The role of twinning in the cavitation erosion of cobalt single crystals

The deformation characteristics contributing to the superior cavitation erosion properties of HCP cobalt single crystals have been determined. Results indicate that its erosion response is highly orientation sensitive. A homogeneous distribution of and glide occurs in {0110} crystals, whereas slip in the (0001) crystals is much more heterogeneous and consists mainly of dislocations. Continued exposure to cavitation nucleates a large number of twins, predominantly on the and planes in the and (0001) crystals respectively. The former twins are finer and more needle-like than the latter. The crystals are also significantly more erosion resistant than the (0001) crystals. The twin density increases continuously with cavitation exposure until a dense network of twins spans the entire exposed area. This fine-scale twinning is considered responsible for the superior erosion resistance of the metal.     

Crystallography of directionally solidified magnesium alloy AZ91

The crystallographic direction of growth in directionally solidified magnesium alloy AZ91 has been studied by TEM and EBSP techniques in SEM. The main direction of growth is found to be . The dendrites have sixfold symmetry around the main direction, with secondary arms lying along the traces of the (0001), , and -planes, respectively. The secondary arms lying in the basal plane are crystallographically of the same type as the main direction: and .     

Modulated microstructures inβ Cu-Zn-Al

β Cu-Zn-Al samples with different surface orientations have been examined by electron microscopy and diffraction. The nature of the modulated microstructure or tweed and the accompanying diffuse streaking has been studied. Tweed in samples without surface martensite is confirmed to be a subsurface phenomenon, and the diffuse streaking is attributed to both dynamic and static {110} displacement waves. The image contrast in samples with surface martensite is shown to be determined essentially by the surface martensite features and probably tweed in the surface martensite. Possible reasons for the observed tweed are discussed. Formerly at Division Metales, Centro Atómico Bariloche, Argentina.     

Phase Transition and Texture Evolution in the Ni-Mn-Ga Ferromagnetic Shape-Memory Alloys Studied by a Neutron Diffraction Technique

The phase transition and influence of the applied stress on the texture evolution in the as-cast Ni-Mn-Ga ferromagnetic shape-memory alloys were studied by the time-of-flight (TOF) neutron diffraction technique. The neutron diffraction experiments were performed on the General Purpose Powder Diffractometer (Argonne National Laboratory). Inverse pole figures were determined from the neutron data for characterizing the orientation distributions and variant selections of polycrystalline Ni-Mn-Ga alloys subjected to different uniaxial compression deformations. Texture analyses reveal that the initial texture for the parent phase in the as-cast specimen was composed of , , , and , which was weakened after the compression deformation. Moreover, a strong preferred selection of martensitic-twin variants (and ) was observed in the transformed martensite after a compression stress applied on the parent phase along the cyclindrical axis of the specimens. The preferred selection of variants can be well explained by considering the grain/variant-orientation-dependent Bain-distortion energy. This article is based on a presentation given in the symposium entitled “Neutron and X-Ray Studies for Probing Materials Behavior,” which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee.     

Deformation in Ti3Al fatigued at room and elevated temperatures

The fatigue behavior of Ti₃Al in the temperature range 23 to 900 °C was investigated. With increasing temperature the fracture mode changed from transgranular cleavage to cleavage mixed with increasing intergranular failure. At the highest temperature microcracks parallel to the tensile axis were observed, accompanied by decreased planarity of the fracture surface. Little evidence of cyclic crack propagation was observed. The dislocation substructure was found to consist predominantly of planar tangles of dislocation in edge orientations at temperatures of 500 °C and above. No appreciable activity of dislocations having other Burgers vectors was observed. As a consequence of the limited number of active slip systems, fracture was controlled by slip incompatability at grain boundaries.     

