assisted above.4.3.2. Application of Magnetic Field on Fe-Based Amorphous Alloys

So far, magnetic field, as a very common applied field energy, has been applied in various fields with certain success, and it is no exception in the case of Fe-based amorphous alloys. Numerous researchers have added magnetic field to their studies on Fe-based amorphous alloys and recorded the changes on Fe-based amorphous alloys after adding magnetic field.

Chao and Zhang [191] optimized the performance of Fe78Si9B13 amorphous strip by treating it with low-frequency pulsed magnetic fields. Their method of treating Fe-based amorphous alloys is the magneto-crystallization method, because the nano-crystallization of amorphous alloys is a spontaneous process with reduced energy, and the magnetic field provides the energy for the transition from the amorphous state to the crystalline state. As mentioned in Section 3.3, nano-crystallization may enable the desired unique properties of Fe-based amorphous alloys and can therefore be used to optimize their performance. Jin et al. [192] also used a pulsed magnetic field of certain intensity to treat Fe52Co34Hf7B6Cu1 samples. Wang [13] also adopted the same method, and investigated how the crystallization and nano-crystallization of Fe-based amorphous alloys would be

affected if the intensity of the magnetic field was increased under the applied magnetic field. After treating the Fe-based amorphous alloy with a magnetic field, Wang used transmission electron microscopy to discover nanocrystals, which are about 10 nm in size as shown in Figure 28a,b. And Wang, finally, concluded that the crystallization of the Fe-based amorphous alloy increases monotonically with increasing magnetic field strength. The nature of this magneto-crystallization phenomenon has been studied. Among them, Guo et al. [193] derived from the theory of phase transition kinetics that when a pulsed magnetic field acts on an amorphous alloy, in order to satisfy the minimum free energy, the amorphous alloy will undergo linear magneto-striction. Macroscopically, the workpiece changes in the length direction, and the magneto-striction strengthens the vibration of the internal atomic cycle, provides a driving force for nucleation, and promotes the crystallization of amorphous alloys to produce nanocrystalline. Both Yoshizawa [91] and Suzuki [194] considered this nano-crystallization phenomenon as an effective machining method to improve the soft magnetic properties of Fe-based amorphous alloys. *Processes* **2022**, *10*, x 31 of 43

**Figure 27.** (**a**) Modeling of the machine layout when Khalil et al. [190] use magnetic field assist. (**b**) Modeling of the machine layout when Yip and To use magnetic field assistance. (**c**) Schematic of the distribution of permanent magnet and workpiece positions when using magnetic field assist, adapted with permission from Ref. [190] 2022, Elsevier. (**d**) Actual machine layout when Yip and To use magnetic field assist, adapted with permission from Ref. [186] 2017, Elsevier. **Figure 27.** (**a**) Modeling of the machine layout when Khalil et al. [190] use magnetic field assist. (**b**) Modeling of the machine layout when Yip and To use magnetic field assistance. (**c**) Schematic of the distribution of permanent magnet and workpiece positions when using magnetic field assist, adapted with permission from Ref. [190] 2022, Elsevier. (**d**) Actual machine layout when Yip and To use magnetic field assist, adapted with permission from Ref. [186] 2017, Elsevier.

So far, magnetic field, as a very common applied field energy, has been applied in various fields with certain success, and it is no exception in the case of Fe-based amorphous alloys. Numerous researchers have added magnetic field to their studies on Febased amorphous alloys and recorded the changes on Fe-based amorphous alloys after

Chao and Zhang [191] optimized the performance of Fe78Si9B13 amorphous strip by

of amorphous alloys is a spontaneous process with reduced energy, and the magnetic field provides the energy for the transition from the amorphous state to the crystalline state. As mentioned in Section 3.3, nano-crystallization may enable the desired unique properties of Fe-based amorphous alloys and can therefore be used to optimize their performance. Jin et al. [192] also used a pulsed magnetic field of certain intensity to treat Fe52Co34Hf7B6Cu1 samples. Wang [13] also adopted the same method, and investigated how the crystallization and nano-crystallization of Fe-based amorphous alloys would be affected if the intensity of the magnetic field was increased under the applied magnetic field. After treating the Fe-based amorphous alloy with a magnetic field, Wang used transmission electron microscopy to discover nanocrystals, which are about 10 nm in size as shown in Figure 28a,b. And Wang, finally, concluded that the crystallization of the Fe-

adding magnetic field.

