Subsequent Martensitic Transformation Research Articles

Ultrahochfeste thermomechanisch gewalzte Stähle – verbesserte Eigenschaften und Anwendungen

The beneficial combination of thermomechanical rolling and subsequent martensitic transformation is described as convenient method to receive ultra high strength hot rolled material with well balanced ductility and toughness. Using examples from laboratory experiments and investigations on hot rolled material produced in industrial scale the influence of austenite conditioning on the material behaviour after quenching with and without subsequent tempering is shown. Using this method for production of strip and cut length sheets or plates yield strength levels from 900MPa to 1100MPa can be reached with very low carbon contents between 0.08 and 0.17 m%. Therefore excellent welding behaviour is obtained.

Kinetics of isochronal austenization in modified high Cr ferritic heat-resistant steel

Employment of high Cr ferritic steels as a main structural material is considered as a way to achieve economical competitiveness of main steam pipe and nuclear reactors in power plants. Differential dilatometry and microstructure observation were employed to investigate the isochronal austenitic transformation of the modified high Cr ferritic steel. The kinetics of the isochronal austenitic transformation were described by a phase-transformation model involving site saturation (pre-existing nuclei), diffusion-controlled growth, and incorporating an impingement correction. The experimental results and kinetic analysis indicate that an increase of the heating rate promotes the diffusion-controlled austenitic transformation. The dissolving degree of precipitates during the austenization process affects the activation energy for diffusion and the undissolved precipitates lead to an increase of the onset temperature of the subsequent martensite transformation upon cooling.

Investigation on the precipitation behavior of M 3C phase in T91 ferritic steels

As traditional 9–12% Cr heat-resistant ferritic steels, T91 steels have been considered as candidate reduced-activation materials for nuclear engineering applying due to its excellent creep resistance and high resistance to void formation during neutron irradiations at elevated temperature. Needle-like M 3C precipitates are produced during the routine normalizing process before tempering. Differential scanning calorimetry and infrared radiation thermometer have been employed to study the precipitation behavior of the secondary M 3C particles upon subsequent cooling process after austenization. Various austenization conditions (holding time, temperature and the subsequent cooling rate) were carried out to clarify effect of normalizing condition on the formation of the M 3C phase. In spite of various austenization conditions applied, it is found that the precipitation of M 3C phase is depends only on the cooling rate applied. Furthermore, the precipitation of M 3C phase occurs before the onset of the martensite transformation, which is contrary to the previous statement that it takes place during the auto-tempering stage after martensitic transformation. The above observation points out that the precipitation of M 3C would produce an effect on the subsequent martensitic transformation behavior, leading to the formation of wide martensite laths with a low dislocation density.

Structure of domains formed upon martensitic transformations B2 → B19 and B2 → B19′ in alloys with a B2 superstructure

An analysis of symmetry changes upon the structural B2-ISS2 phase transition with subsequent martensitic transformations into the B19 and B19′ phases has been performed. An interrelation between the atomic displacements and the lattice deformation has been shown to exist. It has been established that the structures with longitudinal and transverse displacements have the same symmetry and can pass into one another via a twinning operation. A sequence of transformations of the tensor of lattice deformations consistent with the transformations of the atomic displacements has been obtained. It has been found that the operations that provide these transformations are the operations of twinning.

Effect of Ni 4Ti 3 precipitation on martensitic transformation in Ti–Ni

Precipitation of Ni 4Ti 3 plays a critical role in determining the martensitic transformation path and temperature in Ni–Ti shape memory alloys. In this study, the equilibrium shape of a coherent Ni 4Ti 3 precipitate and the concentration and stress fields around it are determined quantitatively using the phase field method. Most recent experimental data on lattice parameters, elastic constants, precipitate–matrix orientation relationship and thermodynamic database are used as model inputs. The effects of the concentration and stress fields on subsequent martensitic transformations are analyzed through interaction energy between a nucleating martensitic particle and the existing microstructure. Results indicate that R-phase formation prior to B19′ phase could be attributed to both direct elastic interaction and stress-induced spatial variation in concentration near Ni 4Ti 3 precipitates. The preferred nucleation sites for the R-phase are close to the broad side of the lenticular-shaped Ni 4Ti 3 precipitates, where tension normal to the habit plane is highest, and Ni concentration is lowest.

