Influence of Manganese and Nickel on the α´ Martensite Transformation Temperatures of High Alloyed Cr‐Mn‐Ni Steels

The martensite start temperature (M_s), the martensite austenite re‐transformation start temperature (A_s) and the re‐transformation finish temperature (A_f) of six high alloyed Cr‐Mn‐Ni steels with varying Ni and Mn contents in the wrought and as‐cast state were studied. The aim of this investigation is the development of the relationships between the M_s, A_s, A_f, T₀ temperatures and the chemical composition of a new type of Cr‐Mn‐Ni steels. The investigations show that the M_s, A_s and A_f temperatures decrease with increasing nickel and manganese contents. The A_f temperature depends on the amount of martensite. Regression equations for the transformation temperatures are given. The experimental results are based on dilatometer tests and microstructure investigations.     

Modeling the ductile-brittle transition behavior in thermomechanically controlled rolled steels

The Charpy impact transition temperature (ITT) is well modeled for hot-rolled or normalized steels having uniform grain size using empirical equations. However, the ITT of nonhomogeneous steel microstructures, such as duplex (mixed fine and coarse) grain sizes, and the scatter in experimental Charpy energy values, observed in the transition region, are not accurately modeled. This article describes research on the microstructure-fracture property relationship and the prediction of the ITT using a cellular automata finite element (CAFE) model in thermomechanically controlled rolled (TMCR) Nb-microalloyed steels. The ferrite grain size distributions for two TMCR steel plates were analyzed and used for the prediction of the local fracture stress (σ _F ) values based upon the Griffith model. It was found that the coarse grain size distribution could be used to predict the range of σ _F values observed. The CAFE model was used to predict the ITT using the predicted σ _F distribution for a TMCR steel. Results showed that the CAFE model realistically predicted the Charpy ITT; in particular, it was able to reproduce the scatter in values in the transition region. Within the model, the percentage of brittle failure and the upper shelf ductile energy were predicted well. However, the lower shelf brittle energy was overestimated due to computational limitations in the commercial FE software used with the current CAFE model.     

Effects of specimen thickness on the temperature of nucleation of martensite in Fe−Ni alloys

M _S (M_b) temperatures of thin Fe?Ni foils were measured as a function of thickness. It is found that two opposing effects yield a scatter ofM _s (M_b) to lower as well as to higher temperatures. These effects can be related to the distribution of nuclei per unit volume and to the effects of image forces on martensite nuclei in thin foils.     

Transformation behavior of TRIP steels

True-stress (σ), true-strain (ε) and volume fraction martensite(f) were measured during both uniform and localized flow as a function of temperature on TRIP steels in both the solution-treated and warm-rolled conditions. The transformation curves(f vs ε) of materials in both conditions have a sigmoidal shape at temperatures above M_s ^σ (maximum temperature at which transformation is induced by elastic stress) but approach initially linear behavior at temperatures below M_s ^σ where the flow is controlled by transformation plasticity. The martensite which forms spontaneously on cooling or by stress-assisted transformation below M_s ^σ exhibits a plate morphology. Additional martensite units produced by strain-induced nucleation at shear-band intersections become important above M_s ^σ. Comparison of σ-ε andf-ε curves indicate that a “rule of mixtures” relation based on the “static” strengthening effect of the transformation product describes the plastic flow behavior reasonably well above M_s ^σ, but there is also a dynamic “transformation softening” contribution which becomes dominant below M_s ^σ due to the operation of transformation plasticity as a deformation mechanism. Temperature sensitivity of the transformation kinetics and associated flow behavior is greatest above M_s ^σ. Less temperature-sensitive TRIP steels could be obtained by designing alloys to operate with optimum mechanical properties below M_s ^σ.     