Orientation relationships between M2C carbide and the austenite matrix in an Fe-Mn-AI-Mo-C alloy

M₂C carbides were observed to precipitate within the austenite matrix of an Fe-24.6Mn-6.6Al-3. IMo-1.0C alloy after quenching from 1200 °C and aging at 750 °C. By means of transmission electron microscopy (TEM) and diffraction techniques, the orientation relationships between M₂C (p) and the austenite (γ) matrix were determined to be: (0001)p//(111)γ, (11– 0)p// (1 0)γ, ( 100)p//(11 )γ. M₂C carbide has been reported by many researchers to precipitate from the ferrite matrix or along austenite/ferrite boundaries in alloy steels containing Mo. However, little information concerning the formation of M₂C in the austenite matrix has been provided. This investigation presents the first evidence for the existence of M₂C carbide wholly within the austenite matrix and its relationship to the austenite. The energy-dispersive spectrometry (EDS) analyses were performed on M₂C carbides, and the results indicate that the solubility of the M₂C carbide for foreign atoms other than Mo is very limited.     

Competition among basal,prism, and pyramidal slip modes in hcp metals

The competition of slip among , and slip modes of hcp metals has been analyzed geometrically in terms of a critical resolved shear stress, CRSS, criterion. Under the action of an applied stress slip systems of one or more modes may be activated depending on the value of the CRSS and on the orientation of the slip systems with respect to the applied stress. If the CRSS of a given slip mode should exceed a limiting value relative to the CRSS of the other modes, however, the given mode becomes inoperative even under the most favorably stressed conditions. It is found by an examination of the yield loci that basal slip is inoperative if α₂ < cos θ; prism slip is inoperative if α₂ < α₁ sin θ; and pyramidal slip is inoperative if α₂ > cos θ + α₁ sin θ where and are, respectively, the ratios of CRSS for prism and pyramidal slips relative to basal slip, and ϕ is the angle between the (0001) and normals. Since the value of ϕ is a function ofc/a, the limiting values of α₁ and α₁ depend on thec/a ratio of the crystal structure.     

Deformation of a Burgers oriented bimetallic bicrystal of alpha-beta (Ti-13Mn)

The deformation behavior of a Burgers oriented α-β-Ti-13Mn bimetallic bicrystal was studied at two plastic strains, 0.52 and 2.08 pct. Two single crystals, α and β, each corresponding to the orientation of its respective bicrystal component were also investigated. The stress axes were and [1218]_β. The interface planes were and and lay in the x’-z’ plane. The deformation behavior of the a component differed from that of the a single crystal because of plastically induced stresses,T _y’z’ ,T _x’z’ ,T _x’y’ , and σ _x’x’ . Prismatic slip and twinning were found in the single crystal α whereas the bicrystal revealed additionally pyramidal andc + a slip, the latter at the interface. Slip on the front and back surfaces was different and both thec + a and twinning systems acted to maintain compatibility. Slip in the β single crystal and the β bicrystal component were quite similar. However, there were differences in the intensity and amount of primary slip, (231) , on the front and back surfaces. The diminished amount of (231) slip on the back surface was due to plastically induced stresses, and on the front surface the primary slip cross slipped to slip which triggeredc + a slip in α. On the back surface the dominant slip system was which acted in response to the plastically induced stresses. An approximate calculation revealed that the interface deformation zone had about twice the flow stress of the average bicrystal stress. Formerly a Graduate Student in the Department of Physical and Engineering Metallurgy at Polytechnic Institute of New York, Brooklyn, NY     

Interdiffusion in Ni-rich,Ni-Cr-Al alloys at 1100 and 1200 °C: Part II. Diffusion coefficients and predicted concentration profiles

Ternary interdiffusion coefficients were measured in the Ni solid solution γ (fcc) phase of the Ni-Cr-Al system at 1100 and 1200 °C. Extensive use was made of both γ/γ and γ/γ + β (β-NiAl structure) diffusion couples. Two analysis techniques were employed to calculate the interdiffusion coefficients. When the Matano planes for Al and Cr were not coincident, numerous integral calculations were made to determine an average diffusion coefficient and to assess the effect of the noncoincidence of the Matano planes. The results of the diffusivity measurements showed that is approximately four times greater than , while and are of the same magnitude. For all concentrations, is two to three times greater than . Both and increase with increasing Al concentration, whereas and show little concentration dependence on Cr alone. A ternary, finite-difference interdiffusion model was employed to predict concentration profiles for the γ/γ couples utilizing the concentration dependence of the measured diffusivities. Good agreement was observed between the predicted and measured concentration profiles for both 1100 and 1200 °C.     