4.3.2. Application of Magnetic Field on Fe-Based Amorphous Alloys

based amorphous alloy increases monotonically with increasing magnetic field strength. The nature of this magneto-crystallization phenomenon has been studied. Among them, Guo et al. [193] derived from the theory of phase transition kinetics that when a pulsed magnetic field acts on an amorphous alloy, in order to satisfy the minimum free energy, the amorphous alloy will undergo linear magneto-striction. Macroscopically, the workpiece changes in the length direction, and the magneto-striction strengthens the vibration of the internal atomic cycle, provides a driving force for nucleation, and promotes the crystallization of amorphous alloys to produce nanocrystalline. Both Yoshizawa [91] and Suzuki [194] considered this nano-crystallization phenomenon as an effective machining

method to improve the soft magnetic properties of Fe-based amorphous alloys.

**Figure 28.** Images of Fe52Co34Hf7B6Cu1 after pulsed magnetic field treatment with nano-crystallization of the workpiece, adapted with permission from Ref. [192] 2006, Elsevier. (**a**) TEM micrograph of Fe52Co34Hf7B6Cu1. (**b**) Distinct diffraction rings are generated. It can be determined that the samples are nano-crystallized after the pulsed magnetic field treatment. (**c**) The values of saturation magnetic flux density, Bs, as function of the immersion time in corrosive environment (0.1 M of H2SO4 solution) for amorphous (A) and nanocrystalline (N) samples, adapted with permission from Ref. [195] 2002, Elsevier. Corrosion enhancement was obtained for both Fe73Nb3Si15.5B7.5Cu1 and Fe73.5Cu1Nb3Si15.5B7.5 after nano-crystallization. **Figure 28.** Images of Fe52Co34Hf7B6Cu1 after pulsed magnetic field treatment with nanocrystallization of the workpiece, adapted with permission from Ref. [192] 2006, Elsevier. (**a**) TEM micrograph of Fe52Co34Hf7B6Cu1. (**b**) Distinct diffraction rings are generated. It can be determined that the samples are nano-crystallized after the pulsed magnetic field treatment. (**c**) The values of saturation magnetic flux density, Bs, as function of the immersion time in corrosive environment (0.1 M of H2SO4 solution) for amorphous (A) and nanocrystalline (N) samples, adapted with permission from Ref. [195] 2002, Elsevier. Corrosion enhancement was obtained for both Fe73Nb3Si15.5B7.5Cu1 and Fe73.5Cu1Nb3Si15.5B7.5 after nano-crystallization.

In many cases, nano-crystallization of amorphous alloys results in the strengthening

