Diffusionless Nucleation of Lonsdaleite and Diamond in Hexagonal Graphite under Static Compression

We have used transmission electron microscopy to study the initial stages of diffusionless formation of lonsdaleite and diamond crystals when pyrolytic graphite is subjected to static compression. We consider the effect of plastic deformation of the matrix phase on development of the transformation. We propose a dislocation model for nucleation of dense phases, making it possible to explain the reason for the formation of a metastable lonsdaleite phase, the nature of its structural disordering, and also the possibility of diffusionless nucleation of diamond directly from hexagonal graphite.     

The structural aspect of high-pressure superhard phase synthesis

Structural aspects are considered for the direct phase transitions in carbon and boron nitride at high pressures from the viewpoint of martensite and diffusion transitions. The mechanism for the transitions of graphite and graphite-type BN into superhardphases is controlled primarily by the crystalline perfection of the initial structures that show martensite transformations to metastable phases (lonsdaleite and BN wurtzite allotrope), while highly defective ones show diffusion transformation to high-pressure stable phases (diamond and cubic boron nitride). The perfection in the initial structure has a very marked effect on the transformation mechanism during shock compression, which is the main technique in the commercial production of superhard phases.Institute of Materials Science, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7/8(380), pp. 83–92, July–August, 1995.     

Stress-induced martensitic phase transformations in polycrystalline CuZnAl shape memory alloys under different stress states

The effect of different uniaxial and triaxial stress states on the stress-induced martensitic transformation in CuZnAl was investigated. Under uniaxial loading, it was found that the compressive stress level required to macroscopically trigger the transformation was 34 pct larger than the required tensile stress. The triaxial tests produced effective stress-strain curves with critical transformation stress levels in between the tensile and compressive results. It was found that pure hydrostatic pressure was unable to experimentally trigger a stress-induced martensitic transformation due to the large pressures required. Traditional continuum-based transformation theories, with transformation criteria and Clausius-Clapeyron equations modified to depend on the volume change during transformation, could not properly predict stress-state effects in CuZnAl. Considering a combination of hydrostatic (volume change) effects and crystallographic effects (number of transforming variants), a micromechanical model is used to estimate the dependence of the critical macroscopic transformation stress on the stress state.     

The effect of composition on the pressure-induced HCP (∈) transformation in iron

The pressure-induced phase transformations in iron-rich Fe?Mn and Fe?Ni?Cr alloys were studied using an opposed diamond anvil high-pressure X-ray diffraction unit and a liquidmedium hydrostatic pressure apparatus. Transformations occurring with both increasing and decreasing pressure were studied. It was found that alloy additions of manganese and of nickel plus chromium significantly reduce the formation pressure of the hcp phase and can in some cases stabilize the phase enough to prevent it from transforming into some other phase during pressure release. All of the transformations are shown to be martensitic. Pressurization of prepolished surfaces, a large transformation pressure hysteresis, and the “abaric” formation of the ∈ phase establish the transformation as martensitic.     

Texture evolution during strain-induced martensitic phase transformation in 304L stainless steel at a cryogenic temperature

The strain-induced martensitic phase transformation during quasi-static uniaxial compression testing of a 304L stainless steel was investigated at 300 and 203 K using time-of-flight neutron diffraction to study the evolution of transformation texture. A number of specimens were precompressed to different strain levels at 300 and 203 K and the texture was investigated. At 203 K, the newly formed martensites are bcc and hcp phases and the texture analysis shows that the martensites are highly textured due to the grain-orientation-dependent phase transformation. The bcc {100} planes are mostly oriented with their plane-normal parallel to the loading direction at the beginning of the phase transformation and this texture is weakened during the subsequent compressive deformation. In the case of fcc to hcp transformation, it is less dependent on the grain orientation, although the fcc grains with {111} plane-normal at an angle close to 40 deg to the loading direction transform easier and the {0001} plane-normal of the newly formed hcp phase tends to rotate toward the loading direction during the texture evolution. The final texture of bcc and hcp martensites is the result of the interaction between deformation texture and transformation texture.     