Quantification of precipitates in advanced creep resistant 9-12% Cr steels

In this paper a procedure is introduced for the quantification of precipitates appearing in 9-12°Cr-steels. Results gained from conventional transmission electron microscopic (TEM) investigations are compared with results from energy filtering transmission electron microscopic (EFTEM) investigations. The study was performed on a creep rupture specimen of the cast material G-X12CrMoWVNbN10-1-1 exposed at 600°C and ruptured after 33000 h. The size distribution of M₂₃C₆ carbides, MX phase and Laves phase were measured for both the un-stressed head and the stressed shank (gauge length) part of the investigated specimen. In particular, problems and sources of uncertainty concerning the quantitative determination of particle parameters in this type of steel are discussed. It is shown that quantification of the MX-particles by means of TEM bright field images is hardly possible. The size distributions of M₂₃C₆ carbides and MX phase overlap significantly which makes a separation based only on their size nearly impossible. Only Laves phase occupies a different size range. The difference between the size distributions of head and shank is pronounced only for M₂₃C₆ carbides. The measured particle distributions obey more closely a log-normal distribution rather than a normal distribution.     

Observations of aging effects in a Cu-Sn shape memory alloy

A Cu-15.0 at. pct Sn alloy has been chosen as a model alloy for the study of aging effects in copper-based shape memory alloys. Different thermal aging treatments were carried out to determine the effects of both parent phase and martensite aging on the amount of shape recovery and the characteristic transformation temperaturesM _s ,A _s , andA _f . Aging of the martensite reduces both the amount of shape recovery and the extent of the reverse martensite → parent transformation. High martensite heating rates promote complete shape recovery and reverse transformation while the aging occurring during slow heating can inhibit or prohibit both. But irrespective of the martensite heating rate the transformation temperature hysteresis as given by (M _s -A _s ) is large for the Cu-15 pct Sn alloy compared to other shape memory alloys exhibiting thermoelastic behavior. On the other hand, some beneficial effects were noted when the Cu-15 pct Sn alloy was aged in the parent phase condition prior to subsequent transformation to martensite. TheM _s ,A _s , andA _f were lowered following prior parent phase aging, possibly because of a change in long range order, but prior parent phase aging was found to diminish the deleterious effect of martensite aging. Both shape recovery and the extent of the reverse martensite → parent transformation are enhanced by prior parent phase aging. The enhancement is greater the higher the aging temperature or the longer the aging time at a given temperature. J. D. STICE, formerly Research Assistant at the University of Illinois     

Effect of high magnetic fields on the martensite transformation

The effect of high magnetic fields up to 132 kOe on the martensite transformation has been investigated in two alloy steels, 52100 bearing steel and a type 410 stainless steel. In both cases the martensite start temperature is raised by the application of a magnetic field, and the increase inM _s is linear with field. The rate of formation of martensite is not affected by the field. Numerical values for the entropy of the austenite-martensite reaction can be obtained from the experimental results, and are in reasonable agreement with previous results and with theoretical calculations. Richard Fields was formerly a student.     

Martensitic and Magnetic Transformation Behaviors in Heusler-Type NiMnIn and NiCoMnIn Metamagnetic Shape Memory Alloys

Martensitic and magnetic transformation behaviors of Ni₅₀MnIn, Ni₄₅Co₅MnIn, and Ni_42.5Co_7.5MnIn Heusler alloys were investigated by differential scanning calorimetry (DSC), vibrating sample magnetometry (VSM), and transmission electron microscopy (TEM). The martensitic transformation starting temperature (M _s ) decreases with increasing In composition, while the Curie temperatures (T _c ) of the parent phase are almost independent in each alloy series. On the other hand, the addition of Co resulted in a decrease of the M _s and an increase of the T _c , and the degree of the decline of M _s was accelerated by magnetic transformation of the parent phase. The M _s temperature change induced by the magnetic field was also confirmed. It was found that the degree of M _s change is strongly related to the entropy change by the martensitic transformation, which shows a correlation with T _c -M _s . These behaviors can be qualitatively explained on the basis of thermodynamic considerations. This article is based on a presentation made in the symposium entitled “Phase Transformations in Magnetic Materials,” which occurred during the TMS Annual Meeting, March 12–16, 2006, in San Antonio, Texas, under the auspices of the Joint TMS-MPMD and ASMI-MSCTS Phase Transformations Committee.     

