Fatigue mechanism in Ni3Fe single crystals with L12 superlattice structure

Ni₃Fe single crystals with the L1₂ structure were cyclically deformed with various loading axes at different plastic strain amplitudes. The crystals demonstrated initial strong cyclic hardening followed by rapid cyclic softening at any orientation tested. There existed a typical plateau region in the cyclic stress–strain curves of the crystals oriented for single slip, while the saturation shear stress (τ_s) monotonically increased with increasing plastic shear strain amplitude (γ_pl) for double-slip oriented crystals. Persistent slip bands (PSBs) with cell-like dislocation structure developed in fatigued Ni₃Fe single crystals. The relationship between the volume fraction of PSBs and γ_pl suggests that Winter’s two-phase model can be applied to explain the cyclic stress–strain response in Ni₃Fe single crystals. Activation of the secondary slip is closely related to the cyclic hardening/softening behaviour and the formation of PSB in Ni₃Fe single crystals.     

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.     

The influence of atomic order on H2-induced environmental embrittlement of Ni3Fe intermetallics

The different sensitivity to H₂-induced environmental embrittlement for the ordered and disordered Ni₃Fe alloys has been investigated. The results show no environmental embrittlement in disordered Ni₃Fe in gaseous H₂ when tested at room temperature. However, the H₂-induced environmental embrittlement for the ordered Ni₃Fe becomes severer as the degree of order increases. The results of testing on simultaneous hydrogen charging show that disordered Ni₃Fe embritted as hydrogen atoms are forced into the material, implying that the embrittlement of ordered Ni₃Fe in gaseous H₂ is due to the acceleration of the kinetics of catalytic reaction to produce more atomic hydrogen. Further more, the hydrogen adsorption test of Ni₃Fe powder shows that the amount of chemically adsorbed hydrogen in the ordered state at room temperature is significantly larger than that adsorbed by the disordered materials, indicating that the more sensitive to H₂-induced embrittlement in the ordered Ni₃Fe is essentially due to accelerated catalytic reaction to produce more atomic hydrogen.     

Some features of the formation and destruction of dislocation barriers in intermetallic compounds: III. Thermoactivated straightening of dislocations along a preferred direction in Ni<Subscript>3</Subscript>Al

Based on an analysis of the dislocation structure, the process of self-blocking, i.e., the transformation of glissile superdislocations into dislocation barriers without the effect of an external stress, has been investigated in the intermetallic compounds with an L1₂ superstructure. Using different regimes of heating without stress after preliminary deformation both below and above the temperature T _max corresponding to the peak in the yield-stress temperature dependence, a characteristic feature of self-blocking has been revealed in the intermetallic compounds on the basis of Ni₃Al, namely, the barriers were formed, but they were not destroyed. This feature was observed not only for single crystals of Ni₃(Al,Nb), but also for single crystals of the two-phase complex intermetallic alloy VKNA-4U, which contains 90% γ′ phase. By adjusting the temperature of heating and the duration of heating after preliminary low-temperature deformation, it was possible to observe the initial stages of the process of self-blocking of superdislocations for single crystals of Ni₃(Al,Nb). In experiments on the high-temperature deformation of intermetallic compounds, the process of extension (straightening) of dislocations along a preferred direction has been revealed and its thermoactivated nature has been proved. Based on the example of a well ordered alloy Ni₃Fe, the possibility of using the experiments with heating of preliminarily deformed intermetallic compounds for rapid analysis of the nature of the anomaly of the temperature dependence of yield stress has been demonstrated. In particular, for Ni₃Fe the observed anomaly is not connected with the transformations of superdislocations into barriers.     

H2-induced environmental embrittlement in ordered and disordered Ni3Fe: An electronic structure approach

The different behaviors in H₂-induced environmental embrittlement in ordered and disordered Ni₃Fe are associated with differences in their electronic structures. The experimental study on electronic structures of ordered and disordered Ni₃Fe has been carried out by electron energy-loss spectroscopy (EELS). The onset energy of Ni L_2,3 edges from ordered phase is 0.3 eV lower than that from disordered phase, while the 3d occupancy of Ni atoms in ordered phase is 0.07 electrons/atom less than that in disordered phase. Severe H₂-induced environmental embrittlement in ordered phase is attributed to rather negative dissociative adsorption energy of hydrogen at surfaces, which arises from upward shifting of the valence band center of Ni.     

