Roles of dislocations and grain boundaries in martensite nucleation

In order to elucidate roles of dislocations and grain boundaries in martensite nucleation, the transformation temperature (M_s) of specimens austenitized at various temperatures and subjected to prestrain has been measured, using Fe-Ni, Fe-Ni-C, and Fe-Cr-C alloys. It is concluded that the plastic accommodation, in austenite, of the shape strain of the transforming martensite is a vital step in the nucleation event. Any factors impeding such plastic accommodation, such as the lack of dislocations, work hardening, and grain refinement, suppress the transformation. Contrary to the general belief, dislocations themselves do not act as favorable nucleation sites. Grain boundaries provide nucleation site, but only certain types of grain boundaries are qualified to be potential nuclei. A quantitative analysis shows that the increasing difficulty for the plastic accommodation with decreasing grain size is the main factor to depress M_s in fine-grained specimens.     

Isothermal Transformation of a CMnSi Steel Below the MS Temperature

After applying an austenitizing heat treatment above A _c3 followed by quenching to a temperature below the martensite-start (M _S ) temperature, an isothermal transformation was observed by means of dilatation measurement in a low-alloyed, low-carbon steel. The precise nature of this isothermal transformation below the M _S temperature is still unclear. The present contribution is a comprehensive comparison of the main difference between the isothermal transformation below the M _S temperature and the athermal martensitic transformation using electron microscopy and internal friction measurements. The mechanical properties of the transformation product also were characterized. The observations revealed that the isothermal transformation product below the M _S temperature had its own characteristic microstructure with a Kurdjumov–Sachs orientation relationship with the parent austenite and without carbide precipitation.     

Internal friction in ferrous martensites

Internal friction measurements were made at 1 Hz in the temperature range of 25 to 500°C on quenched, tempered and cold-worked Fe-Ni-C martensites. The alloys, which contained 15 to 30 wt pct Ni and 0.1 to 1.0 wt pct C, andM _s temperatures <0°C, transformed to martensite with a twinned (259)_γ habit and exhibited a relaxation peak at ~160°C. These results could be contrasted with those for Fe-C martensites, which form above room temperature, have predominantly dislocated (111)_γ or (225)_γ habits, and exhibit an internal friction peak at about 250°C. The literature on substructures, tempering and internal friction of all ferrous martensites and cold-worked ferrites was utilized in the interpretation of the 160°C peak. Several dissimilarities between the 160°C peak and the 250°C peak in Fe-C martensites or the cold-work peak in ferrite were noted such that models of dislocation-interstitial interaction for these peaks could not explain the 160°C peak. It was concluded that the 160°C peak is associated with the stress-induced motion of twin boundaries containing mobile carbon atoms. Such a mechanism was shown to be consistent with the present experimental observations and all other available data.     

On bainite formation

Driving force calculations for Fe-C, Fe-X-C, and Cu-Zn alloys show that the formation of bainite by a shear mechanism is thermodynamically impossible. There exist superledges on the broad faces of bainite in steels, revealing that the thickening of bainite probably proceeds by a ledge mechanism. In some Fe-Ni-C alloys and commercial steels, no simple relationship was found between the strength of austenite and theBs temperature; however, there is a linear relationship betweenBs and the diffusion coefficient of carbon and iron in austenite, as well as between the incubation period and the function containing D _Fe ^γ . The bainite reaction seems to be controlled by diffusional processes. In a low-carbon Ni-Cr steel, the morphology of the bainite/matrix interface boundary is different from that of the martensite, and the habit plane of the bainite (1 7 11)_α deviates 13.3 deg from that of the martensite (1 1 0)_α, indicating that the mechanism of the bainite reaction is not necessarily analogous to that of the martensitic transformation. At temperatures nearMs, as the driving force will be large enough, the growth of bainite by shear may be able to occur, and evidence is given by the morphology of bainite showing shear characteristic. X-ray diffraction study in Ag-Cd and internal friction measurements in Ag-Cd and Cu-Zn-Al alloys, 18CrNiWA steel, and its decarburized specimen reveal that the nucleation process occurs within the incubation period of bainite formation. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Stress-Assisted and strain-induced martensites in FE-NI-C alloys

