File Name: phase transformations in metals and alloys .zip
This article introduces the mechanism of diffusion and the common types of heat treatments such as annealing and precipitation hardening, which are applicable to most ferrous and nonferrous systems. Three distinct processes occur during annealing: recovery, recrystallization, and grain growth.
- Mechanism of Phase Transformations in Metals
- Solid–solid phase transitions via melting in metals
- Phase Transformations in Steels
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Mechanism of Phase Transformations in Metals
Observing solid—solid phase transitions in-situ with sufficient temporal and spatial resolution is a great challenge, and is often only possible via computer simulations or in model systems. Recently, a study of polymeric colloidal particles, where the particles mimic atoms, revealed an intermediate liquid state in the transition from one solid to another.
While not yet observed there, this finding suggests that such phenomena may also occur in metals and alloys. We observe this transition in a bulk glass-forming metallic system in-situ using fast differential scanning calorimetry. We investigate the corresponding transformation kinetics and discuss the underlying thermodynamics.
The mechanism is likely to be a feature of many metallic glasses and metals in general, and may provide further insight into phase transition theory.
Nearly all classes of materials show solid-state phase transitions. Understanding transformations from one crystal to another is a key topic in materials science because in many cases they determine material properties.
The transition from graphite to diamond 1 , the property design of steels 2 or the formation of metastable phases to strengthen aluminium 3 are just a few well-known examples of solid—solid transitions in crystalline materials. Solid-state transitions from amorphous to crystalline or even vice versa 4 , 5 have also been reported in metals, and they are also of great importance in other fields. For example, pharmaceutical substances can drastically alter their bioavailability in the human body via transitions from a metastable to a stable polymorphic modification 6.
These mechanisms are complex and in most cases difficult to investigate via in-situ experiments. The atomistic mechanisms of solid—solid phase transitions are therefore still poorly understood.
In the past this limitation was circumvented by deploying colloidal model systems 7 , where the motion of particles can be directly observed via video microscopy. Recently, one such study on micron-sized polymer particles discovered an intermediate liquid state in the transition from one solid to another 8.
However, because of the spatial localization and rapid kinetics of such transitions 9 , this phenomenon has not yet been demonstrated in metallic systems. Orava et al. In their study, small-scale samples on a chip sensor with low thermal mass facilitated precise measurements of thermodynamic data at high speed 11 , The slowest known transformation kinetics within metallic materials has been obtained for metallic glasses The first metallic glass discovered was a Au—Si system Since then Au-based bulk metallic glasses BMGs with very slow crystallization kinetics have been found 15 , 16 , and their suitability for FDSC investigations has recently been demonstrated 17 , They are also of interest for future applications, for example, in small-scale devices for information technology 19 , 20 , and are known to form various metastable crystalline states By deploying a Au 70 Cu 5.
With this method we have been able to demonstrate unambiguously that a metallic system can transform from a metastable crystalline state into a more stable crystalline state via the formation of a metastable liquid at a temperature far below the deepest equilibrium eutectic temperature. This transformation differs from the usual crystallization pathway of metallic glasses, which often occurs via primary crystallization at low temperature and eutectic crystallization of the residual supercooled liquid which now has a composition different from that of the original glass at elevated temperature.
This behaviour agrees well with results for Au 70 Cu 5. At low rates, a glass transition T g , exothermic phase transition peaks T 1 and T 2 and endothermic melting T m can be seen. The phase transition temperatures T 1 and T 2 shift to higher temperatures with increasing heating rate, and at sufficiently high rates an unexpected endothermic peak at T e arises after T 1 and just before T 2.
The onset of the endothermic effect shows no rate dependence, which indicates melting, whereas the T 2 peak shifts to higher temperatures with increasing rates and changes its shape and size if located above T e.
While the peak at T 2 shifts to higher temperatures when the rate increases, the endothermic effect shows no rate dependence, which is a common sign of melting.
Note that T e is far below T m , that is, the temperature T e corresponds to that of a metastable liquid. This metastable liquid re-transforms very rapidly within a few ms towards a more stable crystalline state within the subsequent exothermic event at T 2.
For comparison, Fig. Figure 3b illustrates how the T e endothermic peak is separated from the T 2 exothermic peak. Because the enthalpy of melting at T m remains similar for all heating curves in Fig. The metastable state appears to consist of one major metastable phase, which transforms via metastable melting see also Supplementary Fig. The latter can be concluded from the melting event at T m , which contains one sharp endothermic peak corresponding to eutectic melting , followed by a broader endothermic event see Fig.
R 2 denotes the high quality of the fit. The red curve shows the crystallization enthalpy of the observed partial peak, using a horizontal baseline, and the black curve shows the mathematically separated individual crystallization enthalpy H x2. Error bars are defined as standard deviations. In the following we present further evidence for the unusual solid—solid phase transition via melting.
We first prepared glassy Au 70 Cu 5. We then processed the alloys according to the upper image in Fig. The lower images in Fig. The blue dash-dotted curve indicates heating with H 1 to T freeze1 , which is above T 1 but slightly below T e.
In this case a glass transition occurs, followed by the formation of a metastable crystalline state at T 1.
This implies that the supercooled liquid was mostly crystallized at T freeze1. If the material is frozen during the formation of the metastable liquid at T freeze2 , partial crystallization occurs upon freezing dotted red curve and a small glass transition and a small first crystallization event are observed upon reheating solid red curve.