The substructure of (252)f martensite formed in an Fe- 8Cr- 1 C alloy

This work deals with a transmission electron microscopy investigation of plate martensite in an Fe-8Cr-lC alloy, which exhibits a typical (252)_f martensite transformation. It was found that macroscopic martensite plates, such as those used for optical microscopy determination of the habit plane and the shape strain, actually consist of many individual small plates having the same crystallographic orientation and habit plane. Individual plates within a macroscopic plate appear to form by auto-catalytic nucleation. The coalescence plane between individual plates is usually parallel to (011)_b = (lll)_f because the growth of the plates is restricted by stacking faults and twins on (lll)_f, formed as a result of accommodation of the shape strain. Deformation of the martensite at the coalescence sites, mainly on the twin system (112)_{b b}, leads to a complex substructure of the macroscopic plates. Stacking faults and twins in the austenite may be inherited in the martensite, which leads to further complexity. Prior to the coalescence, the substructure of individual martensite plates is simple, and consists of a low density of irregularly distributed twins on (112)_{b b} and dislocations having Burgers vector a_b/2 . These defects are probably caused by accommodation deformation. The martensite/austenite interface contains a set of parallel dislocations having a spacing of about 13 A. These dislocations are likely to be screw dislocations with Burgers vector a_b/2 = a_f/2 , which accomplish the complementary shear in the phenomenological crystallographic theory. Formerly with the University of Illinois     

Interaction of deformation twin and 120 deg-rotational order fault domain boundary in the lamellar structure of two-phase TiAl-based alloys

Interactions between deformation twin and 120 deg-rotational domain boundary were studied by transmission electron microscopy in a two-phase TiAl-based alloy with fully lamellar structure deformed at room temperature. Three types of the interaction were observed, depending on the interaction geometry and crystallography faced by the incident twinning Shockleys. It was found that the incident twinning shear could be accommodated into the barrier domain by a reaction involving emission of 1/2 {111} _B slip in all the three types of interactions presumably since the slip required a small critical resolved shear stress (CRSS) and was always favored by the pile-up stress. Several reaction schemes involving 1/2 {111} _B slip for each type of the interactions were proposed by considering whether the reaction resulted in a reduced elastic energy and if the dissociated dislocations were able to glide away to minimize the total elastic energy associated with a long-range stress field of a pileup of the incident twinning partials. It is suggested that whether a reaction scheme is feasible would depend on behavior of other product dislocation except 1/2 {111} _B .     

Determination of the interdiffusion coefficients in the Fe-Ni and Fe-Ni-P Systems Below 900 °C

Analytical electron microscopy was used to measure the interdiffusion coefficients, , in the Fe-Ni and Fe-Ni-P systems between 925 and 610 °C in austenite,γ, and between 850 and 550 °C in ferrite,α. The values of binaryγ Fe-Ni follow the extrapolated high temperature values of Goldsteinet al. The values of binaryα Fe-Ni are as much as two orders of magnitude lower than previously determined tracer diffusion measurements for the ferromagnetic region between 700 and 550 °C. In both ternaryγ Fe-Ni-P and ternary ferromagneticα Fe-Ni-P, the values are increased over the equivalent binary values at the same temperature. This increase is related to the ratio of the P content in the diffusion couple to the maximum P solubility in theα orγ phase at the diffusion temperature. The increase is most likely due to the effect of the electropositive solute atom P on the vacancy formation energy of the solvent, as described by LeClaire.     

