Measurement of Steel Phase Transformation Kinetics by Dilatometry and In-Situ Neutron Diffraction – A Comparative Study Zhenzhen Yu, Zhili Feng, Ke An, Wei Zhang, Eliot D. Specht, Jian Chen, Xun-li Wang, and Stan David Oak Ridge National Laboratory, Oak Ridge, TN, USA Emails:
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Abstract Solid-state phase transformation kinetics between ferrite (α) and austenite (γ) phases in a C-Mn steel were investigated by the dilatometry and in-situ neutron diffraction methods, respectively. Dilatometry is commonly used to study the phase transformation kinetics. It infers the allotropic phase transformation from the overall dimensional changes, e.g., the diameter changes of the specimen, during heating and/or cooling. In this regard, it is an indirect measurement technique. Alternatively, neutron diffraction directly detects and measures the individual phases as they present during different stages of phase transformation. A comparative study was made in this work between the dilatometry and the in-situ neutron diffraction methods. The comparison shows that dilatometry analysis would cause the under-estimation of austenite phase evolution as a function of temperature due to the drastic deviation of linear expansion assumption of austenite as function of temperature.
Keywords Phase transformation, dilatometry, neutron diffraction
Introduction Advanced high-strength steels (AHSS) are an integral part of the materials solution for automotive industry to produce highly crash-resistant body structures while reducing the vehicle weight for increased fuel efficiency. The manufacture of nearly all types of AHSS relies on carefully designed thermo-mechanically controlled processing (TMCP) route to achieve the target properties [1, 2]. The TMCP route often involved multiple steps of fast heating and cooling. Accurate measurement of the phase transformation kinetics of AHSS as a function of the TMCP parameters plays a critical role in the development and application of AHSS. However, our knowledge of phase transformation is very limited in a complex alloy system during fast heating and cooling conditions. This is largely due to the lack of direct experimental measurement techniques to identify and quantify the transformation process.
Dilatometry is commonly used to study the phase transformation kinetics. It infers the allotropic phase transformation from the overall dimensional changes, e.g., the diameter changes of the specimen, during heating and/or cooling [3, 4]. Alternatively, diffraction methods, such as xray/synchrotron [1, 5, 6] and neutron diffraction [7-9], directly detects and measures the individual phases as they present during different stages of phase transformation. In this work, we experimentally measured the phase transformation behavior in a high strength steel by the dilatometry and in-situ neutron diffraction methods, respectively, in order to identify their comparability.
Principles of Experimental Methods The dilatometry technique relies on the analysis of sample volume change associated with the phase transformation to determine the transformation behavior. A linear relationship between the volume expansion/contraction of a phase and temperature is typically assumed. In other words, the expansion or contraction of a phase is only a linear function of temperature, and often the complex influence of alloying concentration is ignored. As illustrated in the schematic diagram of Figure 1, during heating up, Phase 1 transforms to Phase 2. The starting and finishing temperatures for the new phase, Phase 2, are marked as Ts and Tf. Dashed lines represent the assumption of linear expansion of the two phases in co-existence region. The level rule is used to calculate the volume fraction of each phase during the transformation process, e.g., the volume fraction of Phase 2 at point C can be calculated by the equation below: (1) In terms of diffraction methods, the measured diffraction pattern can be analyzed either by single peak or whole profile fitting, to obtain the information including the peak position and intensity. Peak position determines the lattice spacing of different phases, and the relative peak intensity is used to determine the volume/weight fraction of different phases. Details of such analysis method can be found in literature [1, 5-8].
Experimental Procedures The material used in this study is a commercial DP980 steel. The nominal chemical composition in mass percentage is Fe0.15C-0.32Si-1.32Mn-0.04Al-0.03Cr-0.02Cu-0.01Ni. DP980 has an initial dual phase microstructure, i.e., ferrite (bcc) and martensite (bct) phases at room temperature. Based on the optical microscopic examination, the volume fraction of martensite phase is roughly 40%.
phase cannot be distinguished from that of ferrite (bcc, α), since the lattice parameters of α and αM are fairly close [15]. As the temperature increases, the austenite (fcc, γ) phase starts to form and eventually replaces the original phases. More details about the neutron data results was published elsewhere [9]. Based on the diffraction pattern evolution, it is observed that the phase transformation process starts to form at 719°C and finishes at 851°C.
1.4
For dilatometry measurements, a resistance heating system, DSI Gleeble 3500, equipped with laser dilatometer was used. Samples with a dimension of 2mm in thickness, 10mm in width, and 100mm in length were prepared.
DP980
1.2
Gleeble
dL/L0 (%)
1.0 0.8
α+γ
α+αM
0.6
γ
0.4 0.2 0.0
0
200
400
600
800
1000
Temp(oC)
Fig.2 Dialotometry curve during heating process obtained from Gleeble measurement with laser dilatometer.
(310) (220)
(211)
(200)
(111)
FCC
The samples for both dilatometry and diffraction measurements were heated to 1050°C under a controlled rate of 3°C/s, and then cool down freely to room temperature in argon gas atmosphere to prevent oxidization.