Stress- and strain-induced formation of martensite and its effects on strength and ductility of metastable austenitic stainless steels

The effects of deformation-induced formation of martensite have been studied in metastable austenitic stainless steels. The stability of the austenite, being the critical factor in the formation of martensite, was controlled principally by varying the amounts of carbon and manganese. The formation of martensite was also affected by different test and rolling temperatures, rolling time, and various reductions in thickness. The terms “stress-induced” and “strain-induced” formation of martensite are defined. Experimental results show that low austenite stability resulted in stress-induced formation of martensite, high work-hardening rates, high tensile strengths, low “yield strengths,” and low elongation values. When the austenite was stable, plastic deformation was initiated by slip, and the work-hardening rate was too low to prevent early necking. A specific amount of strain-induced martensite led to an “optimum” work-hardening rate, resulting in high strengthand high ductility. For best results processing should be carried out aboveM _d and testing betweenM _d andM _s. Mechanical working aboveM _d had a negligible effect on the yield strength betweenM _d andM _s when the austenite stability was low, but its effect increased as the austenite became, more stable. Serrations appeared in the stress-strain curve when martensite was strain induced.     

Analysis of size distributions of primary oxide inclusions in Fe-10 mass Pct Ni-M (M=Si, Ti, Al, Zr, and Ce) alloy

The planar and spatial size distributions of primary oxide inclusions were measured in an Fe-10 mass pct Ni alloy on deoxidation with 0.2 mass pct M (M=Si, Ti, Al, Zr, and Ce). It was found that the size distribution of inclusions obtained at a certain magnification in microanalysis was accurate only in a limited range of particle size, due to the limit of observation for the minimum size and observed area. The inclusion characteristics such as mean spatial diameter, , the number of particles per unit volume, N _v, and the volume fraction of particles, f _v, were estimated from the size distribution obtained in a planar cross section by using different methods. The and N _v values and the size distribution estimated from the cross-sectional method using different magnifications were compared with those obtained from the extraction method. The f _v values obtained from size distribution were found to be considerably greater than those from chemical analysis of oxygen or metal element as inclusion. Furthermore, the behavior of the size distribution as a function of holding time at 1873 K is discussed.     

A comparison of common and new methods to determine martensite start temperature using a dilatometer

This paper analyses the start of the martensitic transformation in 4140 steel from the point of view of six definitions, and discusses in detail the implications based on the better understanding of progression of the transformation. The application of two relatively new techniques (cooling curve analysis-CCA and dilation curve analysis-DCA) is among the methods studied. These new techniques allow for a more rigorous quantification of microstructural constituents at each step of the transformation. Experiments consisted of dilatometric analysis of 12 samples of 4140 steel with prior austenite grain sizes from 16 to 44?µm that were rapidly quenched in the dilatometer to form martensite. The results indicate that DCA and CCA are superior to traditional methods used to determine the martensite start temperature. The practical choice of 10% martensite fraction in CCA and DCA yielded M_s values statistically undistinguishable from ASTM A1033 or the tangent method. The practical choice of 1% martensite fraction in CCA and DCA yielded M_s values comparable to the offset method. The important implication of this finding is that M_s values determined with empirical methods should not be confused with the temperature of first appearance of martensite; instead, they correspond to martensite fractions of the order of 10%.     

Shear localization-martensitic transformation interactions in Fe-Cr-Ni monocrystal