A metallographic study was made of the martensite formed during plastic straining of metastable, austenitic Fe-Ni-C alloys withM _s temperatures below 0°C. A comparison was made between this martensite and that formed during the deformation of two TRIP steels. In the Fe-Ni-C alloys two distinctly different types of martensite formed concurrently with plastic deformation. The large differences in morphology, distribution, temperature dependence, and other characteristics indicate that the two martensites form by different transformation mechanisms. The first type, stress-assisted martensite, is simply the same plate martensite that forms spontaneously belowM _s except that it is somewhat finer and less regularly shaped than that formed by a temperature drop alone. This difference is due to the stress-assisted martensite forming from cold-worked austenite. The second type, strain-induced martensite, formed along the slip bands of the austenite as sheaves of fine parallel laths less than 0.5μm wide strung out on the {111}_γ planes of the austenite. Electron diffraction indicated a Kurdjumov-Sachs orientation for the strain-induced martensite relative to the parent austenite. No stress-assisted, plate martensite formed in the TRIP steels; all of the martensite caused by deformation of the TRIP steels appeared identical to the strain-induced martensite of the Fe-Ni-C alloys. It is concluded that the transformation-induced ductility of the TRIP steels is a consequence of the formation of strain-induced martensite. Formerly a graduate student at Stanford University     

A study on morphology and plate mean dimensions in Fe-Ni and Fe-Ni-Cr alloys

A systematic study has been carried out in Fe-Ni as well as Fe-Ni-Cr alloys to understand whether the factors related to the burst temperature or alloy composition are responsible for various morphologies as well as mean plate dimensions. A systematic procedure has been adopted to give a set of Fe-Ni-Cr alloys with constant transformation burst temperature (M _b ). While maintaining theM _b constant, it is found that Cr replaces more and more Ni with further additions. The martensitic transformation progresses predominantly through a spreading-out process. The aspect ratio (c/r) follows a parabolic relation withM _b , whereas it is linear with respect to Ni content in the case of Fe-Ni alloys. In Fe-Ni-Cr alloys, the aspect ratio, however, does not show any relationship with composition at 133 K. The appearance of different morphologies and the variation in different features within a morphology are seen in alloys with different compositions having the sameM _b .     

Stability and mechanical properties of some metastable austenitic steels

The relation between austenite stability and the tensile properties, as affected by testing temperature and processing, was studied for a series of alloys of increasing compositional complexity, viz., the Fe-Ni, Fe-Ni-C, and Fe-Ni-Cr-Mn-C systems. The “stress” and “strain induced” modes of transformation to martensite differed significantly in their influence on the shape of the stress-strain curve. Under certain testing conditions, unusually low yield strengths and high work hardening rates were observed in some of these alloys. Maxima in yield strengths were observed for all austenitic alloys containing carbon that were processed at deformation temperatures between 200° and 300°C. Evidence gleaned from electron microscopy and magnetic and mechanical testing suggested that the maxima were due to the formation of carbon atmospheres on dislocations during processing. The influence of austenite stability on the mechanical properties of steels, varied by systematic changes in test temperature (22° to -196°C), composition (8 pct, 12 pct, 16 pct, and 21 pct Ni) and deformation temperature (25° to 450°C), was evaluated quantitatively. An erratum to this article is available at .     

Bainite formation in a silver-cadmium alloy

The changes in X-ray diffraction intensity and internal friction within and beyond the incubation period of bainitic transformation on the isothermal TTT diagram in a silver-cadmium (43.3 wt pct Cd) alloy were studied. Experimental results show that the parent-phase X-ray diffraction intensity and the internal friction value increase immediately at the beginning of isothermal holding and that the internal friction reaches its maximum value,Q ⁻¹ _max within the incubation period. It then gradually decreases. It is suggested that the prebainitic transformation within the incubation period is the nucleation process of bainite.     

Internal friction in ferrous martensites

Internal friction measurements were made at 1 Hz in the temperature range of 25 to 500°C on quenched, tempered and cold-worked Fe-Ni-C martensites. The alloys, which contained 15 to 30 wt pct Ni and 0.1 to 1.0 wt pct C, andM _s temperatures <0°C, transformed to martensite with a twinned (259)_γ habit and exhibited a relaxation peak at ~160°C. These results could be contrasted with those for Fe-C martensites, which form above room temperature, have predominantly dislocated (111)_γ or (225)_γ habits, and exhibit an internal friction peak at about 250°C. The literature on substructures, tempering and internal friction of all ferrous martensites and cold-worked ferrites was utilized in the interpretation of the 160°C peak. Several dissimilarities between the 160°C peak and the 250°C peak in Fe-C martensites or the cold-work peak in ferrite were noted such that models of dislocation-interstitial interaction for these peaks could not explain the 160°C peak. It was concluded that the 160°C peak is associated with the stress-induced motion of twin boundaries containing mobile carbon atoms. Such a mechanism was shown to be consistent with the present experimental observations and all other available data.     