Biaxial path dependence of deformation substructure of type 304 stainless steel

Although martensitic transformations in austenitic stainless steels have been studied rather thoroughly for uniaxial monotonie and cyclic loading, data are scant for biaxially loaded specimens. In particular, recent nonproportional straining experiments have indicated a significant increase in cyclic hardening beyond that observed in uniaxial tests at equivalent strain levels. In this paper, a link is made between the additional hardening and microstructural uniformity of transformation product. This link is expressed through a micromechanical viewpointvia increased latent hardening associated with rotation of the principal stress and plastic strain rate directions.     

Shear deformation and compaction of nickel aluminide powders at elevated temperatures

Powder compacts of nickel aluminide were compressed under uniaxial load above 1373 K, using a method that allowed the material to move laterally. Lateral and axial displacements were measured by means of three LVDTs. The resulting data fully described the applied stress state and the strain state as a function of time. That allowed us to obtain simultaneous measurements of the time dependent, and density dependent shear and densification behavior of the powder compact. The shear rate was non-linear in stress suggesting a dislocation flow mechanism. A model for densification by power law creep was applied to the data. It greatly overestimated the measured densification rates. Interestingly it was found that it is difficult to densify the powder to a value more than about 0.80 (relative) using uniaxial compression. In further experiments the powder was hot-pressed in a constraint cavity, allowing large hydrostatic pressures to be applied to the specimen. Near theoretical densities were obtained, presumably because the hydrostatic pressure promoted the diffusional transport mechanism of densification. The hot-pressing data were combined with the sinter forging data to obtain the correlation between densification rate and applied pressure. The diffusional mechanism of densification gave a good quantitative explanation for the densification behavior. In a broader context, we think that powder consolidation techniques ought to be optimized with a view to both shear strain as well as hydrostatic pressure. The shear strain can promote microstructure refinement through dynamic recrystallization, while pressure provides a driving force for diffusional densification.     

Computer simulation of martensitic transformations in constrained,two-dimensional crystals under external stress

This article reports a computer simulation study of the microstructures produced by martensitic transformations. In the present work, the transformation strain is dyadic, and the transformation is athermal and irreversible. The transformation occurs in a two-dimensional crystal that is constrained in a matrix that has no net transformation strain and may be subject to external stress. The crystal is divided into elementary cells. The transformation is simulated by computing the elastic strain energy in the linear elastic approximation and transforming the most-favored cell in each step to generate the minimum-energy transformation path. The simulation generates the microstructure at each step of the transformation and plots a temperature-transformation (TT) curve by computing the chemical driving force required to maintain the transformation and assuming that it is proportional to the undercooling. The results show that the matrix constraint causes complex, multivariant microstructures and separatesM _sandM _f. Multiple variants partly relax the shear part of transformation strain but interfere so that the transformation is difficult to maintain. The dilational part of the transformation strain produces interesting microstructures, such as “butterfly martensite,” in partially transformed crystals. It also increases ΔM since it produces a hydrostatic stress that cannot be compensated by mixing variants. The applied stress can be divided into hydrostatic and deviatoric components. The hydrostatic component changesM _swithout altering the microstructure or ΔM. The deviatoric stress changes the relative energies of the variants and produces a microstructure that is rich in the favored variant. It also increases ΔM, since single-variant transformations must be sustained against an accumulating, uncompensated shear. The thermal resistance (ΔM) increases with the magnitude of the deviatoric stress until a high-stress limit is reached and only one variant appears. The microstructure is most complex at intermediate stress, where both variants develop in a complex internal stress field. Cyclic stress dramatically increases the extent of transformation at given maximum load. The martensite that has already formed becomes a source of intense internal stress when the stress is reversed, promoting further transformation.     

Isothermal martensitic transformation under hydrostatic pressure in an FeNiC alloy at low temperatures

The isothermal martensitic transformation under applied hydrostatic pressure has been investigated with an Fe21.5Ni0.95C alloy single crystal by electrical resistivity, magnetic susceptibility, X-ray diffraction and optical microscopy. The martensitic transformation starting temperature, M_s, is lowered by applied hydrostatic pressure. The isothermal martensitic transformation occurs first as a burst under a critical pressure and it is continued by further transformation with decreasing hydrostatic pressure. The critical pressure under which the isothermal martesitic transformation starts changes curvilinearly with decreasing temperature. The morphology of the isothermal martensite formed under hydrostatic pressure is similar to that of athermal martensite. The temperature dependence of the critical pressure under which martensitic transformation starts has been calculated based on the homogeneous nucleation model and the heterogeneous nucleation model.     

