For example, rapid solidification has been widely used to obtain metallic glass and solid-state phase transformation via thermal treatment has also been often employed to introduce strengthening phases in commercial alloys 34, 35, 36, 37. The phase formation in alloys can be well controlled through the phase transformation kinetics 32, 33. Besides the thermodynamic method, kinetics of phase transformation also greatly affects the phase selection, usually by two stages: solidification and solid-state phase transformation. Thermodynamic ways of annealing are the common way to alter the phases in alloys. Tailoring the phases in HEAs will bring more metallic materials with high performance. The very recent publications 30, 31 showed that multi-phase HEAs have better mechanical properties than HEAs with single solid solution phase, but many multi-phase HEAs consist of too many phases and this is bad for mechanical properties. Thus methods to control the phases of HEAs are needed for a better performance. Intermetallic compounds, like σ, μ, Laves and R phases occurred in these HEAs. For example, HEAs like CoCrFeNiTi x 24, 25, CoCrFeNiMo x 26 and CoCrFeNiNb x 27, 28, 29 consisting of complex microstructures and phases.
![trc diagram jmatpro trc diagram jmatpro](https://cdn.coverstand.com/11623/658213/article_assets/43ed454254215f4d7b11f3b8d3366fa5b2edf466.jpg)
These previous efforts are helpful to search for single phase HEAs, however, it was found that only several alloys consist of single solid solution while most HEAs are with multi-phases. Moreover, researchers also tried to use CALPHAD method and ab initial calculations to predict phases of HEAs 22, 23. Some researchers concentrated on the competition of mixing enthalpy (∆H) and ideal mixing entropy (∆S) to develop criteria of phase stability 3, 20, 21. Some of the existing work focused on the extension of Hume-Rothery rules, such as atomic size (δ, γ), electronegativities (∆χ), and valence electron concentration (VEC) 9, 10, 13, 17, 18, 19. In the past decade, great efforts have been made to explore HEAs with thermally stable single solid solution phase. Phase selection is a crucial issue in HEAs. HEAs differentiate from conventional alloys by equal or near-equimolar chemical compositions and attracted a lot of attentions due to their potential applications as well as the extended opportunity of alloy design 8, 9, 10, 11, 12, 13, 14, 15, 16. Different from the traditional alloys, a new class of alloys was proposed by Cantor and Yeh et al.
![trc diagram jmatpro trc diagram jmatpro](http://www.solution-lab.co.kr/images/sub03/sub_2/04.png)
The minor elements are with strict restriction on the concentration to prevent the formation of complex microstructures, especially the intermetallic compounds leading to a fragile mechanical behavior. For a long time, the development of alloys was restricted to this manner. The microstructural evolution showed a microstructure of carbide-free bainitic ferrite along with film and island of retained austenite.Conventional alloys always consist of one principal component with several minor elements added to tailor the properties.
![trc diagram jmatpro trc diagram jmatpro](https://ars.els-cdn.com/content/image/1-s2.0-S0921509319314947-fx1.jpg)
Finally, an industrially hot-forged sample cooled at 1 K/s was investigated. The microstructures varied from a mixture of bainite and blocky martensite/austenite islands at lower cooling rates to a bainitic ferrite with martensite at higher cooling rates. It was found that a bainitic microstructure can be obtained at cooling rates from 0.15 to 8 K/s. Based on the dilatometric tests, hardness, and microstructural investigation, a deformation-CCT diagram was plotted. In addition, critical temperatures were calculated using Thermo-Calc, JMatPro, and empirical formulations. It was tried to predict the phase transformations via a continuous cooling transformation (CCT) diagram calculated by JMatPro using the results of prior austenite grain size measurement. The microstructure evolution was characterized using light microscope (LM), scanning electron microscope (SEM), transmission electron microscope (TEM), x-ray diffraction (XRD), and electron backscatter diffraction (EBSD).
![trc diagram jmatpro trc diagram jmatpro](https://academic.hep.com.cn/foms/article/2011/2095-025X/2095-025X-5-2-168/hcm0000265247.jpg)
In this work, the microstructural evolution and the phase transformations on a 0.22 wt.% carbon bainitic forging steel are investigated using a deformation dilatometer.