The effect of aluminium and phosphorus on the stability of individual austenite grains in TRIP steels

We have performed in situ synchrotron X-ray diffraction experiments to assess the influence of aluminium and phosphorus on the austenite stability in low-alloyed transformation-induced plasticity steels during the high-temperature bainitic holding and the subsequent martensitic transformation during cooling to temperatures between room temperature and 100 K. Although the addition of aluminium increases the chemical driving force for the formation of bainitic ferrite plates significantly, the phosphorus exerts a larger influence on the bainitic transformation kinetics. Consequently, the addition of phosphorus leads to a higher degree of carbon enrichment and a narrower grain volume distribution of the metastable austenite. The stability of the individual austenite grains with respect to their martensitic transformation depends on both the local carbon content and the grain volume for austenite grains smaller than 20 μm 3. The presence of aluminium and phosphorus further stabilizes the austenite grains.

Vibrational and magnetic behavior of transforming and nontransforming Ni-Mn-Ga alloys

Temperature dependences of the vibrational and magnetic properties of the high symmetry parent phase in three different alloys ${\mathrm{Ni}}_{51.1}{\mathrm{Mn}}_{24.9}{\mathrm{Ga}}_{24.0}$, ${\mathrm{Ni}}_{49.7}{\mathrm{Mn}}_{24.1}{\mathrm{Ga}}_{26.2}$, and ${\mathrm{Ni}}_{49.6}{\mathrm{Mn}}_{21.9}{\mathrm{Ga}}_{28.5}$ are measured in the temperature range between 4.2 and $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. While in the first two alloys an intermediate transformation precedes the subsequent martensitic transformation (MT), the MT is not detected in the analyzed temperature range in the third one. The features of both the intermediate phase transformation and the intermediate phase in these alloys are determined. It is found that the elastic characteristics of the high symmetry phase are similar independent of the closeness of the martensitic transformation. Besides, the magnetic susceptibility shows also a similar behavior in the parent phase of the three alloys, while in the intermediate phase, this behavior differs in the nontransforming alloy. In the latter case, the susceptibility shows an anomalous increasing during cooling down to $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The continuous increase of the resonance frequency measured by resonant ultrasound spectroscopy and the anomalous increase of the magnetic susceptibility during cooling must be an intrinsic characteristic of the intermediate phase with no obvious relationship to the appearance of the martensitic transformation.

High resolution transmission electron microscopy study on the nano‐scale twinning of θ‐NiMn precipitates in an Fe–Ni–Mn maraging alloy

AbstractPrecipitation of nanometer‐sized fct θ‐NiMn intermetallic compound was found to cause age hardening in Fe–10Ni–7Mn (wt%) maraging alloy during isothermal aging at 753 K. High resolution transmission electron microscopy was used to study the substructure of the fct θ‐NiMn precipitates in the alloy after aging for 0.36 and 86.4 ks. Nano‐scale twins on (111) planes were found in the precipitates of intial and later stages of aging. Precipitation of bcc β‐NiMn at 753 K and subsequent martensitic transformation to fct θ‐NiMn during cooling to room temperature was suggested. It was also indicated that precipitation of fct θ‐NiMn at bcc iron matrix gives rise to high coherency strain energy. The (111) twinning is proposed to accommodate the strain energy of either martensitic transformation or coherency strain of precipitation reaction. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Martensitic transformations in shape-memory Cu-Zn-AI alloys with the α-phase particles and the bainite crystals

The martensitic transformations in Cu-Zn-Al alloys undergoing the decomposition of β solid solution with the formation of rod-like α-phase or of plate-type bainite crystals have been studied by optical microscopy, electron microscopy and electron-probe microanalysis. The fine structure of the phases formed upon the partial decomposition of the β-phase and upon the subsequent martensitic transformation was studied. The β→α transformation was shown to occur in a crystallographically ordered manner and to obey the Kurdyumov-Sachs orientation relationships. The β→α transformation includes features of both diffusional and martensitic transformations. The α-phase formation includes a crystals lattice reconstruction and the nucleation dislocations in periodically arranged slip planes. At the same time, the shape of the α-phase crystals and their coalescence and changes in chemical composition are typical of diffusional transformation. The interactions of martensite crystals with particles of the a phase and bainite crystals were studied.

Bainitic Transformation of Au-47.5at%Cd Alloy

The formation of bainite in a Au-47.5at%Cd alloy during isothermal aging above Af temperature was directly identified for the first time by in-situ TEM observation. Bainite forms in the form of thin plates with rapid initial growth and followed by a slower growth. The diffraction pattern of bainite showed 1/3 spots, being different from those of the γ 2 ' martensite of this alloy. This suggests that the structure of the bainite is different from the martensite. With DSC measurement, we found that the transformation temperatures of subsequent martensitic transformation decrease with increase of aging times; at prolonged aging time the DSC curve showed two peaks, corresponding to B2-γ 2 ', B2-ζ 2 ' martensitic transformation, respectively. These results indicates that bainite is depleted in Cd.

