High-resolution transmission electron microscopy investigation of the face-centered cubic/hexagonal close-packed martensite transformation in Co-31.8 wt pct Ni alloy: Part 2. Plate intersections,extended defects,and nucleation mechanisms

The face-centered cubic/hexagonal close-packed (fcc/hcp) martensite phase transformation in a Co-31.8 wt pct Ni alloy was studied by high-resolution transmission electron microscopy (HRTEM). The HRTEM was used to study the structure and properties of intersections between martensite plates and other defects observed in the alloy such as stacking fault tetrahedra (SFT) and Z-type defects. The HRTEM was also used to attempt to determine if various proposed mechanisms for the fcc/hcp martensite transformation were operating. There is evidence to suggest that the reflection mechanism proposed by Bollmann and the dipole mechanism proposed by Hirth are active in the fcc/hcp martensitic transformation, although the evidence is not completely certain in either case. Growth of the hcp phase by a four- or six-plane mechanism as proposed by Mahajanet al. is possible in theory but was not observed in this study. Transformation by previously proposed pole mechanisms was also not observed in this study, although evidence for a new type of pole mechanism was found. The formation of SFT along the fcc/hcp martensite interface was observed to occur by the cross-slip of Shockley partial dislocations out of the fcc/hcp interface onto conjugate fcc matrix planes, followed by further cross-slip to form the SFT, as previously observed for grain boundaries in fcc alloys.     

High-resolution transmission electron microscopy investigation of the face-centered cubic/hexagonal close-packed martensite transformation in Co-31.8 wt pct Ni alloy: Part 1. Plate interfaces and growth ledges

The face-centered cubic/hexagonal close-packed (fcc/hcp) martensite transformation in a Co-31.8 wt pct Ni alloy was studied by high-resolution transmission electron microscopy (HRTEM). High-resolution transmission electron microscopy was used to study the structure and properties of growth ledges, the tips of martensite plates, and martensite nucleation sites. The HRTEM image simulations were performed in order to determine the effects of both beam and crystal tilt on the experimental images. In the investigation, it was determined that the fcc/hcp martensite transformation in Co-Ni occurs by the passage of Shockley partial dislocation ledges (b=1/6〈112〉) along every other (111) plane in the fcc matrix. The hcp martensite thickens by the lateral movement of ledges across the fcc/hcp interface. Although superledges were observed, the majority of the ledges were two (0002) planes and this is the basic ledge height. Image simulations show that both beam and crystal tilt can have a marked effect on HRTEM images of fine hcp martensite plates. The effects of tilt must be minimized in order to unambiguously resolve the interfacial structure.     

A transmission electron microscopy study of the mechanisms of strengthening in heat-treated Co-Cr-Mo-C alloys

Microstructures produced in the Co-Cr-Mo-C alloy H.S.21 were observed by transmission electron microscopy in cast specimens following solutionizing at 1230°C and aging at 650°C and in low-carbon wrought specimens following solutionizing and aging at 650°C and 750°C. In all cases, aging was found to promote the formation of fcc stacking faults and to cause an initial martensitic transformation from the fcc phase to a heavily faulted hep structure. Precipitate formation was observed in hcp areas of the cast material after 20 h at 650°C and in hcp areas of wrought material after 20 h at 750°C. Prolonged aging at 750°C produced a transformation in the hcp structure of wrought specimens, with a relatively fault-free structure replacing the heavily faulted martensitic form. Interruption of fcc slip by both fcc stacking faults and bands of hcp phase was found to be the principal strengthening mechanism activated by aging. Precipitate formation in the hcp plays an increasingly significant role as aging time is increased. This microstructural information is used to explain the observed tensile properties of these alloys after the heat treatments mentioned.     