High temperature creep of alpha iron in terms of effective stress and dislocation dynamics

The dynamics of dislocations in both steady state and transient creep in alpha iron of about 99.5 pct purity was investigated in the temperature interval 773 to 923 K, and the applied stress range 24.5 to 220.5 MN m⁻². The applied stress sensitivity parameter of the steady state creep rate m∲ = (∂ In ε/∂ In σ) _T increased linearly with increasing applied stress σ from about 5 at σ = 24.5 MN m⁻² to about 12 at σ = 196 MN m⁻². The apparent activation energy of steady state creep rate was found to decrease linearly with stress from 89 K cal mol⁻¹ at σ = 98 MN m⁻² to 81 K cal mol⁻¹ at a = 147 MN m⁻². Measurements of the mean effective stress σ^* by the strain transient dip test technique led to a nonlinear relation between σ^* and σ, indicating a dependence of the ratio σ*/σ on the applied stress. The effective stress sensitivity parameter was lower than m′.However, the apparent activation energy was equal toQ. Using the stress sensitivity technique, the relation between transient creep rate and effective stress at various constant internal stresses and temperatures was obtained. The effective stress sensitivity of transient creep rate was found to be lessthan that of steady creep rate.     

The development of the deformation texture in zirconium during rolling in sequential passes

Slip and twinning systems activated by rolling in sequential passes were observed on a coarse grained zirconium polycrystal. At least five independent deformation modes are activated; slip and twinning systems occur simultaneously. For low degrees of deforma-tion the main slip system is prism slip , even when the orientation is not favorable. The lattice rotations caused by slip proceed gradually with increasing defor-mation; they are relatively small although the strain achieved can be large. On the other hand, twinning causes spontaneous, large-scale lattice rotations, even for low degrees of deformation. The type of twinning depends largely on the crystallographic orientation of the matrix. For basal pole orientations of the undeformed grains in the area 0 to 50 deg from the normal direction twinning becomes preferentially operative. For basal pole orientations of the undeformed grains in the area 50 to 90 deg from the normal direction, however, twinning becomes preferentially operative. In both orientation areas as a complementary system twinning is operative. For all deformation sys-tems their operation is independent of the azimutal position of the basal pole in these areas. The lattice rotations alter the orientation of the crystallites in such a way that the basal poles all become aligned more or less in the direction of the deforming compres-sive force. For higher degrees of deformation pyramidal slip with a (c + a) type Burgers Vector can explain why this preferred orientation is maintained as final position, which for zirconium shows a split of basal poles of ±30 to 50 deg towards the transverse direc-tion. The method of following the complicated interactions between different slip and twinning systems in a stepwise deformed, coarse grained sheet by 1) trace analysis of the deformation modes, 2) by correspondingly derived lattice rotations, and 3) by texture measurements leads to an explanation of the texture development in zirconium. It is dis-cussed on the basis of basal pole rotations.     

Interdiffusion in Co-Mn alloys

Interdiffusion coefficient in cobalt-manganese alloys has been determined by Matano's method in the temperature range between 1133 and 1423 K on (pure Co)-(Co-30.28 at. pct Mn alloy) and (pure Co)-(Co-51.76 at. pct Mn alloy) couples. This, ∼D, has been found to increase with the increase of manganese content. However, the activation energy (∼Q) and frequency factor ( ₀) show a maximum at about 10 at. pct Mn. The concentration dependence of and has been discussed taking into account the thermodynamic properties of the alloy. The difference in between the ferro- and paramagnetic phases in Co-5 at. pct Mn alloy has been found to be 24 kJ/mol, which is larger, than that for the diffusion of Mn⁵⁴ in this alloy. Further it has been found that the Kirkendall marker moves toward manganese-rich side, showing that manganese atoms diffuse faster than cobalt atoms. From the marker shift, the intrinsic diffusion coefficients,D _Co andD _Mn, at 33 at. pct Mn have been determined as follows:D _Co=0.22×10⁻⁴ exp(−263 kJ mol⁻¹/RT) m²/s, andD _Mn=0.98×10⁻⁴ exp(−229 kJ mol⁻¹/RT) m²/s.