new locations during cyclic deformation. The volume fraction and width of the PSBs were observed to increase during saturation, which indicates that some of the PSBs become aged and new PSBs form in order to continue to carry the plastic strain. A rule of mixtures model was established to link the cyclic stress–strain response of the monocrystalline composites to the behavior of monolithic single crystals and fibers. The results calculated from the model show very good agreement with the experimental data.     

A comparison of the plastic behaviour of Fe3Al and Fe3Si in the temperature range of 300–973 K

The plastic behaviour of Fe₃Al and Fe₃Si is compared in view of the different diffusivities of Fe atoms in these intermetallic phases. In case of Fe₃Al serrated yielding and vanishing strain rate sensitivity (S) in a range of intermediate temperatures (T=400–800 K) indicates some kind of dynamic strain aging effect. In spite of the higher diffusivity of Fe in Fe₃Si, serrations and S ⩽ 0 have been found only in a small temperature interval around 800 K. Possible reasons are discussed using experimental information on the temperature and strain rate dependences of the critical resolved shear stress and on the slip line development measured and observed during compression tests in vacuum (300–973 K) on near stoichiometric Fe₃Al single and polycrystals and Fe₃Si single crystals. In addition the coarsening of D0₃ domains (in case of Fe₃Al) during TEM in situ heating has been observed.     

Cyclic deformation behaviour of γ′-Ni3(Al,Ti) single crystal containing disordered γ precipitates

Effect of disordered γ precipitates on the cyclic deformation behaviour of γ′-Ni₃(Al,Ti)-based single crystals with L1₂ structure was investigated. The γ precipitates with fine spherical and coarse plate-like shapes were formed in the γ′-based single crystals by annealing at 1073 K for 3 h (peak-aged) and 100 h (over-aged), respectively. The over-aged crystal shows stronger cyclic hardening than does the peak-aged crystal, although the yield stress of the peak-aged crystal is higher than that of the over-aged one. In these two crystals the precipitates were sheared by superlattice dislocations dissociated into superpartials. The separation distance between superpartials increased in the precipitates because there was no APB in the disordered phase and the cyclic hardening was enhanced in the aged crystal. The size of the precipitates in the over-aged crystal was reduced by shear deformation resulting in acceleration of the cyclic hardening. Dissolution of γ phase into γ′ matrix did not occur during cyclic loading and cyclic softening did not appear.     

Cu单晶体驻留滑移带的形成与消失

在循环加载条件下，单滑移取向的Cu单晶体首先出现驻留滑移线(PSL)，然后随着循环周次的增加转变为驻留滑移带(PSB)．在不同温度、不同时间条件下对疲劳Cu单晶进行真空退火处理，观察PSB结构在热激活条件下的变化情况．结果表明，退火处理过程中由于空位浓度差异所产生的渗透力促使位错运动，并使PSB的某些部位逐步细化，以至消失．实现了PSB的分段相消．在退火过程中由于应变能的逐步释放，未观察到再结晶现象．     

Cyclic deformation and fatigue behaviour of 7Mo–0.5N superaustenitic stainless steel—stress–strain relations and fatigue life

The cyclic deformation characteristics and fatigue behaviour of a superaustenitic stainless steel with composition Fe–25Cr–22Ni–7.6Mo–3Mn–0.46N (wt%) have been investigated. Detailed studies were performed on cyclic hardening/softening behaviour, hysteresis loops, waveform, fatigue lifetime, and internal as well as effective stresses during cyclic straining in the total strain amplitude range 2.7·10⁻³–1.0·10⁻². Special attention is paid to the role of nitrogen and the interaction between nitrogen and molybdenum. Immediate cyclic softening takes place at small strain amplitudes, whereas hardening occurs during the first few cycles at large strain amplitudes followed by softening. For all strain amplitudes a virtually stationary state develops after about 10% of the lifetime with only a weak decrease of the peak stresses. In the cyclic stress–strain curve the material hardens linearly during multi step testing, whereas single step testing leads to excessive hardening at the largest strain amplitudes. During strain cycling the internal stresses develop like the total stresses, while the effective stresses decrease with increasing number of cycles for all strain amplitudes and also diminish with increased strain amplitude. This behaviour is discussed in terms of developing dislocation structures, studied in an accompanying paper. A double slope behaviour in Coffin–Manson diagrams is observed. The fatigue lifetime resembles that of AISI 316 with 0.29 wt% nitrogen at high strain amplitudes but is shorter at lower strain amplitudes. However, in stress controlled situations the superaustenitic material is superior.     