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.     

Cyclic hardening of tungsten monofilament-reinforced monocrystalline copper matrix composites

The cyclic hardening of fiber-reinforced metal matrix composites has been explored using tungsten monofilament-reinforced monocrystalline copper composite specimens. This composite showed the same overall hardening tendencies as monolithic copper single crystals, i.e. rapid hardening, momentary softening and saturation, but at much higher stress levels as compared to the behavior in the copper single crystals. The back stress of the composites was found both very high and much larger than the friction stress so that there was a pronounced Bauschinger effect. The high cyclic stress in the composite was predominantly due to the back stress which resulted from the interaction between the fiber and dislocations. The fiber did not have a significant effect on the friction stress.     

Formation of deformation bands and precipitation of Ni3Al(γ′) phase in NiAl(β)-based single crystals by thermomechanical processing

Microstructure and crystal orientation distribution of two-phase NiAl(β)/Ni₃Al(γ′) alloys obtained by thermomechanical processing were investigated. Cylindrical Ni-36 and 38 at.%Al alloy single crystals with various initial loading axes were hot-compressed at a high strain rate of 1.0 s⁻¹ and subsequently annealed in the (β/γ′) two-phase region. After the hot deformation, relatively uniform subgrain structures were formed in initial 〈111〉_β-oriented crystals, while a number of deformation bands perpendicular to the compressive axis developed in 〈100〉_β-, 〈110〉_β- and 〈123〉_β-oriented crystals. Two types of deformation bands with different crystal orientations were alternately arranged against the compressive axis. After annealing in the (β/γ′) two-phase region, a film-shaped γ′ phase with peculiar variants of the Kurdjumov–Sachs orientation relationship preferentially precipitated along the boundary between the deformation bands, resulting in the formation of a (β/γ′) two-phase lamellar structure. Formation process of the deformation bands in β phase and the crystallography of γ′ precipitates along deformation bands were discussed.     

Effect of ordering process on giant pseudoelasticity in Fe3Al single crystals

Pseudoelasticity in Fe₃Al single crystals with the D0₃ structure was investigated focusing on the ordering process and the ordered domain structure. During unloading, a plastic strain of 5.0% was perfectly recovered in Fe–23.0at.%Al single crystals with an appropriate heat treatment, while Fe–28.0at.%Al crystals exhibited little strain recovery. The ordered domain structures in slowly cooled Fe–23.0at.%Al and Fe–28.0at.%Al were different from each other depending on their ordering process. In Fe–23.0at.%Al, the strong interaction between specific domain boundaries and superpartial dislocations with Burgers vector of 1/4<1 1 1> led to the individual motion of the superpartials dragging an antiphase boundary (APB). The surface tension due to the APB resulted in the pseudoelasticity in Fe–23.0at.%Al single crystals. In addition, APB created by an intersection between the domain boundaries and the superpartials also pulled back the superpartials during unloading. A refinement of ordered domains was effective in enhancing the pseudoelasticity in Fe₃Al crystals.     

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.     

Comparison of mechanical properties of Ni3Al thin films in disordered FCC and ordered L12 phases

We report the results of several experiments isolating the effect of long-range order on mechanical properties of intermetallic compounds. Kinetically disordered FCC Ni₃Al (Ni 76%) thin films were produced by rapid solidification following pulsed laser melting. For comparison, compositionally and microstructurally identical films with ordered L1₂ structure were produced by subsequent annealing at 550°C for 2 h. These FCC and L1₂ Ni₃Al thin films were tested by nanoindentation for hardness and Young's modulus, and the critical strain to fracture was measured by straining the substrate under four-point bending. Ni₃Al thin films in the disordered phase were found to have nearly twice the critical strain to fracture, more than three times the fracture toughness, and about 20% lower hardness than in the ordered counterpart. Blunter crack tips and crack bridging observed in the disordered phase also illustrate increased ductility. The increased plasticity of Ni₃Al due to chemical disorder is manifested both within the grains and at the grain boundaries. Young's moduli of the ordered and disordered materials were found to be indistinguishable.     