Isothermal and athermal type martensitic transformations in yttria doped zirconia

The phase transformation from the high temperature tetragonal phase to the low temperature monoclinic phase of zirconia had been long considered to be a typical athermal martensitic transformation until it was recently identified to be a fast isothermal transformation. The isothermal nature becomes more apparent when a stabilizing oxide, such as yttria, is doped, by which the transformation temperature is reduced and accordingly the transformation rate becomes low.Thus it becomes easy to experimentally establish a C-curve nature in a TTT (Time-Temperature-Transformation) diagram. The C-curve approaches that of well known isothermal transformation of Y-TZP (Yttria Doped Tetragonal Zirconia Polycrystals), which typically contains 3mol% of Y2O3. In principle, an isothermal transformation can be suppressed by a rapid cooling so that the cooling curve avoids intersecting the C-curve in TTT diagram. Y-TZP is the case, where the stability of the metastable tetragonal phase is relatively high and thus the tetragonal phase persists even at the liquid nitrogen temperature. On the other hand, the high temperature tetragonal phase of pure zirconia can never be quenched-in at room temperature by a rapid cooling; instead it always turns into monoclinic phase at room temperature. This suggests the occurrence of an athermal transformation after escaping the isothermal transformation, provided the cooling rate was fast enough to suppress the isothermal transformation. Thus, with an intermediate yttria composition, it would be possible to obtain the tetragonal phase which is not only metastable at room temperature but athermally transforms into the monoclinic phase by subzero cooling. The objective of the present work is to show that, with a certain range of yttria content, the tetragonal phase can be quenched in at room temperature and undergoes isothermal transformation and athermal transformation depending on being heated at a moderate temperature or under-cooied below room temperature. Because both of the product phases are essentially the same monoclinic phase, both transformations are regarded as martensitic transformation, i. e. isothermal and athermal martensite. In some steels such as Fe-Mn-Ni and Fe-Ni-C, the occurrence of both isothermal and alhermal martensitic transformations has been reported. However, in these cases, the isothermal transformation occurs at temperatures slightly above the Ms (Martensite start) temperatures, and thus these transformations are considered to conform the same C-curve. On the other hand, the Ms temperature of the present material is well below the C-curve, which suggests that completely different mechanisms are controlling the kinetics of these two modes of transformations. Other aspects on these transformations are also to be reported..     

添加Y2O3的ZrO2的等温和变温马氏体相变

The phase transformation from the high temperature tetragonal phase to the low temperature monoclinic phase of zirconia had been long considered to be a typical athermal martensitic transformation until it was recently identified to be a fast isothermal transformation. The isothermal nature becomes more apparent when a stabilizing oxide, such as yttria, is doped, by which the transformation temperature is reduced and accordingly the transformation rate becomes low.Thus it becomes easy to experimentally establish a C-curve nature in a TTT (Time-Temperature-Transformation) diagram. The C-curve approaches that of well known isothermal transformation of Y-TZP (Yttria Doped Tetragonal Zirconia Polycrystals), which typically contains 3mol% of Y2O3.In principle, an isothermal transformation can be suppressed by a rapid cooling so that the cooling curve avoids intersecting the C-curve in TTT diagram. Y-TZP is the case, where the stability of the metastable tetragonal phase is relatively high and thus the tetragonal phase persists even at the liquid nitrogen temperature. On the other hand, the high temperature tetragonal phase of pure zirconia can never be quenched-in at room temperature by a rapid cooling; instead it always turns into monoclinic phase at room temperature. This suggests the occurrence of an athermal transformation after escaping the isothermal transformation, provided the cooling rate was fast enough to suppress the isothermal transformation. Thus, with an intermediate yttria composition, it would be possible to obtain the tetragonal phase which is not only metastable at room temperature but athermally transforms into the monoclinic phase by subzero cooling.The objective of the present work is to show that, with a certain range of yttria content, the tetragonal phase can be quenched in at room temperature and undergoes isothermal transformation and athermal transformation depending on being heated at a moderate temperature or under-cooled below room temperature. Because both of the product phases are essentially the same monoclinic phase, both transformations are regarded as martensitic transformation, i. e. isothermal and athermal martensite. In some steels such as Fe-Mn-Ni and Fe-Ni-C, the occurrence of both isothermal and athermal martensitic transformations has been reported. However, in these cases, the isothermal transformation occurs at temperatures slightly above the Ms (Martensite start) temperatures, and thus these transformations are considered to conform the same C-curve. On the other hand, the Ms temperature of the present material is well below the C-curve, which suggests that completely different mechanisms are controlling the kinetics of these two modes of transformations. Other aspects on these transformations are also to be reported..     

