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.     

Role of yttrium content in twinning behavior of extruded Mg—Y sheets under tension/compression

The influence of Y content on the grain-scale twinning behavior in extruded Mg—xY (x=0.5, 1, 5, wt.%) sheets under uniaxial tension and compression along the extruded direction was statistically investigated. An automatic twin variant analysis was employed, based on large data sets obtained by electron backscatter diffraction (EBSD), including 2691 grains with 977 twins. The {10

2} tension twinning (TTW) dominance and prevailing anomalous twinning behavior (Schmid factor (m) <0) under both tension and compression were found. The anomalous twinning behavior was more pronounced as Y content increased under tensile loading, indicating a promoted stochasticity of twin variant selection for more concentrated Mg—Y alloys. However, the trend for the Y-content dependent anomalous twinning behavior was opposite in compression. The fractions of the anomalous TTWs were found to be well correlated with the maximum Schmid factor (m_max) values of basal slip and prismatic slip in the corresponding parent grains for compression and tension, respectively, indicating that twinning and dislocation slip might be closely related in the present Mg—Y alloys.     

Deformation Mechanisms of Alpha-Uranium Single Crystals

The operative deformation elements in α-uranium single crystals under compression at room temperature have been determined as a function of the compression directions. The deformation mechanisms noted may be arranged with respect to their frequency of occurrence and ease of operation in the following order: 1—(010)-[100] slip, 2—{130} twinning, 3—{~172} twinning, and 4—under special conditions of stress application, kinking, cross-slip, {~176} twinning, and {011} slip. The composition planes of the {172} and {176} systems were found to be irrational. Cross-slip was shown to be associated with the major (010) slip system, coupled with localized interaction of slip on the (001) planes. The mechanism of kinking was found to be similar to that observed in other metals in that it occurred chiefly when the compression direction was nearly parallel to the principal slip direction [100] and was associated with a lattice rotation about an axis contained in the slip plane and normal to the slip direction: the [001] in the uranium lattice. The resolved critical shear stress for slip on the (010)-[100] system was found to be 0.34 kg per mm². In a single test it was shown that under compression in suitable directions twinning on the {130} also occurs at 600°C     

Dislocation slip and twinning in Ni-based L12 type alloys

We report theoretical results on dislocation slip and twinning in Ni₃ (Al, Ti, Ta, Hf) compositions with L1₂ crystal structures utilizing first-principles simulations. The lattice parameters of Ni₃Al, Ni₃Al_0.75Ta_0.25, Ni₃Al_0.5Ta_0.5, Ni₃Ta, Ni₃Ti and Ni₃Al_0.75Hf_0.25 are calculated, and the crystal structures with lower structural energies are determined. We established the Generalized Stacking Fault Energy (GSFE) and Generalized Planar Fault Energy (GPFE), and calculated stacking fault energies APB (anti-phase boundary) and CSF (complex stacking fault) matched other calculations and experiments. Based on the extended Peierls–Nabarro model for slip and the proposed twin nucleation model, we predict slip and twinning stress and the results show a general agreement with available experimental data. The results show that in the studied intermetallic alloys, twinning stress is lower than slip stress; Ta and Hf ternary addition are substantial to increase flow stress in Ni₃Al. The models proposed in the paper provide quantitative understanding and guidelines for selecting optimal precipitate chemistry and composition to obtain higher mechanical strength in Shape Memory Alloys.     

Effect of substitutational elements on plastic deformation behaviour of NbSi2-based silicide single crystals with C40 structure

Plastic deformation behaviour of binary NbSi₂ and ternary NbSi₂-based silicide single crystals with the C40 structure was examined at high temperatures and compared with the results in MoSi₂ with the C11_b and TiSi₂ with the C54 structure, focusing on anomalous strengthening. Addition of substitutional alloying elements affected the anomalous strengthening in NbSi₂-based silicides; addition of Mo and W strongly affected the anomalous strengthening, while Ti and Al additions showed little effect. The substitutional impurity atoms may gather and form a dragging atmosphere around 1/3〈21̄1̄0] moving dislocations containing a superlattice intrinsic stacking fault in NbSi₂-based silicides, resulting in anomalous stress increase. The effect of the dragging atmosphere and diffusion path of additional elements on an anomalous strengthening is discussed, focusing on the atomic arrangement on slip planes and the phase stability of the C40 structure with respect to the C11_b and C54 structures.     