Bimodal size distribution of Ti 3Ni 4 particles and martensitic transformations in slowly cooled nickel-rich Ti [sbnd]Ni alloys

The TiNi alloys containing more than 50 at.% nickel undergo precipitation of Ti{sub 3}Ni{sub 4}, Ti{sub 2}Ni{sub 3} and TiNi{sub 3} phases during aging. The precipitation phase particles and the following change in the chemical composition of the matrix have a considerable effect on subsequent martensitic transformations. The influence of Ti{sub 3}Ni{sub 4} particles is especially pronounced: characteristic temperatures of martensitic transformations change and the B2 {yields} R transformation appears. It was shown that Ti{sub 3}Ni{sub 4} particles with a bimodal size distribution are formed in Ti{sub 48}Ni{sub 52} during aging under continuous cooling at a rate of 3 C/min in the range 870--500 C and 1.5 C/min in the range 500--200 C. Coarse particles (Widmanstatten crystals) may be as long as 6--7 {micro}m. Dispersed particles, which are located between coarse ones, are two orders of magnitude smaller. Changes in the dimensions and size distribution of the precipitates are important additional factors that affect martensitic transformations and thus the shape memory effect in Ti-Ni alloys. This paper deals with a study of the distribution of Ti{sub 3}Ni{sub 4} particles in Ti{sub 49}Ni{sub 51} and Ti{sub 48}Ni{sub 52} after different thermal treatments and their influence on martensitic transformations.

Effect of Crystal Defects in FeNiC Alloys on Subsequent Martensitic Transformation

Effect of Crystal Defects in FeNiC Alloys on Subsequent Martensitic Transformation

Effect of electron irradiation on the 3C-4H transformation in a Co-Fe alloy

In general, irradiation of materials by high energy electrons, protons or neutrons has been shown to stabilize the existing microstructure against subsequent martensitic transformation. Alloys which have been studied include the binary intermediate phases NiTi and Ni63Al37 and ternary phases between Cu37,5Al62.5 and CU63.6Al36.4 as well as the intermetallic Cu69.01Al17.92 Zn12.17. In these materials, the martensitic transformations are known to be sensitive to small changes in chemical composition and to the degree of long range order. In addition, one should expect the kinetics of martensitic transformations to be sensitive to other forms of structural damage introduced during irradiation also. This spectrum of irradiation-induced phenomena renders the interpretation of results in studies of such concentrated binary and ternary phases rather complicated, therefore.We present here the preliminary results of a study of the effect of electron irradiation on the character of the martensitic transformation in a relatively dilute terminal solid solution of Co+5.75 wt pet Fe, similar in composition to that employed for studies of the transformation mechanism itself.

Computer study of tweed as a precursor to a martensitic transformation of a Bcc lattice

Using molecular dynamics computer simulations we have studied a bcc crystal with an interatomic potential having a second derivative which passes through zero near second neighbor distances. This causes an inherent tetragonal instability which manifests itself as random dynamic tetragonal distortions at high temperature (“dynamic tweed”), quasi-static and somewhat spatially correlated tetragonal distortions at intermediate temperatures (“static tweed”), and an apparent martensitic transformation to a close-packed structure at low temperatures. The effect of simple defects (vacancies and interstitials) on the development and spatial localization of the pretransformation structure (tweed) and the subsequent martensitic transformation will also be discussed.

Deformation and transition behavior associated with theR-phase in Ti-Ni alloys

Deformation behavior associated with theR-phase (rhombohedral phase) transition and the subsequent martensitic transformation was studied systematically in Ti-Ni alloys by tensile testing over a wide temperature range covering belowM f to aboveT′ R (>Af). Since the deformation and transition characteristics showed a strong dependence on thermo-mechanical treatment and Ni-content, internal structures were examined by electron microscopy in specimens with various Ni-content and thermo-mechanical treatment. As a result precipitates and/or dislocations were revealed in the specimens in which theR-phase transition occurs. Based on the above results, the effects of thermo-mechanical treatment and Ni-content on the deformation and transition characteristics were clarified for both theR-phase transition and the martensitic transformation.