Effect of orientation on the peak temperature of the yield-stress anomaly in single crystals of the Ni3Ge alloy

Temperature dependence of the yield stress τ_c(T) and shear stresses, as well as the orientation dependence of the peak temperature T _p of the yield-stress anomaly have been studied on single crystals of the Ni₃Ge alloy. The peak temperature T _p corresponds to the maximum in the τ_c(T) dependence. Profiles of the T _p isolines have been determined based on the equality of the values of the yield stress in the octahedral and cube planes of cross slip. It is shown that in the case of the above equality it is possible to describe the orientation dependence of the peak temperature T _p of the temperature anomaly of the yield stress in single crystals of Ni₃Ge.     

Low cycle fatigue behaviour of a low interstitial Ni-base superalloy

The low cycle fatigue behaviour of precipitation strengthened nickel-base superalloy 720Li containing a low concentration of interstitial carbon and boron was studied at 25, 400 and 650 °C. Cyclic stress response at all temperatures was stable under fully reversed constant total strain amplitude (Δε/2) when Δε/2 ? 0.6%. At Δε/2 > 0.6%, cyclic hardening was followed by softening, until fracture at 25 and 650 °C. At 400 °C, however, cyclic stress plateaued after initial hardening. Dislocation–dislocation interactions and precipitate shearing were the micromechanisms responsible for the cyclic hardening and softening, respectively. The number of reversals to failure vs. plastic strain amplitude plot exhibits a bilinear Coffin–Manson relation. Transmission electron microscopy substructures revealed that planar slip was the major deformation mode under the conditions examined. However, differences in its distribution were observed to be the cause for the bilinearity in fatigue lives. The presence of fine deformation twins at low Δε/2 at 650 °C suggests the role of twinning in homogenization of cyclic deformation.     

Deformation and fracture of the PWA 1472 superalloy single crystal

The deformation and fracture of a PWA 1472 single crystal oriented in a soft orientation were studied by analyzing the evolution of slip bands and detailed fractography of tensile specimens tested at low (10⁻⁴/s) and high (1.0/s) strain rates at room temperature. The results of this study indicated that the slip did not start on the system with the highest Schmid factor. This yielding anisotropy has been attributed to the deformation of the γ′-Ni₃Al precipitates. The plastic deformation was heterogeneous and occurred by the propagation of a Lüders band. Cleavage fracture occurred along the kink bands within a coarse shear band that developed after the activation of the secondary slip system. An increase in the strain rate did not affect the yield strength significantly, but it enhanced the deformation localization, and hence, decreased the ductility and increased the percentage of the cleavage facets. The effect of strain rate is discussed in terms of cross-slip processes.     

Tension–compression asymmetry of the stress–strain response in aged single crystal and polycrystalline NiTi

The purpose of this work is to thoroughly understand tension–compression asymmetry in precipitated NiTi using unique experimental results and micro-mechanical modeling. For the first time, tensile and compressive stress–strain behaviors were established on aged single crystals ([100], [110], and [111] orientations) and polycrystalline NiTi. The single crystal and polycrystalline Ti–50.8 at.% Ni materials were given both peak aged and over aged heat treatments. The drawn polycrystalline NiTi has a strong texture of the 〈111〉{110} type, thus it deformed in a manner consistent with the [111] single crystals. In contrast to the phenomenological theory of martensitic transformations (analogous to Schmid's law), the critical resolved shear stress required to trigger the transformation, τ_crss, in the peak-aged single crystals was dependent on both the stress direction and crystallographic orientation. Using micro-mechanical modeling, the deviation from Schmid's law was attributed to the unique orientation relationship that exists between the Ti₃Ni₄ precipitates (their coherent stress fields) and the 24 martensite correspondence variant pairs. The over-aged single crystals generally obeyed Schmid's law within experimental error, consistent with the proposed micro-mechanical model. Qualitatively, the tension–compression asymmetry and orientation dependence of the recoverable strain level, ε₀, was consistent with the phenomenological theory for martensitic transformations. However, the peak- and over-aged single crystals generally both demonstrated smaller ε₀ magnitudes than predicted. The differences for both crystals were attributed to the inhibition of martensite detwinning coupled with several unique microstructural effects.     