A Fe-15 wt pct Cr-15 wt pct Ni alloy monocrystal was deformed dynamically (strain rate ∼10⁴ s⁻¹) by the collapse of an explosively driven thick-walled cylinder under prescribed initial temperature and strain conditions. The experiments were carried out under the following conditions: (a) alloy in austenitic state, temperature above transformation temperature; (b) alloy in transformed state; and (c) alloy at temperature slightly above M _s , propitiating concurrent shear-band propagation and martensitic transformation. The alloy exhibited profuse shear-band formation, which was a sensitive function of the deformation condition. Stress-assisted and strain-induced martensitic transformation competes with shear localization. The alloy deformed at a temperature slightly above M _s shows a significantly reduced number of shear bands. The anisotropy of plastic deformation determines the evolution of strains and distribution of shear bands. The different conditions showed significant differences that are interpreted in terms of the microstructural anisotropy. Calculated shear-band spacings based on the Grady-Kipp (GK) and Wright-Ockendon (WO) theories are compared with the observed values. The microstructure within the shear bands was characterized by transmission electron microscopy. Regions of sub-micron grain sizes exhibiting evidence of recrystallization were observed, as well as amorphous regions possibly resulting from melting and rapid resolidification. This article is based on a presentation given in the symposium “Dynamic Deformation: Constitutive Modeling, Grain Size, and Other Effects: In Honor of Prof. Ronald W. Armstrong,” March 2–6, 2003, at the 2003 TMS/ASM Annual Meeting, San Diego, California, under the auspices of the TMS/ASM Joint Mechanical Behavior of Materials Committee.     

Influence of austenite strength on martensite start temperature Ms

It is known that austenite strength determines the morphology of the new phase during martensitic transformation. As the strength of austenite influences the growth of a martensite crystal, i.e. the movement of the austenite/martensite interface, a correlation between strength of the parent phase and M_s has to exist. M_s depends on thermodynamical and mechanical properties of the alloys. To distinguish the individual variables, austenite strength was changed by different hardening mechanisms: solid solution hardening, plastic deformation or both.     

Influence of strain-induced martensitic transformations on fatigue crack growth rates in stainless steels

The fatigue crack growth rates (FCGR) of two unstable austenitic stainless steels (Fe-16 Cr-13Ni) and (Fe-18Cr-6.5Ni-0.19C) were determined in theM_s-M_d temperature range where a strain induced μ → α′ martensitic transformation occurs near the crack tip. These FCGR were compared to the rates measured in the stable austenitic phase of a Fe-31.5Ni and a Fe-34 Ni alloy and in the martensitic phase obtained by quenching the Fe-31.5 Ni alloy below M_s. In the Fe-31.5 Ni, the FCGR are an order of magnitude higher in the martensitic than in the austenitic structures for ΔK ≤ 40 ksi in. The FCGR of the stainless steels decrease markedly when the test temperature approachesM _s in theM _s - M_d range. The FCGR for the alloy Fe-18Cr-6.5 Ni-0.19 C in a warm-worked condition are consistently higher than for the same alloy in the annealed condition for ΔK ≤ 40 ksi √in.. The results are discussed in terms of the influence of phase structures, stacking fault energy and work hardening exponent on the FCGR.     

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.     

Some Effects of Parent Phase Aging on the Martensitic Transformation in a Cu- AI- Ni Shape Memory Alloy

Transmission electron microscopy observations have been carried out for a Cu-14 pct Al-4 pct Ni (wt pct) alloy aged in the thin foil state in an electron microscope. It was found that large cuboidal precipitates of theγ ₂ phase and many small domains of a highly ordered phase form in the DO₃ matrix during aging. The small ordered domains form preferentially on matrix antiphase boundaries as well as within the antiphase domains. The formation ofγ ₂ and the highly ordered phase, both of which are rich in alloy content, depletes the matrix of solute and thus raises the transformation temperaturesM _s andM _f. The small domains of the highly ordered phase prevent the propagation and reversion of martensite plates, leading to higherM _s-M_f andA _fins-A_f temperature intervals.     

Stored energy,microstructure, and flow stress of deformed metals

The stored energy of plastic deformation has been estimated from transmission electron microscope measurements of dislocation boundary spacings and misorientation angles using Al (99.99 pct) cold rolled to reductions of 5 to 90 pct as an example system. In order to obtain the most accurate estimate of stored energy, it is necessary to take into account the presence of two classes of dislocation boundary, considering the boundary misorientation angle distribution and the stereology of each class independently. Stereological relationships are developed to predict the stored energy estimates that would result from electron backscatter pattern (EBSP) investigations on these microstructures. The calculations show that EBSP investigations can be used to estimate the stored energy, but that at low strains, the limited angular resolution will lead to a significant underestimation. A relationship between the flow stress (0.2 pct offset) and the stored energy is found, though the relationship differs significantly for the low and high strain regimes. At low strains, the flow stress is linearly related to the square root of the stored energy (E _s ) according to σ − σ ₀ = Mα[(G/K)E _s ]^0.5, where G is the bulk modulus, M is the Taylor factor, and K and α are constants.     