A transmission electron microscopy study of the mechanisms of strengthening in heat-treated Co-Cr-Mo-C alloys

Microstructures produced in the Co-Cr-Mo-C alloy H.S.21 were observed by transmission electron microscopy in cast specimens following solutionizing at 1230°C and aging at 650°C and in low-carbon wrought specimens following solutionizing and aging at 650°C and 750°C. In all cases, aging was found to promote the formation of fcc stacking faults and to cause an initial martensitic transformation from the fcc phase to a heavily faulted hep structure. Precipitate formation was observed in hcp areas of the cast material after 20 h at 650°C and in hcp areas of wrought material after 20 h at 750°C. Prolonged aging at 750°C produced a transformation in the hcp structure of wrought specimens, with a relatively fault-free structure replacing the heavily faulted martensitic form. Interruption of fcc slip by both fcc stacking faults and bands of hcp phase was found to be the principal strengthening mechanism activated by aging. Precipitate formation in the hcp plays an increasingly significant role as aging time is increased. This microstructural information is used to explain the observed tensile properties of these alloys after the heat treatments mentioned.     

Co-Al二元系中的fcc/hcp马氏体相变和形状记忆效应

It is known that pure Co undergoes martensitic transformation from γ phase (fcc) to ε phase (hcp) by the movement of a/6<112> Shockley partial dislocations at around 400 ℃, however, there have been few systematic works on the SM effect in Co and Co-based alloys. In this study, the fcc/hcp martensitic transformation and the SM effect were investigated in Co-Al binary alloys(mole fraction of Al=0～16%).The γ/ε martensitic transformation temperatures were found from the DSC measurements to decrease with increasing Al content, while the transformation temperature hystereses were observed to increase from 60 ℃ at x(Al)=0 to 150 ℃at x(Al)= 16%. The SM effect evaluated by a conventional bending test was enhanced by the addition of Al over 4%(mole fraction) and Co-Al alloys containing over 10%(mole fraction) exhibit a good SM effect associated with the hcp →fcc reverse transformation above 200 ℃. The SM effect was significantly improved by precipitation ofβ (B2) phase and the maximal shape recovery strain of 2. 2% was obtained, which can be explained by precipitation hardening. The crystallographic orientations between theβ, ε and γ phases were also determined. Finally, the magnetic properties were investigated and it was found that the Curie temperature and saturation magnetization of Co-14% Al(mole fraction) are 690 ℃and 120 emu/g, respectively. It is concluded that the Co-Al alloys hold promise as new high-temperature and ferromagnetic SM alloys.     

Prismatic slip in α-titanium single crystals

Single crystals of Ti, oriented favorably for prismatic slip and containing two levels of interstitial impurities, have been deformed in tension at temperatures between 78 to 1120 K. The stress-strain curves exhibit three stages of hardening similar to those of Zr and of fcc crystals. The work hardening rate in Stage II, θ_II/G is lower in Ti than in Zr. From the strain rate dependence of the stress at the onset of Stage III, the stacking fault energy on the prism planes of Ti has been estimated as 0.145 Joules/m². The relative values of stacking fault energy for Ti and Zr are consistent with a dissociation model which is based on the hcp ai bcc transformation. The thermally activated prismatic slip below 250 K is controlled by the interaction of dislocations with interstitial solute atoms. Above 250 K the dislocation mechanism for plastic flow is not clearly understood.     

The Hexagonal Close-Packed (HCP)???Face-Centered Cubic (FCC) Transition in Co-Re-Based Experimental Alloys Investigated by Neutron Scattering

Co-Re-based alloys have been developed to supplement the Ni-base superalloys used in gas turbine applications at high temperatures (1473?K [1200?°C] bare metal temperature). Unlike other commercial Co-based alloys, the Co matrix in the Co-Re alloys has a stable hexagonal close-packed (hcp) structure at room temperature. In situ neutron diffraction measurements on experimental Co-Re alloys hardened by carbide precipitates showed that the matrix undergoes an hcp ? face-centered cubic (fcc) allotropic transformation after heating to high temperatures. Furthermore, it was found that this transformation has a large hysteresis (~100?K). Thermodynamic calculations were undertaken to study the high-temperature phase stability and transformations in the complex multicomponent, multiphase Co-Re-Cr-C system with or without the addition of Ta. The results show that the minor phases (Cr₂₃C₆-type carbides and the Cr₂Re₃-type ?? phase) play an important role in the hcp ? fcc hysteresis by influencing the partitioning of Cr and Re between the matrix and the other phases.     