Magnetic contribution to the interdiffusion coefficients in bcc (<Emphasis Type="Italic">α</Emphasis>) and fcc (<Emphasis Type="Italic">γ</Emphasis>) Fe-Ni alloys

The interdiffusion coefficients in bcc (α) and fcc (γ) Fe-Ni alloys below their Curie temperatures have been calculated based on the magnetic contribution to the free energy for interdiffusion. The free energy for interdiffusion due to magnetic ordering in bcc Fe-Ni alloys is positive. The calculated interdiffusion coefficients in bcc Fe-Ni alloys fit the experimental data quite well. In fcc Fe-Ni alloys, the magnetic contribution to interdiffusion depends on both temperature and composition and is abnormal for Ni compositions in the Invar region. The free energy of vacancy formation is positive and the free energy of vacancy migration is negative, due to the effect of magnetic ordering. The interdiffusion coefficient in the ferromagnetic phase is lower than that extrapolated from the paramagnetic phase for Ni compositions of 50 at. pct and greater and is higher than that extrapolated from the paramagnetic phase for Ni compositions of 40 at. pct and lower.     

Pseudoelastic and anelastic effects due to the motion of twin boundaries in copper-based alloys

Copper alloys, e.g. with a few at.% of Ge, are known to deform by slip and twinning when subjected to a tensile test at room temperature. Such alloys exhibit a large hysteresis loop or pseudoelasticity in a loading-unloading cycle, which is believed to be due to reversible motion of twin boundaries. The mobility and the annealing behaviour of twinning dislocations in several copper alloys with Ge, Si or Al have been studied by low-frequency internal friction measurements, electrical resistivity measurements and electron microscopy. Deformed samples show a characteristic anelastic effect; internal friction increases linearly with temperature in the range from 80 to 280 K and falls down rapidly above room temperature. The magnitude of the anelastic effect depends on the solute concentration and on the degree of deformation similarly to that of the pseudoelastic effect. The internal friction is tentatively explained as arising either from l ocal motion of twinning dislocations or from rearrangement of metastable stacking sequences.     

The pre-bainitic transformation in CuZnAlMn alloy

The behaviour of the pre-bainitic transformation in the CuZnAlMn alloy was investigated by using internal friction (Q⁻¹) measurements and TEM. The results show that there always exists an internal friction peak associated with the segregation of solute atoms before the formation of orthorhombic 9R bainite and that the 9R bainite nucleates martensitically in depleted regions of solute atoms in the B₂ phase. The transformation processes mentioned above were also confirmed in isothermal internal friction and TEM experiments.     

Computation of thermodynamic properties of liquid ferrous alloys from structural measurements

Relation between X-ray scattering intensities, mean square thermal fluctuations and thermodynamic properties. High temperature X-ray diffraction study of liquid Fe-Ni and Fe-Si alloys using reflection and transmission geometries. Calculation of the structure factor as a function of wave vector. Extrapolation to zero wave vector. Calculation of the concentration-concentration correlation function defined by A. B. Bhatia and D. E. Thorton. Computation of thermodynamic quantities of mixing ΔG, ΔH and ΔS for the binary alloys. Comparison with direct thermodynamic measurements reported in the literature.     

Crystallization of an Fe-Ni base amorphous alloy

The crystallization of an amorphous Fe-Ni base alloy was studied in a dynamic heating mode from room temperature to 700°C and during isothermal annealing at 400°C. Differential scanning calorimetry, X-ray diffraction, transmission electron microscopy, and hardness measurement were used to characterize the crystallization process under two heating conditions. In the dynamic heating condition, structural relaxation or atomic regrouping was thought to occur belowT _c. AboveT _c, crystallization occurred spontaneously and four crystalline phases were formed. The number of phases and the relative amount of these phases varied with the heating temperature. At a higher temperature, recrystallization occurred which resulted in grain growth. The final matrix phase was observed to coexist with other phases after crystallization. In the isothermal heating condition, it was found that the transformation of the alloy from amorphous state to crystalline state was through the nucleation and growth process. The first crystallization steps were via the formation of metastable phases. The final matrix phase than nucleated from the existing metastable phases. Hardness measurements in both heating conditions indicated that the alloy attained its peak hardness immediately after complete crystallization.     