This is further proof that metastable melting is indeed observed at T e. Heat flow curves are shown after heating the Au 70 Cu 5. The freezing temperature T freeze1 blue curves was chosen to be just above the formation of the metastable crystalline state T 1 , while T freeze2 red curves was chosen to be within the metastable liquid. This indicates that the material was mostly crystallized during the first heating step. Upon freezing from T freeze2 partial crystallization is observed, and upon re-heating with H 2 a small glass transition and crystallization event are seen below T freeze2.
This provides evidence that a metastable liquid has formed during the endothermic event at T e. We now turn to the rate dependence of the formation of the solids. Figure 3a shows a Kissinger plot 24 , from which the effective activation energies of the transitions can be deduced Such activation energies are a common measure of phase transformation kinetics As seen in Fig.
This is illustrated in the insert to Fig. This behaviour is so pronounced that it is still obvious if the transition peak resulting from the overlap of metastable melting and crystallization is integrated using a horizontal baseline.
At even higher rates the melting peak at T e no longer overlaps significantly with the T 2 crystallization peak, which is illustrated in the Supplementary Fig. Here the melting of the metastable crystalline state becomes very obvious and can be clearly distinguished from a c p step, which would result from an additional glass transition.
Conventional DSC was deployed to determine the transition temperatures and specific enthalpies, because of the relatively high uncertainty of the sample mass determination in FDSC. The relative specific enthalpies, as shown in Fig. The values from Fig. In this context it should be mentioned that we work with a four-component BMG to ensure sluggish transformation kinetics. The metastable crystalline state may thus contain more than one phase, but the transformation enthalpies measured reveal clearly that the majority of the material follows the proposed transition path.
Future transmission electron microscopy studies may further illustrate these microstructural features. For better visualization Fig. The mangenta dotted line schematically illustrates the kinetic transition path of the solid—solid transition via melting. Figure 4b thus illustrates that there is a thermodynamic driving force 9 for the formation of a metastable liquid, and from refs 8 , 9 it can be concluded that the liquid—solid interfacial free energy is likely to be lower than that between the solids.
Finally, the transition to the equilibrium liquid occurs at T m. We have thus documented the discovery of a solid—solid transition via melting in a metallic system. The Au 70 Cu 5. With FDSC we are able to explore this new transition path in-situ and, using also conventional DSC, gain new insights into the underlying thermodynamics. In conclusion, we suggest that the observed fundamental solid—solid transition via metastable melting is a feature of many metallic glasses and metals, where different formation sequences of solid phases with varying thermodynamic stability are frequently observed The concept presented can be applied to any metastable to more stable transformation of crystals and does not require the glassy state.
This is illustrated in Supplementary Fig. The possibility of studying solid—solid transitions via melting in real atomic systems demonstrated here is thus expected to provide further insight into phase transition theory, and may also trigger the development of new materials.
For example, FDSC may be deployed to gain a deeper understanding of the reversion of precipitates upon heating in conventional metallurgy Finally, the concept presented may also be of great importance for the understanding of new processing techniques, involving metals and BMGs, where rapid cooling and heating are applied for example, 3D printing of metals and metastable phases form frequently The elements Au purity The FDSC samples were prepared by cutting the melt-spun ribbons into small pieces under a stereomicroscope and then transferred using an electrostatic manipulator hair onto a conditioned and temperature-corrected MultiSTAR UFS1 sensor.
The sample masses were estimated to be 1. Reproducibility was checked by comparing the same thermal cycles at the start and end of each measurement series. Conventional DSC was performed using a differential scanning calorimeter Mettler-Toledo DSC1 to determine the temperatures and enthalpies of the transitions.
The measurements were conducted at a heating rate of 0. Heat flow calibration was performed with indium and zinc. Sapphire was used as reference substance for the specific heat capacity determination.
The measurements were conducted at heating and cooling rates of 0. For the specific heat capacity determination of the liquid phase, heating and cooling measurements were combined to increase the temperature range of the measured data. How to cite this article: Pogatscher, S. Solid—solid phase transitions via melting in metals. Author contributions S.
All authors contributed extensively to the data analysis and discussion. National Center for Biotechnology Information , U. Nat Commun.
Published online Apr Pogatscher , a, 1, 2 D.
Solid–solid phase transitions via melting in metals
Observing solid—solid phase transitions in-situ with sufficient temporal and spatial resolution is a great challenge, and is often only possible via computer simulations or in model systems. Recently, a study of polymeric colloidal particles, where the particles mimic atoms, revealed an intermediate liquid state in the transition from one solid to another. While not yet observed there, this finding suggests that such phenomena may also occur in metals and alloys. We observe this transition in a bulk glass-forming metallic system in-situ using fast differential scanning calorimetry. We investigate the corresponding transformation kinetics and discuss the underlying thermodynamics. The mechanism is likely to be a feature of many metallic glasses and metals in general, and may provide further insight into phase transition theory.
Phase Transformations in Steels
Fundamentals of Materials Science pp Cite as. The manipulation of the microstructure of materials belongs to the heart of the realm of materials science. The goal of the invoked microstructural changes is to bring about favourable values for the material properties of interest in the application of the material concerned. Mechanical treatments in combination with heat treatments, such as cold rolling followed by annealing to induce recrystallization, provide one example, which is discussed in Chap. Very often the microstructure is changed by deliberately generated phase transformations, which are the focal point of interest in this chapter.
Chapter 7: Phase transformations in transformation induced plasticity TRIP -assisted multiphase steels. Chapter Application of modern transmission electron microscopy TEM techniques to the study of phase transformations in steels.
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