Effect of Cerium on Microstructures of Hard Alloys

Microstructures and crack extending characteristic of the YG8R,YT5R and YT14R hard alloysadding a trace cerium were studied by AEM and HVEM with a tensile holder.In the hard alloys addingcerium,the compounds of Ce_2O_3 or Ce_2O_2S could be formed,extending of stacking faults and transforma-tion from fcc Co to hcp Co were suppressed,volume fraction of fcc Co in the alloys was increased,andstrength of Co and(TiW)C phases and grain boundaries were raised.Plastic deformation in Co phase nearedges of the tensile cracks was more strong,and the tensile cracks could pass through some smaller WCgrains.     

Formation of hcp martensite during the isothermal aging of an fcc Co-27Cr-5Mo-0.05C orthopedic implant alloy

The evolution of the microstructure of a Co-27Cr-5Mo-0.05C alloy was investigated during isothermal aging between 650 °C and 950 °C. The main structural change observed as a result of aging was an fcc (metastable)→hcp isothermal martensitic transformation. The relationships between transformation, temperature, and time for this phase transition were determined using two different techniques: (1) room-temperature X-ray diffraction on samples aged after quenching from 1150 °C to 25 °C and (2) high-temperature in situ X-ray diffraction on samples cooled at 50 °C/min from 1150 °C to the aging temperature. The results show that the intermediate water quench significantly retards the kinetics of the phase transition by up to one order of magnitude in time. In addition, it was found that the grain size of the metastable fcc phase prior to aging does not affect the kinetics of the transformation. Age hardening resulting from this transformation varies linearly with the amount of hcp phase formed during the isothermal treatment and does not depend on the aging temperature. It is suggested that local plastic deformation, due to thermal and transformation stresses induced by quenching, reduces the number of hcp martensite embryos formed in the metastable fcc phase. This effect decreases the number of nucleation sites available for the fcc→hcp transformation during isothermal aging and leads to the slower transformation rates observed in water-quenched material.     

Implementation of advanced characterisation techniques for assessment of grinding effects on the surface integrity of WC–Co cemented carbides

Grinding is a key step on the manufacturing process of WC–Co cemented carbides (hardmetals). In this work, an investigation of grinding effects on the surface integrity of hardmetals is conducted. It is done by combining diverse advanced characterisation techniques: X-ray diffraction, field emission-scanning electron microscopy, electron back scatter diffraction, focused ion beam – 3D tomography and transmission electron microscopy. The study is carried out in a fine-grained WC–Co grade. Besides ground state, polished surface finish condition is assessed for comparison purposes. It is evidenced that grinding induces significant alterations: 3D tomography illustrates microcracking exists down to 2.5?μm depth with a highly anisotropic distribution at the subsurface, large compressive residual stresses extending until subsurface levels of about 12?μm, and phase transformation of binder from the original fcc phase into the hcp one, as well as severe plastic deformation observed within the binder at the surface level.

Invited keynote papers from EuroPM2017, Milan.     

Phase transformations in a wrought Co-Cr-Mo-C alloy

The effect of heat treatment on microstructure has been studied in a Co-Cr-Mo-C alloy using transmission electron microscopy. Isothermal aging treatments at 750 °C were found to promote a two stage fcc → hcp transformation, coincident with a discontinuous precipitation of M₂₃C₆ carbides. The variation in morphology of the carbides associated with the fcc → hcp transition is discussed in terms of the nature of the fcc/hcp interface.     

Phase transformation of stainless steel during fatigue

Transformation of austenite during cyclic loading was studied in AISI 301 and 304 alloys whose stability was adjusted by heat treatment and temperature changes. Fatigue life was determined under controlled strain amplitude tension-compression conditions. The amount of transformation to α’ (bcc) martensite was continuously indicated magnetically during testing, and the α’ and ∈ (hcp) phases were observed metallographically at failure. It was found in room temperature testing that at strain amplitudes in excess of 0.4 pct the formation of α’ (bcc) martensite was detrimental to the fatigue life. At 200°F (366 K) the fatigue life of an unstable alloy was increased, while in a completely stable austenitic alloy (20Cr, 6Ni, 9Mn), the life at 200°F (366 K) was less than that at room temperature for the same cyclic strain amplitude. The differing effect of temperature on life of these two types of alloy is attributed to the alteration of the austenite stacking fault energy and the relative free energies of the α’ (bcc), ∈ (hcp) and γ (fcc) phases in the unstable alloys. It has been observed that within the standard composition ranges of the two 300 series stainless steel grades there can be marked differences in the degree of transformation resulting from cyclic loading. This has the implication that for fatigue applications modifications in the specifications for the different grades of stainless would be advantageous.     