Plastic deformation of single crystals of TiSi2

Compression experiments have been performed at high temperatures for single crystals of TiSi₂ with the C54 (oF24) structure. Compression axes chosen are -, b- and c-axes, and intermediate directions between a- and c-axes, and b- and c-axes. Based on the slip line observation and the geometrical consideration, it has been concluded that one of the three slip systems, (001)[110], ( 10)[130] and (0 1)[011], is activated depending on the compression axis; in the former two systems dislocations are assumed to be dissociated into superpartials, i.e. 1/2[1101→1/3[100]1+1/6[130] and 1/2[130]→3 × 1/6[130]. The (001)[110] slip is active at room temperature and the other two slip systems are active only above 1000 K. The thermal-activation analysis of the plastic deformation has shown that the deformation is controlled by the Peierls mechanism for the three slip systems; the total activation enthalpy is 1.5 eV for the (001)[110] slip and 4–5 eV for the ( 10)[130] and (0 1)[011] slips. An asymmetry of the Peierls potential is suggested for the (3 0)[130] slip.     

Compressive response of NiTi single crystals

The deformation of NiTi shape memory single crystals are reported under compression loading for selected crystal orientations and two different Ti₃Ni₄ precipitate sizes. For the [148] orientation, selected for highest recoverable strains, the peak aging treatment decreased the transformation stress from austenite to martensite. At the same time, peak aging raised the flow stress of both the austenite and martensite compared to the overaged case by increasing the resistance of the material to dislocation motion. The transformation proceeds beyond the stress plateau region and extends until martensite yielding occurs. This results in recoverable strain levels equivalent to the theoretical estimate of 6.4%. The [112] orientation was chosen to produce two variant formations and in this case, the transformation proceeded over an ascending stress–strain curve compared to the nearly plateau response for the [148] case. Since the austenite and martensite yield levels are reached at a smaller strain level in this case, the maximum recoverable strain was limited to 3.5% even though the theoretical estimates are near 5.1%. The theoretical estimates of transformation strains were established for Type I and Type II twinning cases to cover all possible habit plane and twin systems. TEM investigations support that slip in austenite occurs concomitant with increasing transformation strains. In the [001] orientation, the unfavorable slip systems for dislocation motion in the austenite inhibit slip and permit recoverable strains similar to the theoretical estimates (nearly 4.2%). The [001] orientation exhibits a continuous increase of flow stress with temperature beyond 360 K unlike any other orientation. The results point out that in order to optimize the material performance, close attention must be paid to the selection of the crystallographic orientation, and the precipitate size through heat treatment.     

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.     

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.     

Twinning stress in shape memory alloys: Theory and experiments

Utilizing first-principles atomistic simulations we present a twin nucleation model based on the Peierls–Nabarro formulation. We investigated twinning in several important shape memory alloys, starting with Ni₂FeGa (14M modulated monoclinic and L1₀ crystals) to illustrate the methodology, and predicted the twin stress in Ni₂MnGa, NiTi, Co₂NiGa, and Co₂NiAl martensites, all of which were in excellent agreement with the experimental results. Minimization of the total energy led to determination of the twinning stress accounting for the twinning energy landscape in the presence of interacting multiple twin dislocations and disregistry profiles at the dislocation core. The validity of the model was confirmed by determining the twinning stress from experiments on Ni₂FeGa (14M and L1₀), NiTi, and Ni₂MnGa and utilizing results from the literature for Co₂NiGa and Co₂NiAl martensites. This paper demonstrates that the predicted twinning stress can vary from 3.5 MPa in 10M Ni₂MnGa to 129 MPa for the B19′ NiTi case, consistent with the experimental results.     

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.     

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.     

Grain-boundary precipitation in Allvac 718Plus

Allvac 718Plus is a newly developed nickel-based superalloy designed to replace Inconel 718 in static and rotating applications in gas turbine engines. Fine γ′ precipitates act as its principal strengthening phase and a second plate-like phase is generally present at grain boundaries. This has been reported to be the δ phase (Ni₃Nb, D0_a, orthorhombic), and is used to pin grain boundaries and improve resistance to intergranular fracture. In Allvac 718Plus non-uniform precipitation of a δ-like phase was observed along the grain boundaries; however, no relation was found between grain-boundary misorientation and the occurrence of precipitation. The crystal structure and chemistry of this phase was found to be different from the orthorhombic Ni₃Nb δ phase reported previously in Inconel 718 and Allvac 718Plus. The true structure was found to be consistent with the hexagonal η-Ni₃Ti D0₂₄ structure, but its chemistry was close to Ni₆AlNb with partial ordering of Al and Nb over the prototype Ti sites. It was found that the serrated boundaries observed in the alloy were a result of discontinuous precipitation of η-Ni₆AlNb, which was a predominant precipitation mechanism throughout the microstructure.     