Modeling the effects of stress state and crystal orientation on the stress-induced transformation of NiTi single crystals

A model that combines the phenomenological theory of martensite with a generalized Schmid’s law has been used to predict the principal stress combinations required to induce the martensitic transformation in unconstrained NiTi shape memory alloy (SMA) single crystals. The transformation surfaces prescribed by the model are anisotropic and asymmetric, reflecting the unidirectional character of shear on individual martensite habit planes. Model predictions of the transformation strain as a function of stress axis orientation for a uniaxial applied stress further demonstrate the anisotropy of the stress-induced transformation in NiTi single crystals. Model results for the uniaxial stress case compare favorably with previously published experimental observations for aged NiTi single crystals.     

Atomic mechanisms of diffusional nucleation and growth and comparisons with their counterparts in shear transformations

An integrated overview is presented of a viewpoint on the present understanding of nucleation and growth mechanisms in both diffusional and shear (martensitic) transformations. Special emphasis is placed on the roles played by the anisotropy of interphase boundary structure and energy and also upon elastic shear strain energy in both types of transformation. Even though diffusional nucleation is based on random statistical fluctuations, use of the time reversal principle shows that interfacial energy anisotropy leads to accurately reproducible orientation relationships and hence to partially or fully coherent boundaries, even when nucleation at a grain boundary requires an irrational orientation relationship to obtain. Since the fully coherent boundary areas separating most linear misfit compensating defects are wholly immobile during diffusional growth because of the improbability of moving substitutional atoms even temporarily into interstitial sites under conditions normally encountered, partially and fully coherent interphase boundaries should be immovable without the intervention of growth ledges. These ledges, however, must be heavily kinked and usually irregular in both spacing and path if they, too, are not to be similarly trapped. On the other hand, the large shear strain energy usually associated with martensite requires that its formation be initiated through a process which avoids the activation barrier associated with nucleation, perhaps by the Olson-Cohen matrix dislocation rearrangement mechanism. During growth, certain ledges on martensite plates serve as transformation dislocations and perform the crystal structure change (Bain strain). However, the terraces between these ledges in martensite (unlike those present during diffusional growth) are also mobile during non-fcc/hcp transformations; glissile dislocations on these terraces perform the lattice invariant deformation. Growth ledges operative during both diffusional and shear growth probably migrate by means of kink mechanisms. However, diffusional kinks appear to be nonconservative and sessile (and therefore resist immediate transmission of elastic shear strain energy), whereas those associated with martensitic growth must be conservative and glissile (and fully transmit such strain energy). The broad faces of both diffusionally and martensitically formed plates contain an invariant line, as emphasized by Dahmen and Weatherly. However, in the diffusional case, minimization of growth ledge formation kinetics seems to be the main role thereby played, whereas in martensitic growth, the main purpose of such an interface is to minimize elastic shear strain energy. The latter minimization requires that martensite forms as plates (or perhaps as laths) enclosed by a pair of invariant line-containing interfaces. During diffusional transformations, on the other hand, other interfaces at which growth ledge formation kinetics are not too much faster than those at the invariant line interface can also comprise a significant portion of the interfacial area, thereby leading to the formation of other, quite different morphologies, such as intragranular idiomorphs and grain boundary allotriomorphs. Critical problems remaining unsolved in diffusional transformations include calculation of critical nucleus shapes when the crystal structures of the two phases are significantly different, highly accurate calculation of the energies of the interphase boundaries thus formed, and direct observation of atomic scale kinks on the risers of growth ledges by means of a yet-to-be-invented three-dimensional (3-D) atomic-resolution form of transmission electron microscopy. Experimental identification and characterization of transformation dislocations and experimental testing of “nucleation” mechanisms are now of special importance in fundamental studies of martensitic transformations.     