纳米压入结合有限元模拟确定金属材料的塑性性能

材料具有相同的弹性模量 E以及代表性应力与代表性应变(σr,εr)时,可以获得相同的纳米压痕加载曲线,而与材料的应变强化指数 n无关。基于此,利用纳米压入结合有限元数值模拟建立一种确定金属材料塑性性能参数的改进方法。首先,不考虑金属材料的加工硬化,通过不断调整代表性应力的假设值,当模拟与实验载荷?位移曲线的加载阶段相吻合时,确定其代表性应力。其次,对金属材料假设不同的应变强化指数,采用相同的方法确定其代表性应变。最后,通过调整应变强化指数的假设值,使模拟曲线与实验曲线的卸载阶段相吻合来确定金属材料的真实应变强化指数,继而利用幂强化本构方程确定金属材料的初始屈服极限。将该方法应用于AISI 304不锈钢、铁及铝合金三种金属,其有效性得到验证。     

Mechanical properties and hardening mechanism of Fe–Al–Ni single crystals containing NiAl precipitates

The microstructures and mechanical properties of Fe–23.0 Al–6.0 Ni (at.%) single crystals containing NiAl precipitates were investigated and the hardening mechanism due to the precipitates was discussed, focusing on the activated slip systems. When these alloys were slowly cooled to room temperature after homogenization at 1373 K, the NiAl phase with the B2 structure precipitated in the body-centered cubic (bcc) Fe–Al matrix, satisfying the cube-on-cube relationship with a small misfit strain. The single crystals containing the NiAl precipitates exhibited a high yield stress above 1 GPa at room temperature. In addition, the activated slip system and deformation behavior depended strongly on the loading axis. For instance, 〈1 1 1〉 slip, which is the primary slip for the bcc matrix, occurred at 〈1 4 9〉 and 〈0 0 1〉 orientations and the NiAl precipitates were sheared by the slip. A critical resolved shear stress of 〈1 1 1〉 slip in the NiAl phase was known to be extremely high, which led to strong precipitation hardening. On the other hand, at 〈5 5 7〉 and 〈0 1 1〉 orientations, 〈0 0 1〉 slip, which is the primary slip system for the NiAl precipitates, forcibly sheared the bcc Fe–Al matrix, also leading to strong hardening. Thus, in the Fe–Al–Ni alloys, the difference in the primary slip system between the bcc Fe–Al matrix and the NiAl precipitates resulted in extreme hardening. This hardening mechanism caused by the NiAl precipitates effectively increased the yield stress even at high temperatures. In fact, the crystals exhibited a high yield stress at ～1 GPa up to 823 K.     

On the temperature dependence of the yield stress in Fe3Al single crystals in the range of 300 to 500 K

The temperature dependence of the critical resolved shear stress of Fe₃Al single crystals exhibits not only the well-known high temperature anomaly with rising stress but also a sharp decrease of the yield stress in a narrow temperature range somewhat above ambient temperature. This drop is connected with a characteristic change of the operative deformation mechanism, resulting in a specific behaviour of slip line evolution on the sample surface, which is monitored by means of an optical microscope during deformation. In addition to the yield stress, the strain rate sensitivity and the work hardening rate show characteristic changes in this particular temperature range.     