FLOW SOFTENING DURING HOT COMPRESSION OF Cu-3.45 wt.% Ti ALLOY

Hot compression tests were performed on supersaturated Cu-3.46 wt.% Ti alloy at the temperatures of 873 K and 973 K. Constant true strain rates of 1.4 × 10⁻⁴ s⁻¹ and 6.9 × 10⁻³ s⁻¹ were used. The flow curves were characterized by a single peak followed by continuous flow softening until the sample fracture. The strain to the flow stress peak was found to depend on both the deformation temperature and the applied strain rate. The flow softening (flow stress decrease) was more pronounced during deformation at 873 K for the used strain rates. Strain hardening and precipitation process were together responsible for the initial hardening of the material until the flow stress maximum has been reached. Moreover, the strain localization in a form of coarse slip and shear bands was intensified with increased strain value. It resulted in additional sites for discontinuous precipitation beside the high angle grain boundaries and might be responsible for the flow stress decrease at larger strains. The microstructure of hot deformed samples did not reveal any evidence for dynamic recrystallization during hot deformation in the presence of precipitates. However, dynamic particles coarsening within shear bands was observed and has been assumed to be responsible for further strain softening of the hot deformed sample. It was also suggested that the flow stress was partly reduced due to dynamic recovery which intensified in the course of discontinuous growth and particles coarsening within shear bands. As expected, the flow stress value was affected by the discontinuous precipitation process more effectively in those samples which were deformed at higher temperatures and low strain rates. Flow localization was significantly reduced during hot deformation of the material containing the structure transformed by discontinuous precipitation. © Acta Metallurgica Inc.     

Hydrogen diffusivity in B-doped and B-free ordered Ni3Fe alloys

The hydrogen diffusion coefficient of the ordered Ni₃Fe–B alloys with and without boron additions was measured by a method of the cathodical precharging with hydrogen. The apparent hydrogen diffusion coefficient decreases with increasing the boron concentration doped in the ordered Ni₃Fe alloy. Comparing with the B-free ordered Ni₃Fe alloy, the activation energy of hydrogen diffusion for the ordered B-doped Ni₃Fe alloy increases by as high as 42% when the boron content is sufficient. The doping boron in the Ni₃Fe alloy is effective in reducing the hydrogen diffusion at the grain boundary.     

Sol-gel preparation and characterization of NiCo and Ni3Fe nanoalloys

Sol-gel process is effective in preparation of metallic oxides. Here we report that sol-gel process based on chelating of citric acid is also effective in fabricating NiCo and Ni₃Fe nanoalloys under N₂ or H₂ atmosphere during heating treatment. With the introducing of surfactant, the average grain size of the nanoalloys is less than 10 nm and the grain size distribution is narrow. The formation of nanoalloys with equilibrium ordered phase rather than metastable disordered phase is confirmed by the occurrence of superlattice diffraction rings obtained by selected area electron diffraction. The Ni₃Fe nanoalloy shows typical ferromagnetic behavior at room temperature.     

Hot Deformation Behavior of NiTiHf Shape Memory Alloy Under Hot Compression Test

In this study, the hot deformation behavior of Ni₄₉Ti₃₆Hf₁₅ alloy was investigated. Compression tests were carried out at temperatures ranging from 800 to 1100?°C and at the strain rates of 0.001?C1/s. The peak stress decreases with increasing deformation temperature and decreasing strain rate, a behavior which can be described by plotting the Zener-Hollomon parameter as a function of stress. It was realized that dynamic recrystallization (DRX) was responsible for flow softening. Most of the samples exhibited typical DRX stress-strain curves with a single peak stress followed by a gradual fall down stress. Microstructure evolution showed that new recrystallized grains formed in the vicinity of grain boundaries. The hyperbolic-sine-type constitutive model of Ni₄₉Ti₃₆Hf₁₅ alloy was obtained to provide basic data for determining reasonable hot-forming process. The activation energy for hot deformation of the Ni₄₉Ti₃₆Hf₁₅ alloy was close to 410?kJ/mol.     