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.     

Kinetics of strain-induced fcc→hcp martensitic transformation

The kinetics of the strain-induced γ (fcc)→ε (hcp) transformationi.e. the amount of phase transformationvs applied strain were determined by density measurements at various temperatures. The transformation curve has a sigmoidal shape and approaches saturation below 100 pct transformation. Assuming that ε-platelets form from stacking faults, the volume fraction can be expressed as an implicit function of strain. The saturation value is constant and can be evaluated from quantitative metallography. The approach to saturation is determined by only one temperature-dependent parameter related to the stacking fault energy. Good agreement with experimental results was obtained. The model was also applied to transformation kinetics after a prestrain inducing both slip and twinning. The prestrain stabilizes austenite with respect to the strain-induced transformation through a block-refining of austenite by the substructure. In addition the nucleation is enhanced through the introduction of stacking faults. This effect vanishes at high applied strains but causes the shape of the transformation curve to become parabolic. It is concluded that decreasing the size of the ε platelets provides a simple means for reducing the temperature dependence of the transformation kinetics.     

Investigation on the cold deformation strengthening mechanism in MP159 alloy

Composite electron-diffraction patterns (EDPs) for any specified fcc matrix orientation containing deformation twin and/or strain-induced hexagonal close-packed ε martensite (shortened as hcp phase or hcp platelet subsequently) related to the matrix by specific orientation relationships have been computed and plotted. These results helped us to select the favorable fcc matrix orientation for distinguishing between deformation twin and hcp phase and to index experimental EDPs obtained by using transmission electron microscopy (TEM). The identification of structure features in cold-drawn MP159 alloy with 48 pct reduction in cross-sectional area was performed by using TEM, combined with the results from the computer-simulated composite EDPs. Investigation results demonstrated that the closely spaced, intersecting network of fine platelets formed during cold drawing is deformation twins, and no reliable evidence has been found for the presence of hcp phase, which was generally believed to explain the high strength attained in cold-drawn MP159 alloy in some literature. It follows that the significant increase in the strength due to cold working in MP159 alloy results predominately from the formation of the intersecting network of thin deformation twin platelets, which act as “cells” or “subgrains” and provide strong barriers to the movement of dislocations over large distances. In addition, the relatively high dislocation density makes a certain contribution to strengthening.     

Application of the theory of martensite crystallography to displacive phase transformations in substitutional nonferrous alloys

It has been demonstrated that the theory of martensite crystallography is capable of accounting successfully for the form and crystallography of a range of plate- or lath-shaped transformation products, even when the formation of the product phase involves significant substitutional diffusion. These transformations include the precipitation of metastable hexagonal γ’ (Ag₂Al) plates in disordered face-centered cubic (fcc) solid-solution Al-Ag alloys, the formation of ordered AuCu II plates from disordered fcc solid solution in equiatomic Au-Cu alloys, and the formation of metastable9R α ₁, plates in ordered(B2) Cu-Zn and Ag-Cd alloys. The application of the theory to these transformations is reviewed critically and the features common to them identified. It is confirmed that, in all three transformations, the product phase produces relief at a free surface consistent with an invariant plane-strain shape change and that the transformations are thus properly described as displacive. The agreement between experimental observations and theoretical predictions of the transformation crystallography is in all cases excellent. It is proposed that successful application of the theory implies a growth mechanism in which the coherent or semicoherent, planar interface between parent and product phases maintains its structural identity during migration and that growth proceeds atom by atom in a manner consistent with the maintenance of a correspondence of lattice sites. In the case of the coherent, planar interfaces associated with γ’ precipitate plates in Al-Ag alloys, there is direct experimental evidence that this is accomplished by the motion of transformation dislocations across the coherent broad faces of the precipitate plates; the transformation dislocations define steps that are two atom layers in height normal to the habit plane and have a Burgers vector at least approximately equivalent to an(α/6)(112) Shockley partial dislocation in the parent fcc structure. However, for AuCu II plates, where the product phase is twinned on a fine scale, and for α₁ plates, for which the lattice invariant strain leads to a substructure of finely spaced stacking faults, the structures of the semicoherent interphase boundaries and thus the details of the transformation mechanism remain less clearly defined. Formerly with the Department of Materials Engineering, Monash University Formerly with the Department of Materials Engineering, Monash University This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.     