Three-dimensional computational-cell modeling of the micromechanics of the martensitic transformation in transformation-induced-plasticity-assisted multiphase steels

A three-dimensional finite-element microstructural cell model involving an inclusion of retained austenite embedded within a ferrite grain, which is surrounded by a homogeneous matrix representing the behavior of a transformation-induced-plasticity (TRIP)-assisted multiphase steel, was developed in order to address the micromechanics of the martensitic transformation in small isolated austenite grains. The transformation of a single martensite plate is simulated after various amounts of prior plastic deformation under different in-plane loading conditions. The values of the mechanical driving force and of the elastic and plastic accommodation energies associated with the transformation are calculated as a function of the externally applied loading conditions. The mechanical driving force and the total accommodation energy are of the same order of magnitude. The mechanical driving force depends upon the stress state and is the highest for plane-strain conditions. The total accommodation energy is almost independent of the stress state. It is affected by the amount of plastic straining prior to transformation and is very much dependent on the level of the shear component of the transformation strain. The results of this study provide guidelines for the development of realistic stress-state-dependent transformation evolution laws for TRIP-assisted multiphase steels.     

A mesoscale study on the thermodynamic effect of stress on martensitic transformation

The effect of stress on martensitic transformation (MT) is addressed with special emphasis on the mechanical driving force (MDF) for triaxial stress states. The mechanical driving force appears to be additional to the chemical driving force in thermodynamic transformation con-ditions derived from a Gibbs free energy formulation for stressed solids undergoing MT. The thermodynamic criterion of Patel and Cohen predicts the change in martensite start temperature if MT occurs in a stress field. This criterion is extended from uniaxial to triaxial stress states and is discussed in the light of emerging microstresses. As a source of microstress, the elastic anisotropy of single crystals is taken into account. Its influence on the martensite start temper-ature is investigated by mesomechanical finite-element modeling. The critical stress for the start of transformation occurring in a stress field is calculated from a transformation condition and compared with results based on statistical theories for stress-assisted nucleation. In the context of martensitic transformation and mechanical effects, the MDF accounts for the Magee effect. The range of temperature for which the Magee effect has an influence on the macroscopic deformation behavior of a specimen is determined in dependence of the level of uniaxial applied stress. Finally, a constitutive equation in incremental form for transformation-induced plasticity (TRIP) derived within the continuum-thermodynamics framework is suggested.     

Synthesis and Structure of Nanocrystalline Powders of Ultrahard Phases

The synthesis of ultrahard nanocrystalline phases is based on the occurrence of direct phase transformations in carbon and boron nitride at high pressures during shock compression. This survey examines the relationship between features of the initial layered structures, the mechanisms of the direct transformations (martensitic, diffusional), and the actual structure of the ultrahard phases formed during shock compression.     

Plastic strain of powder components in unclosed volumes

During free compaction, extrusion, and bending of powder blanks, the material is under conditions of nonuniform stress and strain and is subject to compression and tension. A form of plasticity theory is developed for such a body that includes the tension and compression applied to the material. The trends in this theory form the basis for three technological hot-pressing processes. They have been used to prepare spherical layers for tractor hydraulic pump systems (the component is formed by compaction and at various stages is subject to various deformation schemes: uniaxial tension, free compaction, and hydrostatic nonuniform compression), and the same basis has been applied to gear forks for tractors (forming in a semi-open die in two stages: in the first, the state of stress varies from hydrostatic compression to uniaxial, while in the second the material is densified under conditions of hydrostatic compression), as well as to the polar sprocket for an automobile alternator (the process involves plastic bending of the flat elements, with changes produced under tension and adjustment of the material on a convex free surface followed by compression and densification on a concave one).Institute of Materials Science, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5–6, pp. 25–31, May–June, 1995.     

Effect of the Type of State of Stress on the Stress Relaxation in Titanium Nickelide

The stress relaxation in titanium nickelide specimens subjected to strain controlled uniaxial tension under martensitic anelasticity conditions at a given rate and the limited creep effect observed at the stages of stress controlled steplike loading are studied. The rheonomic properties of a shape memory alloy are found to depend on the type of state of stress, and they are more pronounced in tension than in compression. A simple model is develop to describe the detected effects.     