(220)
(200)
(211)
(110)
BCC+BCT
Fig.3 Neutron diffraction pattern as a function of temperature in DP980 reveals the phase transformation process. 1.0
0.8
V.F. of Austenite
For diffraction measurements, a newly developed in-situ neutron diffraction technique was utilized for “bulk” volumeaveraged measurement of phase transformation phenomenon in the steel [9]. The measurement was performed at the VULCAN beam line of the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL) [10]. A resistanceheating device was built to heat the sample. Samples with dimensions of 2mm in thickness, 8mm in width, and 240mm in length were prepared for neutron diffraction measurements. Special sample designs were utilized to maintain a relatively uniform temperature distribution in the center of the sample, which is within diffraction volume. Rietveld method [7, 13, 14] that fits the whole diffraction pattern was used to analyze the neutron data.
Temp [°C]
Fig. 1 The schematic diagram of dilatrometry curve during a heating process
DP980 3oC/s Neutron Gleeble
0.6
0.4
15oC 0.2
Results and Discussion
0.0
725
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775
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Temp (oC)
Figure 2 shows the dilatometry curve obtained from Gleeble measurement with the laser dilatometer. As marked by the dashed line, the ferrite/martensite to austenite phase transformation starts at about 720°C and ends at 848°C.
Fig.4 Comparison of austenite phase volume fraction calculations from neutron data and dilatometric analysis of Gleeble measurement.
Figure 3 shows the evolution of neutron diffraction patterns in DP980 in the heating process, and the heating rate is 3°C/s. Note that in Figure 3, peak pattern of martensite (bct, αM)
The volume fractions of austenite phase during the phase transformation process calculated from dilatometric analysis of Gleeble measurement and neutron data, repectively, are
More importantly, the newly formed austenite lattice parameter in the early stage of phase transformation (lower than 825°C) exhibits drastic deviations from the linear relationship between d-spacing and temperature obtained from the solely austenite phase region (higher than 850°C), as shown in Figure 6. The d-spacing of austenite phase firstly decreases as temperature increases, until a volume fraction of ~80% is reached. After that, the lattice parameter of austenite starts to increase nearly linearly as a function of temperature. Such lattice parameter change is possibly caused by the changes in carbon concentration in austenite formed at different stages of phase transformation process [9]. For example, at the early stage of transformation process, austenite phase would prefer nucleation sites with high carbon concentration, such as cementite/ferrite interface, causing a higher lattice parameter. As temperature further increases, austenite starts to form at low-carbon concentration sites, such as within the ferrite grains. Hence, the linear expansion assumption made by dilatometry analysis method would cause significant under-estimation of austenite phase lattice spacing. It in turn determines the calculated austenite volume fraction would be smaller than the actual value at a given temperature, based on the level rule expressed in Eq. (1), which is in agreement with the observation in Figure 4.
2.912 2.908 2.904
o
2.900 2.896
aα=2.8633+4.9040x10-5T
2.892
α+αM
2.888 500
600
P.T. 700
800
900
1000
Temp (oC)
Fig.5 Lattice parameter of ferrite as a function of temperature obtained from neutron data. P.T. denotes phase transformation region. 3.660
1.0
V.F. of γ 0.8 3.656 0.6
aγ
0.4
3.648
0.2
0.0 700
3.652
Austenite Lattice (A)
However, in phase transformation region, the expansion of ferrite phase doesn’t follow the linear relationship. In dilatometry analysis, an extension of this linear relationship would be usually assumed in the phase transformation region, as represented by the red dashed line. A slight deviation of the measured lattice parameter of ferrite phase from the red dashed line can be observed.
The ferrite to austenite phase transformation behavior in DP980 steel was investigated by in-situ neutron diffraction and dilatometry methods. Changes in carbon concentration in austenite phase formed at early stages of phase transformation process causes a decrease in lattice spacing as temperature increases, followed by a linear increase in lattice spacing during the later stages of phase transformation. It invalidates the linear expansion assumption commonly adopted by the dilatomety analysis method. In other words, the nonlinear lattice spacing changes with increasing temperature could lead to inaccurate measurement of phase transformation kinetics when using the dilatometric technique.
aα (A)
The discrepancy between the two methods may be caused by the linear expansion assumption adopted by the dilatometry analysis method. It is worth noticing that based on the evolution of neutron diffraction pattern, as shown in Figure 3, during the heating process, there is a gradual increase in lattice spacing as a function of temperature. Figures 5 and 6 summarize the lattice spacing variations of the ferrite and austenite phases as a function of temperature during the complete heating process. As shown in Figure 5, before the phase transformation starts, lattice spacing of ferrite phase expands linearly as a function of temperature along the red line. The linear relationship can be expressed by the equation below: 𝑎∝ = 2.8633 + 4.9040 × 10−5 𝑇 (Å) (2)
Conclusions
V.F. of Austenite
compared in Figure 4. A discrepancy is clearly observed between the volume fraction changes obtained from the two different methods. For the calculated results, there is a 15°C difference in temperature values obtained from the two methods that correspond to approximately 20% of austenite phase formed in the material. Also, a two-stage phase transformation behavior with a transition point where ~20% of austenite is present is observed in the neutron measurement, as shown in Figure 4. However, such phenomenon is not obvious in the dilatometry analysis result.
750
800
850
900
950
1000
Temp (oC)
Fig.6 Lattice parameter and volume fraction variation of austenite phase as a function of temperature obtained from neutron data.
Acknowledgments This research was financially supported by the Laboratory Directed Research and Development (LDRD) program at Oak Ridge National Laboratory (ORNL). This work was also benefited from the use of ORNL’s Spallation Neutron Source (SNS), which is sponsored by the Scientific User Facilities
Division, Office of Basic Energy Sciences, U.S. Department of Energy.
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