金纳米颗粒烧结的分子动力学模拟

采用分子动力学模拟方法研究了不同尺寸Au纳米颗粒在烧结过程中晶型转变及烧结颈长大机制.研究发现纳米颗粒的烧结颈生长主要分为两个阶段:初始烧结颈的快速形成阶段和烧结颈的稳定长大阶段.不同尺寸纳米颗粒烧结过程中烧结颈长大的主要机制不同:当颗粒尺寸为4 nm时,原子迁移主要受晶界（或位错）滑移、表面扩散和黏性流动控制;当尺寸在6nm左右时,原子迁移主要受晶界扩散、表面扩散和黏性流动控制;当颗粒尺寸为9 nm时,原子迁移主要受晶界扩散和表面扩散控制.烧结过程中Au颗粒的fcc结构会向无定形结构转变.此外,小尺寸的纳米颗粒在烧结过程中由于位错或晶界滑移、原子的黏性流动等因素会形成hcp结构.      

Kinetics of the fcc → hcp Phase Transformation in Cu-Ge Solid Solutions Upon Isothermal Aging

The kinetics of the ζ-phase formation from a supersaturated α-Cu(Ge) solid solution (i.e., transformation from the fcc crystal structure to the hcp crystal structure) containing 10.8 at. pct Ge [at isothermal temperatures of 573 K, 613 K, and 653 K (300 °C, 340 °C, and 380 °C)] were studied by X-ray diffraction (XRD) for phase fraction determination. Both in situ and ex situ annealing experiments were performed. The transformation kinetics were modeled on the basis of a versatile modular model. The transformation kinetics complied with a site-saturation nucleation mode and strongly anisotropic interface-controlled growth mode in association with a corresponding impingement mode: diffusion of Ge (towards the stacking faults, SFs) does not control the transformation rate. Transmission electron microscopy (TEM) investigations showed that segregation of Ge at the stacking faults (SFs) takes place (relatively fast) prior to the structural transformation (fcc → hcp).     

Fcc/Hcp martensitic transformation in the Fe-Mn system: Experimental study and thermodynamic analysis of phase stability

A new experimental study ofA _s andM _s in the Fe-Mn system has been performed by using two complementary experimental techniques,viz., dilatometry and electrical resistivity measurements, which are applied to the whole composition range where the transformation can be detected,i.e., between 10 and 30 pct Mn. We used theA _s andM _s temperatures as input information in an analysis based on thermodynamic models for the Gibbs energy of the face-centered cubic (fcc) and hexagonal close-packed (hcp) phases. In these models, the magnetic contribution to Gibbs energy is accounted for, which allows us to study, by calculation, the influence of the entropy of magnetic ordering upon the relative stability of the phases. The picture of magnetic effects upon the fcc/hcp transformation that emerges from our work is as follows. At low Mn contents, the martensitic transformation temperatures are larger than the Néel temperature of the fcc phase, and bothA _s andM _s decrease linearly with increasing Mn. This encourages an extrapolation to zero Mn content, and we use that to critically discuss the available information on the fcc/hcp equilibrium temperature for Fe at atmospheric pressure. At sufficiently large Mn contents, we haveM _s <T _N ^y , which implies that the fcc phase orders antiferromagnetically before transforming to the hcp phase. Since hcp remains paramagnetic down to lower temperatures, the ordering reaction in fcc leads to a relative stabilization of this phase, which is reflected in a drastic, nonlinear decrease ofM _s.