Microstructure and plastic deformation behavior of Ni3(Ti,X) (X = Nb,Al) single crystals with long-period geometrically closely packed crystal structures

The crystal structure, phase stability and plastic deformation behavior of Ni₃(Ti_1−xNb_x) [x = 0, 0.01, 0.03, 0.05 or 0.10] and Ni₃(Ti_1−yAl_y) [y = 0.16 or 0.28] single crystals were investigated. The substitution of Ti by Nb in Ni₃Ti induced the formation of various long-period stacking ordered (LPSO) structures with 18-fold, 10-fold or 9-fold stacking sequences of closely packed plane (CPP) depending on the Nb content. In compression tests, the yield stress anomaly (YSA) appeared in ternary LPSO crystals as well as in binary Ni₃Ti by slip on the CPP. The change in stacking sequence of CPP in the LPSO phases strongly affects the YSA behavior due to the change in APB energy on the non-closely packed plane.     

The influence of boron-doping on the effectiveness of grain boundary hardening in Ni3Al

The influence of boron-doping on the effectiveness of grain boundary hardening in Ni₃Al has been investigated by measuring microhardness profiles across grain boundaries of binary and boron-doped Ni₃Al bicrystals. It was found that although boron gives rise to significant solution strengthening in Ni₃Al, the effectiveness of grain boundary hardening in Ni₃Al is lessened by the addition of boron. Furthermore, the contribution of grain boundary hardening to the overall strength decreases as the segregation extent of boron at the grain boundary increases. A theoretical model of grain boundary hardening considering the various effects of boron-doping has been developed. Application of the model can deconvolute the individual effects of boron-doping on solution hardening, distribution of microcavities along grain boundaries and the interaction of dislocations on different slip systems. Analyzing the experimental results with the model suggests that boron-doping can (i) improve the transfer efficiency of shear stress across a grain boundary by reducing the amount of microcavities along the grain boundary; (ii) suppress the hardening effect from the interaction of dislocations moving on different slip systems and (iii) cause a significant solution hardening effect.     

Interphase boundary fracture and grain boundary precipitation of Ni3Al(γ′) phase in β-NiAl bicrystals

The effect of the crystallography of film-like Ni₃Al(γ′) precipitates along grain boundaries of NiAl(β) on the fracture stress at room temperature was examined using β bicrystals with controlled orientations. The selected variant of γ′-film satisfied the Kurdjumov–Sachs (K–S) relation with a neighbouring β crystal, and deviated from the relation with an adjacent β crystal. In the course of tensile deformation at ambient temperature, fracture occurred preferentially at the (β/γ′) interphase boundary deviating from the K–S relation, which showed no plastic flow, and the fracture stress decreased with increasing deviation angle. In contrast, slip transfer from γ′-film to β crystal across coherent (β/γ′) interface was observed, when the variant of γ′-film satisfied the K–S relation with both neighbouring β crystals. To clarify the relation between the interphase boundary fracture and the deviation angle from the K–S relation, the boundary structure of incoherent (β/γ′) interfaces was discussed using O-lattice theory and transmission electron microscopic observations.     

Operative slip systems and anomalous strengthening in Ni3Nb single crystals with the D0a structure

The plastic deformation behavior of Ni₃Nb single crystals was examined in tension and compression to determine the operative slip and twinning systems, and to explore the anomalous strengthening behavior. A strong temperature dependence of the CRSS for both the slip and twinning systems was observed, which was dependent on the sample orientation. Anomalous flow behavior was also observed in Ni₃Nb crystals deformed by (010)[100] and (001)[100] slip. The anomalous strengthening mechanism is discussed on the basis of both the anisotropy of APB energy and the formation of dragging atmosphere around moving dislocations.     

A crystal plasticity based work-hardening/softening model for b.c.c. metals under changing strain paths

Metal forming processes typically involve changes of strain paths, which are accompanied by transients in softening or hardening behaviour. The physical cause of these transients in stress–strain responses can be attributed to the evolution of the underlying microstructural details. A crystal plasticity based model is presented to capture the complex hardening/softening transients observed in deformation of b.c.c. polycrystals at low homologous temperatures. For each crystallite, the microstructure, i.e. the cell block boundaries and cell structure, is modelled with three dislocation densities. The cell block boundaries are treated as geometrical obstacles to slip on non-coplanar slip systems. This model is implemented in a Full Constraints Taylor model to obtain the response of a polycrystal from the response of the constituent single crystals. It was found that several important features observed in the experimental stress–strain curves of b.c.c. polycrystals during complex strain paths could be reproduced.