of the workpiece itself. For example, Souza [195] investigated the properties of two Febased amorphous alloys, Fe73Nb3Si15.5B7.5Cu1 and Fe73.5Cu1Nb3Si15.5B7.5, after nano-crystallization and found that when they underwent nano-crystallization, their corrosion resistance was somewhat improved, as shown in Figure 28c. Although there is evidence that the formation of nanocrystals enhances the performance of Fe-based amorphous alloys, contrary opinions have been presented. Gostin et al. [196] found that when the matrix precipitation phase of Fe-based amorphous alloys is α-Fe, Fe carbide, and Fe boride or their mixtures, nano-crystallization of Fe-based amorphous alloys will reduce their corrosion resistance. The results show that the addition of a magnetic field can pro- In many cases, nano-crystallization of amorphous alloys results in the strengthening of the workpiece itself. For example, Souza [195] investigated the properties of two Febased amorphous alloys, Fe73Nb3Si15.5B7.5Cu1 and Fe73.5Cu1Nb3Si15.5B7.5, after nanocrystallization and found that when they underwent nano-crystallization, their corrosion resistance was somewhat improved, as shown in Figure 28c. Although there is evidence that the formation of nanocrystals enhances the performance of Fe-based amorphous alloys, contrary opinions have been presented. Gostin et al. [196] found that when the matrix precipitation phase of Fe-based amorphous alloys is α-Fe, Fe carbide, and Fe boride or their mixtures, nano-crystallization of Fe-based amorphous alloys will reduce their corrosion resistance. The results show that the addition of a magnetic field can promote the nanocrystallization of Fe-based amorphous alloys, and the intensity of the magnetic field can affect the degree of nano-crystallization. Therefore, magnetic field-assisted machining can make good use of this phenomenon to improve the machining performance of Fe-based amorphous alloys.

#### *4.4. Other Assisted Machining Methods*

nm

Amorphous alloys are considered as ideal materials for wear and corrosion resistant coatings due to their excellent properties [197], especially Fe-based amorphous alloys with high strength, high hardness, and excellent wear and corrosion resistance. The protection mechanism of this coating may also be a good way to improve the machining performance of Fe-based amorphous alloys, as coating technology has been an important way to improve tool wear resistance in the machining field [198]. Chemical wear of Fe and C deteriorates the machining performance of Fe-based amorphous alloys under SPDT. Brinksmeier and Glabe [199] demonstrated the potential of TIC and TIN coatings on diamond tools to eliminate chemical wear. Xiao [200] et al. showed that the nano-SiC/Ni composite coating can further protect the diamond from graphitization and can result in higher bending strength and wear resistance of the diamond turning tool bit. It can be seen that coatingassisted technology has the potential to improve the machining performance of Fe-based amorphous alloys under SPDT. However, the hardness of the coating is often lower than that of diamond, and the machined surface quality is not ideal. Therefore, the application of coating technology to improve the surface quality of Fe-based amorphous alloys and other difficult-to-machine materials still has a lot of room for development.

Diamond tools used in SPDT are the hardest known material and are widely used in ultra-precision machining, but diamond suffers from severe tool wear affecting the surface quality when machining Fe and other transition metal alloys. Implanting the near-surface of diamond with ion implantation of other elements to modify its surface mechanical and chemical behavior is considered as a promising assisted method to address this wear [201]. Already in 1999, Klocke and Krieg [198] suggested applying a protective coating to diamond tools to create a diffusion barrier. As an Fe-based amorphous alloy with Fe as the main element, it is also promising to improve the machining performance of Fe-based amorphous alloy under SPDT with the aid of ion implanted modified diamond technology. Wear occurrences are compared between ion implanted diamond and unmodified diamond by Lee et al. [202], as shown in Figure 29. The cutting tool is considered to be worn when it is incapable of achieving the desired surface finish of the work material. After machining a distance of 350 m with the ion implanted tool, a clear reflection of the emblem can still be observed, signifying that the ion implanted tool can operate at a further distance in comparison to the unmodified tool (please see Figure 29c). Correspondingly, the surface roughness measurements showed a similar magnitude of 2.6 times increase in *R*a and *R*q for the surface produced by the unmodified cutting tool (please see Figure 29b). However, there are still obstacles that must be overcome in the technique of diamond ion implantation such as the removal of radiation damage after ion implantation without causing graphitization of the diamond [203]. These obstacles also greatly affect the widespread application of ion implantation modified diamond tools as an assisted technology. *Processes* **2022**, *10*, x 34 of 43