Failure mechanisms of laminated carbon-carbon composites—I. Under uniaxial compression

Failure mechanisms under uniaxial compressive loading are determined for several types of two dimensional (2D)carbon-carbon composites. Compressive strengths ranging from 90 to 180 MPa are observed. As expected, under uniaxial compression, a shear band is formed across the specimen diagonal. Remarkably, no inelastic deformation, either in the form of micro-buckling or microcrack nucleation was observed prior to the terminal shear failure. The crimp angles associated with the weaving pattern are significant not only in pre-setting the angle of the shear band but also in regard to the local kink boundary angle β in the fiber bundles. Hitherto unreported values of β ranging from 50 to 60°, and the kink inclination angle α in the range 35–40°, are observed. A theoretical analysis relating the peak compression load to α and β, with crimp angles as initial imperfections, explains the experimental observations.     

Comparison of shape memory characteristics of a Ti-50.9 At. Pct Ni alloy aged at 473 and 673 K

The effect of aging on transformation and deformation behavior, i.e., the transformation temperatures, shape memory behavior, and multistage martensitic and R-phase transformations, was investigated for a Ti-50.9 at. pct Ni alloy aged at a low temperature (<600 K) rarely used for practical applications and at a high temperature (>600 K) conventionally used for practical applications. It was found that there are many differences between aging at 473 and 673 K. The martensitic and R-phase transformation temperatures significantly varied depending on aging time and temperature. It is found that two-stage R-phase and multistage martensitic transformations appear in both the specimens aged at 473 and 673 K, respectively. The two-stage R-phase transformation appeared by aging at 473 K over 36 ks, while the multistage martensitic transformation (MSMT) appeared by aging at 673 K in the range of aging times between 1.2 and 36 ks. It is found that the critical stress for slip increases with increasing aging time in specimens aged at 473 K, while that of specimens aged at 673 K increases with increasing aging time until reaching a maximum, then it decreases with a further increase in aging time. It is also found that the critical stress for slip is superior for specimens aged at 473 K than that for specimens aged at 673 K. It was confirmed that dense and fine lenticular precipitates of about 10 nm in length were formed through aging, resulting in superior shape memory characteristics.     

Effect of stress state on the stress-induced martensitic transformation in polycrystalline Ni-Ti alloy

The effect of stress state on the character and extent of the stress-induced martensitic transformation in polycrystalline Ni-Ti shape memory alloy has been investigated. Utilizing unique experimental equipment, uniaxial and triaxial stress states have been imposed on Ni-Ti specimens and the pseudoelastic transformation strains have been monitored. Comparisons between tests of differing stress states have been performed using effective stress and effective strain quantities; a strain offset method has been utilized to determine the effective stress required for transformation under a given stress state. Results of the tests under different stress states indicate that (1) despite the negative volumetric strain associated with the austenite-to-martensite transformation in Ni-Ti, effective stress for the onset of transformation decreases with increasing hydrostatic stress; (2) effective stressvs effective strain behavior differs greatly under different applied stress states; and (3) austenite in Ni-Ti is fully stable under large values of compressive hydrostatic stress.     

Microstructure and martensitic transformations in a dual-phase α/β Cu−Zn alloy

The martensitic transformations in a dual-phase α/β Cu−Zn shape-memory alloy, containing 15 pct by volume of α particles, were studied during subcooling and deformation. The crystal structure and characteristics of the martensitic transformation of a dual-phase Cu−Zn alloy were found to be similar to those of a single-phase alloy. Both the thermal martensite formed by subcooling and the stress-induced martensite (SIM) formed by loading possessed an M9R long-period stacking-order (LPSO) structure, with internal stacking faults on the (001) basal plane. Upon subcooling, the α particles were deformed in order to accommodate the shape strain accompanying the martensitic transformation. Although most of them are deformed by slip, deformation twins have, nevertheless, been found in a few α particles. Upon loading, the SIM with an M9R structure nucleates and grows at a given temperature; subsequently, another martensite phase (α_S) possessing an fct structure is formed, with a shear developing on the basal plane of the initial M9R SIM during further loading. However, during unloading, both the α_S and SIM are transformed and follow the reverse sequence back to the parent phase. However, some residual SIM and α_S were found at zero load, due to a constraint effect of the deformed α particles and grain boundaries. The α_S martensite may be formed by two intersecting plates of SIM or by advanced deformation on a single plate of SIM. In addition to the residual SIM and α_S martensite, an α_S lamellar martensite was found in the deformed specimen.