Mechanical behaviour associated with the premartensitic rhombohedral-phase transition in a Ti50Ni47Fe3alloy

Abstract The mechanical behaviour of a Ti50Ni47Fe3 alloy, which exhibits a well-defined premartensitic transition and a subsequent martensitic transformation, was studied systematically using tensile tests over a wide temperature range covering M s to above T R. Two-stage yielding was observed in the temperature range between M f and a critical temperature, denoted by T X(T X > T R). The first stage was confirmed to be associated with the premartensitic R-phase (rhombohedral phase) transition, while the second stage was associated with the martensitic transformation. The temperature dependences of the critical stress and strain indicated that the first stage divides into two temperature regimes at about T R. In the lower-temperature regime the rearrangement of the R-phase was found to be responsible for the first stage, while in the higher regime the stress-induced transition of the R-phase was responsible for the first stage. As with alloys exhibiting a thermo-elastic martensitic transformation, similar mechanical effects associated with the R-phase transition have been found such as the shape memory effect, the two-way memory effect and transformation pseudo-elasticity. This is reasonable, considering that the R-phase transition is associated with a rhombohedral lattice distortion. Overall, the present results were found to be consistent with the recent theory of the premartensitic R-phase transition developed by Salamon, Meichle, Wayman, Hwang and Shapiro (1979).

Cold plastic deformation of chromium-manganese austenitic metastable steels

1. CPD stabilizes austenite in relation to the γ→α-transformation during subsequent loading if during this more than 50% martensite is formed or there is formation of a cellular dislocation structure. 2. CPD activates subsequent martensite transformation if in this way there is formation of extended dislocations, a certain amount (up to 20%) of martensite, and ɛ-phase. 3. The highest level of strength and ductility properties after CPD may be obtained only with a certain degree of austenite stability. With increased metastability, or conversely austenite stability, CPD with a reduction of more than 20% leads to a marked reduction in steel ductility.

Electron microscopy studies of martensitic transformations in Ti 50Ni 50− xCu x alloys. Part I. Compositional dependence of one-third reflections from the matrix phase

A transmission electron microscopy study has been made of the high temperature phase of Ti 50Ni 50− x Cu x alloys, where x was varied from 1 to 19, the Ti content being essentially constant. The high temperature phase was confirmed to have B2-type ordered structure over the composition range studied. Contrary to previous results, however, the M s temperature of the alloys was found to vary considerably, ranging from room temperature to 150°K. On cooling alloys with lower Cu content ( x ≤ 3), the one-third reflections, considered to be associated with a rhombohedral distortion of the B2 structure, appeared prior to the subsequent martensitic transformation, as in binary TiNi alloys. Their appearance was accompanied by microstructural changes, i.e., the formation of needles and domains. The domains were observed to persist in the martensite. In the alloy x = 19, however, the one-third reflections were not observed, even close to the M s temperature. This suggests that the B2 → R (rhombohedral) transformation may be suppressed when the substitution of Cu for Ni exceeds a certain value. The absence of the B2 → R transformation is expected to affect significantly the crystallography of the martensites subsequently produced.

Diffuse electron scattering and “streaming” effects

Various anomalies are found in the parent phase of some alloys (especially B2 and DO3 type β-phase alloys) which undergo martensitic transformations, including lattice softening, streaks, weak diffuse reflections of the 2H-type (or ω-like) an X phase, and “streaming”. These are often referred to as premonitory effects prior to a subsequent martensitic transformation, without clear evidence. This chaotic situation results because various phenomena occur concurrently in these materials, which makes it difficult to isolate individual effects. A critical discussion of these phenomena is presented from an experimental point of view. On the nature of the diffuse streaks and diffuse patterns in β-phase alloys, the presence of intense *β rel rods has been confirmed unambiguously. From the direction of the rel rods and conditions for their extinction, it is concluded that they result from the presence of a low frequency transverse acoustic phonon mode, * β. A possible relation between the rel rods and a subsequent transformation is considered. In contrast to earlier reports, the streaming effect has now been detected in a variety of materials, including those which do not undergo transformations. Thus, it is clear that streaming does not have a direct relation with transformations. A possible origin of this effect is discussed and compared with observed characteristics. It is also shown that the 2H type phase is not a premartensitic one, but rather another metastable phase which competes with a subsequent martensitic transformation. Overall, it is concluded that no clear evidence has been obtained as yet in electron microscopy and diffraction to show a truly premonitory effect for a subsequent martensitic transformation.

Recrystallization of Reversed Austenite and Subsequent Martensitic Transformation in 18%Ni Maraging Steel

Recrystallization of Reversed Austenite and Subsequent Martensitic Transformation in 18%Ni Maraging Steel