The velocity of solidification of highly undercooled nickel

At large undercoolings (τ;10 pctT _M, present theories relating solidification velocity to degree of undercooling do not agree well with reported experimental data for the solidification velocity of nickel as a function of undercooling. The present work shows that this discrepancy is due to two factors. First, the majority of previously reported results overestimate the solidification velocity of nickel at large undercoolings. Second, the scatter in experimental data is so large that a functional relationship between undercooling and velocity is not evident. In this study, the solidification velocity of undercooled nickel was measured using a linear array of 38 photodiodes. The results indicate that the velocity of the thermal field generated by the solid/liquid interface approaches a maximum velocity of 20 m s⁻¹ atΔT} ≈ 10 pctT _M (173 K) and men remains constant with increasing undercooling. This suggests that the velocity of the solid/liquid interface, at undercoolings greater than 10 pctT _M, could be limited by attachment kinetics at the interface. GABRIEL CARRO, formerly Research Associate, Department of Applied and Engineering Sciences, Vanderbilt University     

Stabilization mechanisms of retained austenite in transformation-induced plasticity steel

Three stabilization mechanisms—the shortage of nuclei, the partitioning of alloying elements, and the fine grain size—of the remaining metastable austenite in transformation-induced plasticity (TRIP) steels have been studied by choosing a model alloy Fe-0.2C-1.5Mn-1.5Si. An examination of the nucleus density required for an athermal nucleation mechanism indicates that such a mechanism needs a nucleus density as large as 2.5 · 10¹⁷ m⁻³ when the dispersed austenite grain size is down to 1 μm. Whether the random nucleation on various heterogeneities is likely to dominate the reaction kinetics depends on the heterogeneous embryo density. Chemical stabilization due to the enrichment of carbon in the retained austenite is the most important operational mechanism for the austenite retention. Based on the analysis of 57 engineering steels and some systematic experimental results, an exponential equation describing the influence of carbon concentration on the martensite start (M _s) temperature has been determined to be M _s (K)=273+545.8 · e ^−1.362w _c(mass pct). A function describing the M _s temperature and the energy change of the system has been found, which has been used to study the influence of the grain size on the M _s temperature. The decrease in the grain size of the dispersed residual austenite gives rise to a significant decrease in the M _s temperature when the grain size is as small as 0.1 μm. It is concluded that the influence of the grain size of the retained austenite can become an important factor in decreasing the M _s temperature with respect to the TRIP steels.     

Shock-induced martensitic transformations in near-equiatomic NiTi alloys

Shock-impact generated tensile-stress pulses were used to induce B2-to-monoclinic martensitic transformations in two near-equiatomic NiTi alloys having different martensite transformation start (M _s ) temperatures. The NiTi-I alloy (M _s ≈+27 °C) impacted at room temperature at 2.0 and 2.7 GPa tensile stress-pulse magnitude, showed acicular martensite morphology. These martensite needles had a substructure containing microtwins, typical of “stress-assisted” martensite. The NiTi-II alloy (M _s ≈−45 °C) showed no martensite formation when shocked with tensile-stress pulses of 2 GPa. For tensile stresses of 4.1 GPa, the alloy showed spall initiation near the region of maximum tensile-stress duration. In addition, monoclinic martensite needles, with a well-defined dislocation substructure, typical of “strain-induced” martensite, were seen clustering around the spall region. No stress-assisted martensite was formed in this alloy due to its very low M _s temperature. The present article documents results of the use of a metallurgical technique for generating large-amplitude tensile stress pulses of finite duration for studies of phase transformations involving changes from a high density to a low density state.