**Figure 29.** Images of the machined iron surface quality with the magnified observation using a SEM and the respective surface roughness measurements: (**a**) standard requirements achieved after machining 50 m, (**b**) after machining with an unmodified diamond tool over a distance of 350 m, and (**c**) after machining with a gallium irradiated diamond tool over a distance of 350 m, reprinted with permission from Ref. [202] 2019, Elsevier. In addition to the above−mentioned diamond tool coating techniques and ion im-**Figure 29.** Images of the machined iron surface quality with the magnified observation using a SEM and the respective surface roughness measurements: (**a**) standard requirements achieved after machining 50 m, (**b**) after machining with an unmodified diamond tool over a distance of 350 m, and (**c**) after machining with a gallium irradiated diamond tool over a distance of 350 m, reprinted with permission from Ref. [202] 2019, Elsevier.

tive improving the machining performance of Fe-based amorphous alloys.

chining of the microstructure of Fe-based amorphous alloys.

**5. Summary and Outlook** 

plied to improve the machining performance of difficult-to-machine materials.

plantation diamond modification techniques. Inert gas-assisted machining methods and electric field-assisted machining methods [204] are also favored by researchers often ap-

This paper systematically reviews the properties and machining performance of amorphous alloys. As a special case of amorphous alloy, the preparation, application and machining of Fe-based amorphous alloy are systematically summarized in this review. It is found that single-point diamond turning (SPDT) is a promising machining method to overcome the extremely high hardness of Fe-based amorphous alloys. However, under SPDT, the problems of high machining temperature, machining crystallization and chemical wear still greatly deteriorate the machining performance of Fe-based amorphous alloys. Assisted machining methods such as tool-assisted machining, low-temperature lubrication assisted machining and magnetic field assisted machining et al. are found effec-

Ultrasonic vibration assisted machining is expected to reduce the high cutting temperature of Fe-based amorphous alloys through periodic intermittent machining, thereby reducing the impact of high cutting temperatures on the crystallization and oxidation. Meanwhile, ultrasonic vibration assisted machining can effectively reduce tool wear and cutting force, whereby effectively improve the machining performance of Fe-based amorphous alloys. However, the machining efficiency of ultrasonic vibration assisted machining is low. In addition, fast tool servo (FTS) and slow tool servo (STS) are expected to help Fe-based amorphous alloys to achieve ductile removal, and they can effectively assist ma-

In addition to the above−mentioned diamond tool coating techniques and ion implantation diamond modification techniques. Inert gas-assisted machining methods and electric field-assisted machining methods [204] are also favored by researchers often applied to improve the machining performance of difficult-to-machine materials.

#### **5. Summary and Outlook**

This paper systematically reviews the properties and machining performance of amorphous alloys. As a special case of amorphous alloy, the preparation, application and machining of Fe-based amorphous alloy are systematically summarized in this review. It is found that single-point diamond turning (SPDT) is a promising machining method to overcome the extremely high hardness of Fe-based amorphous alloys. However, under SPDT, the problems of high machining temperature, machining crystallization and chemical wear still greatly deteriorate the machining performance of Fe-based amorphous alloys. Assisted machining methods such as tool-assisted machining, low-temperature lubrication assisted machining and magnetic field assisted machining et al. are found effective improving the machining performance of Fe-based amorphous alloys.

Ultrasonic vibration assisted machining is expected to reduce the high cutting temperature of Fe-based amorphous alloys through periodic intermittent machining, thereby reducing the impact of high cutting temperatures on the crystallization and oxidation. Meanwhile, ultrasonic vibration assisted machining can effectively reduce tool wear and cutting force, whereby effectively improve the machining performance of Fe-based amorphous alloys. However, the machining efficiency of ultrasonic vibration assisted machining is low. In addition, fast tool servo (FTS) and slow tool servo (STS) are expected to help Fe-based amorphous alloys to achieve ductile removal, and they can effectively assist machining of the microstructure of Fe-based amorphous alloys.