Mechanical properties and oxidation characteristics of TiNiAl(Nb) intermetallics

The structural evolution, mechanical properties and the oxidation characteristics of Ti₅₀Ni_50−xAl_x alloys were investigated. The influence of additive Nb on the microstructure, mechanical properties as well as the high temperature oxidation resistance was examined. It is found that the substitution of Al for Ni can remarkably reinforce the TiNi-based intermetallics by precipitation of (Ti,Al)₂Ni phase and by solid-solution strengthening effects. It is also found that the addition of Nb has beneficial influence on the mechanical properties of TiNiAl intermetallics, and that the oxidation resistance of TiNiAl(Nb) alloys can be effectively improved by the Nb addition. The effect of Nb on the high temperature oxidation behavior of TiNiAl(Nb) alloys has been discussed.     

Mechanical properties and dislocation character of YB4 and YB6

YB₄/YB₆ two-phase alloys were prepared by arc-melting of high purity elemental materials. The Young's moduli of YB₄ and YB₆ were evaluated by nanoindentation technique. During loading of nanoindentation, pop-in events were clearly observed for YB₄ and YB₆, suggesting that both have some capability of dislocation emission at room temperature. TEM observation depicts that dislocations have formed in YB₄ and YB₆ due to the thermal stress that developed during cooling following arc-melting. The Burgers vectors of these dislocations were identified to be [001] for YB₄, and 〈100〉 for YB₆. Potential slip systems that are indicated to operate were (100)[001] for YB₄ and {100}〈001〉 for YB₆.     

The Corrosion Behavior of Ni<Subscript>3</Subscript>(Si,Nb) Alloys in Boiling 70 wt.% Sulfuric Acid

Corrosion-resistant Ni₃(Si,Nb) alloys are promising materials of construction for hydrogen-production systems based on the sulfur-iodine thermochemical cycle. In this work, the corrosion rates of three different Ni₃(Si,Nb) alloys were measured in boiling 70 wt.% sulfuric acid and a three-stage corrosion mechanism was identified, based on the composition and morphology of surface scale that developed. The α(Ni) + β(Ni₃Si) eutectic constituent of the alloy microstructure was selectively attacked by acid and, when present, is detrimental to corrosion resistance. The G-phase (Ni₁₆Si₁₇Nb₆) is more passive than the β-matrix and seems to contribute to a lower steady-state corrosion rate.     

Deformation structure during creep deformation in soft orientation PST crystals

Deformation microstructure of soft polysynthetically twinned (PST) crystals Ti–48A1 was investigated. The soft orientation with the lamellar plates oriented 35° to compression axis was deformed at 1150 K under applied stresses of 100–251 MPa. Macroscopically anisotropic deformation was observed after creep deformation. Deformation took place in the maximum shear stress direction in the soft orientation. Dislocation structures in γ domains of six different variants were examined by transmission electron microscopy. Operative slip and twinning systems were also analysed. Macroscopic plastic strain and strain compatibility at domain and lamellar boundaries were discussed in terms of the slip systems in each domain. Refinement of lamellar occurred by mechanical twinning during creep deformation.     

Influence of Fe substitutions on the deformation behavior and fault energies of Ni3Ge–Fe3Ge L12 intermetallic alloys

Ni₃Ge exhibits a yield strength anomaly, whereas the yield strength of Fe₃Ge shows a normal decline with temperature, and there is a gradual transition from anomalous to normal behavior as Fe content increases. A dramatic strengthening for 77 K deformation has also been noted to occur in these alloys as a result of increasing Fe content. The combined use of transmission electron microscopy (TEM) and image simulations has facilitated identification of the operative deformation mechanisms and allowed for a quantitative measure of superdislocation dissociations. A transition from octahedral glide and Kear–Wilsdorf locking to cube glide of superdislocations has been observed to coincide with an increase in either deformation temperature or Fe content. The low-temperature strengthening has been correlated with enhanced cross-slip, which is aided by a significant lowering of the cube-plane antiphase boundary energy with increasing Fe content. It is proposed that the strengthening and the transition to cube glide are promoted by an increase in the complex stacking fault energy, which enhances both cross-slip and cube-plane mobility.     

Mechanism of formation of shear bands upon cold deformation of a commercial Fe-3% Si alloy

A model of the formation of shear bands upon cold deformation of {111}〈112〉 crystals of a commercial Fe-3%Si alloy is proposed. The model suggests a two-stage mechanism for the formation of a shear band and its fine structure. At the first stage, there occurs an anomalous twinning in the {114}〈221〉 system; at the second stage, an almost complete secondary twinning of the band in two standard {112}〈111〉 systems occurs. As a result of these transformations, the shear band consists of regions with an almost cube-on-edge orientation {11 11 1}〈1 1 22〉 and an “octahedral” {111}〈112〉 orientation, which is symmetrical with respect to the initial {111}〈112〉 orientation. A crystallographic mechanism responsible for an anomalous twinning in the {114}〈221〉 system by the slip of (8a/18)〈221〉 partial dislocations is proposed. It is supposed that the shear bands observed consist of groups of different numbers of shear microbands.