The internal friction of the pearlitic,bainitic and martensitic transformations in FeNiC alloys

The low frequency internal friction of the pearlitic, bainitic and martensitic transformations in three FeNiC alloys has been studied. The results show that the internal friction of the martensitic transformation is in correspondence with the model based on the hysteresis loss mechanisms associated with stress-induced motion of the interface dislocations, while that of the pearlitic and bainitic transformations obeys Postnikov's model. At a constant frequency, the internal friction peaks of the pearlitic and bainitic transformations increase as the cooling rate increases, but the temperature of the internal friction peaks decreases, and the Q_max⁻¹ is directly proportional to the T/f_mT_m. When the frequency increases, the Q_max⁻¹ of the pearlitic and bainitic transformations lowers apparently, and the temperature of the internal friction peaks increases. The internal friction of the isothermal bainitic transformation in a FeNiC alloy has also been studied, which proves that there appears a damping peak in the incubation period, and which may be attributed to the nucleation of bainite. Consequently it infers that the mechanism of the bainite reaction seems similar to that of pearlite.     

Carbide precipitation during stage I tempering of Fe-Ni-C martensites

Microstructural changes which accompany the first stage of tempering have been studied by transmission electron microscopy (TEM) and electrical resistometry in two Fe-Ni-C alloys that form platelike martensite. The ε-carbide transition phase in these alloys adopts a platelike shape with a habit plane near {012=_α. Electron diffraction data indicate that the carbide may be partially ordered, resulting in orthorhombic symmetry, and therefore, this phase is designated as ε′- carbide. The carbide particles contain a fine internal substructure which appears to represent an internal accommodation deformation (faulting) on the carbide basal plane. Detailed analysis of the kinematics of carbide precipitation suggests that the observed habit plane and accommodation deformation permit an invariant-plane strain transformation which minimizes elastic strain energy. The apparent selection of only a limited number of possible orientation variants is explained in terms of the symmetry of the parent martensitic phase, which is known to undergo spinodal decomposition prior to the nucleation of the transition carbide. The martensitic substructure is not found to exert any significant influence on this overall precipitation behavior. formerly with the Massachusetts Institute of Technology formerly with the Massachusetts Institute of Technology     

Shape memory behavior of FeNiCoTi single and polycrystals

We present experimental and theoretical evidence of thermoelastic martensites in Fe29Ni18Co4Ti alloys. In this class of alloys, the high strength in the austenite domains limits the slip deformation as verified with transmission electron microscopy. The restriction of slip permits a higher degree of recoverability of the transformation. Using both single crystals with [123] orientation and polycrystals, the appearance of martensite plates upon deformation, and their reversion back to austenite upon heating (the shape memory effect), is revealed with in-situ optical microscopy. Theoretical results for the transformation strains and the detwinning of martensite are presented, which demonstrate convincingly the potential of these classes of alloys. Electrical resistance measurements identified the stress and temperature levels at the onset of forward and reverse transformations in isothermal deformation and thermal cycling experiments, respectively. The return of the electrical resistance to its reference value, upon austenite to martensite followed by martensite to austenite transformation, verified the recovery in the transformation strains measured in the experiments.     

Effects of manganese doping on magnetocaloric effect in Ge-rich Gd_5Ge_（2.05）Si_（1.95） alloy

The structure, magnetic and magnetocaloric properties of the Ge-rich Gd5Ge2.05-xSi1.95-xMn2x (x=0.01 and 0.03) alloys were investigated by scanning electron microscopy, X-ray powder diffraction, differential scanning calorimeter (DSC) and magnetization measurements. The results of energy dispersive X-ray analysis (EDX) and X-ray diffraction analyses showed that the composition and crystal structure of the alloys were desired. DSC measurements were performed to determine the transformation temperatures for each alloy. Both alloys exhibited the first order phase transition around room temperature. The alloys showed an anti-ferromagnetic transition around 60 K. The isothermal magnetic entropy changes of the alloys were determined from the isothermal magnetization measurements by using the Maxwell relation. The maximum values of isothermal magnetic entropy change of the Gd5Ge2.05-xSi1.95-xMn2x alloy with x=0.01 was found to be -12.1 and -19.8 J/(kg·K) using Maxwell equation around 268 K in applied fields of 2 and 5 T, respectively.