Cyclic deformation mechanisms in precipitated NiTi shape memory alloys

Results are presented on the cyclic deformation of single crystal NiTi containing Ti₃Ni₄ precipitates of various sizes. Mechanical cycling experiments reveal that the cyclic degradation resistance of NiTi is strongly dependent on crystallographic orientation. Under compression, orientations approaching the [100] pole of the stereographic triangle possess the highest fatigue resistance. Orientations approaching the [111] pole of the stereographic triangle demonstrate the lowest fatigue resistance. Aging to produce small coherent Ti₃Ni₄ precipitates (10 nm) improves the fatigue resistance of NiTi compared to the other heat treatments (solutionized or overaged) for nearly all orientations. NiTi with 10 nm Ti₃Ni₄ precipitates consistently showed stabilized martensite due to mechanical cycling, and an absence of dislocation activity. Samples with large incoherent Ti₃Ni₄ precipitates (500 nm) consistently showed significant dislocation activity due to mechanical cycling in addition to stabilized martensite colonies. The first cycle stress–strain hysteresis was found to correlate to the fatigue resistance of the material. Samples demonstrating large inherent hysteresis, with different heat treatments and orientations, showed poor fatigue performance. Rational for the observed behaviors is discussed in terms of operant deformation mechanisms and ramifications on modeling the cyclic deformation of NiTi are presented.     

The strain rate and prestrain dependences of the flow stress of Ni3Ga single crystals

Two Ni₃Ga single crystals were tested in compression by introducing upward strain rate jumps over the stress strain curve at temperatures ranging from 290 to 840 K. A Cottrell-Stokes-type law is observed below the yield stress peak temperature when the strain rate sensitivity parameter, is plotted against the workhardening increment τ_h. With increasing temperature, the slopes decrease indicating an increase in the activation volume at a given τ_h. The microstructure sensitivity of this ordered L1₂ alloy is demonstrated by two prestrain experiments. First, a high temperature prestrain reduces the initial values of β, on subsequent deformation at room temperature, compared to that of a virgin specimen at room temperature. Second, a room temperature prestrain weakens the anomaly of the yield stress. The results of the present study are shown to be in broad agreement with earlier results on Ni₃(Al,Hf)B single crystals.     

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.     

Hardness and shear band evolution in bulk metallic glasses after plastic deformation and annealing

Strain-induced hardening and annealing-induced softening are typical in crystalline metals. Bulk metallic glasses (BMG) exhibit the opposite behavior, namely, strain-induced softening and annealing-induced hardening. In addition, reversible softening–hardening–softening occurs in a BMG subjected to a three-step deformation–annealing–deformation process. The hardness changes after deformation and annealing can be correlated with the shear band patterns around/underneath Vickers indents. Shear bands produced during indentation of as-cast BMG are semicircular and radial, consistent with the stress distribution beneath the indenter. In contrast, the shear bands in the pre-strained BMG are irregular and convoluted, and appear to be a mixture of the shear bands produced during the prior compression and those in the as-cast BMG. After annealing, the shear bands tend to recover their semicircular and radial shapes consistent with the annealing-induced hardening.     

An athermal deformation model of the yield stress anomaly in Ni3Al

An athermal deformation model was proposed for the yield stress anomaly in Ni₃Al based on the concept that dislocation multiplication versus the immobilization by the Kear–Wilsdorf (KW) locking controls the plastic deformation. In the model dislocations, superkinks lying between two KW locks multiply through expansion in an athermal manner. Regarding the KW locking, the entire screw segment cross-slips onto (010) to form long KW locks in one thermal activation event. The model can explain the macroscopic characteristics of the deformation, the strain-rate independent of the yield stress and the compliance with Schmid's law, which were recently revealed in binary, stoichiometric Ni₃Al single crystals. Quantitative assessment proved that the model reproduces the temperature dependence of the yield stress with reasonable fitting parameters. The driving force for the KW locking, one of the most important parameters for the yield stress anomaly, was evaluated at 127 mJ m⁻². This value is in good agreement with those calculated from the anti-phase boundary energies on {111} and {100} reported in the literature.