Low-temperature lubrication assisted machining can greatly reduce the cutting temperature of Fe-based amorphous alloys. It is expected to control the cutting temperature of Fe-based amorphous alloys not to exceed glass transition temperature point (*T*g) through low-temperature lubrication assisted machining, thereby eliminating the effects of crystallization and high-temperature oxidation on machining. Meanwhile, the nitrogen atmosphere can effectively reduce the surface hardness of the Fe-based amorphous alloys, which is helpful for better machining. The form of cryogenic gas (CG)+minimum quantity lubrication (MQL) has good flexibility and can effectively adapt to various cooling and lubrication requirements in the machining of Fe-based amorphous alloys. However, surface rebound of the workpiece caused by temperature changes has enormous deteriorated the finish machining accuracy.

The presence of magneto-crystallization in Fe-based amorphous alloys has shown promise for improving machining performance by promoting nano-crystallization on workpiece surfaces through magnetic fields. The magnetic field promotes the thermal conductivity of Fe-containing materials and improves the excessive machining temperature of Fe-based amorphous alloys caused by low thermal conductivity, and also facilitates the collection of chips from magnetic materials. The eddy current damping effect caused by the magnetic field at the workpiece can effectively suppress overall machining vibration and tool vibration. Magnetic field-assisted machining has achieved good results in the machining of many ferromagnetic materials because the implementation of simple equipment also has good economic benefits. Unfortunately, magnetic field-assisted machining has not yet formed a systematic theory, and the experimental process is not easy to control.

In addition to the above three highlighted assisted machining methods, traditional coating protection methods and novel ion implantation modified diamond tools are also effective ways of improving the machining performance of Fe-based amorphous alloys. The use of coating protection can alleviate tool wear to a certain extent. However, since the hardness of the coating is not high enough, the effect is often not ideal when machining high hardness materials such as Fe-based amorphous alloys. Direct machining with diamond tools can well overcome the problem that the tool hardness is not high enough. However,

the chemical affinity of diamond and Fe-based amorphous alloys can cause chemical wear and aggravate tool wear. Ion-injected diamond modification can form a wear-resistant and inert barrier layer for the cutting edge of the tool, which helps to improve the wear of this tool and improve the surface quality. However, the ion implantation technology is not mature enough and can radiate damage to the diamond.

The combination of SPDT and assisted machining methods is a promising method for machining Fe-based amorphous alloys. However, assisted machining methods also cannot fully provide favorable factors, combined with the machining process problems, a reasonable combination of different assisted machining technology may be able to achieve better results. For example, instead of degrading the effect, the combined use of CG and MQL can compensate each other. Therefore, when machining Fe-based amorphous alloys or even amorphous alloys, it is advisable to improve their machining performance more often in the form of combinations based on assisted machining methods. The critical dimension of Fe48Cr15Mo14C15B6Er2 is 12 mm, and the critical dimension of Fe41Co7Cr15Mo14C15B6Y2 is the largest 16 mm. In the study of the machining of Fe−based amorphous alloys, it is suggested that these two samples can be used to facilitate the analysis of the machining mechanism. Meanwhile, Fe77.5Si17.5B15 has been proved that its glass removal not only reduces the magnitude of internal stress, but also significantly reduces the magnetostriction, which is suitable as an object for machining.

**Author Contributions:** Z.H.: Writing-Original Draft, Writing-Review and Editing, Conceptualization. G.Z.: Supervision, Writing—Review and Editing, Funding acquisition. J.H.: Methodology, Software. J.W.: Investigation, Supervision. S.M.: Data Curation, Resources. H.W.: Funding acquisition. All authors have read and agreed to the published version of the manuscript.

**Funding:** The work described in this paper was supported by the National Natural Science Foundation of China (Grant No. U2013603, 51827901), the Shenzhen Natural Science Foundation University Stability Support Project (Grant No. 20200826160002001, 20200821110721002), and the Postgraduate Innovation Development Fund Project of Shenzhen University (Grant No. 315-0000470813).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** No conflict of interest exists in this submitted manuscript, and the manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole, or in part.

#### **References**


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