Evolution of Microstructure, Phase Composition, and Mechanical Properties During Thermomechanical Treatment of Co-Cr-Mo Alloy

Abstract

Co-Cr-Mo alloys are in high demand as materials for medical implants. However, hot processing of these alloys is quite difficult due to the need to maintain narrow temperature range of deformation to achieve the required mechanical properties and structure of the products. The features of formation of structure, phase composition and mechanical properties of Co-Cr-Mo alloy at the main stages of thermomechanical treatment were considered in this study. The results demonstrated a significant enhancement in the strength characteristics of the alloy during processing in both forging and radial shear rolling (RSR). At the same time, radial shear rolling processing simultaneously increased the strength and ductility of the alloy. According to the XRD analysis data, the phase composition changes from single-phase structure (FCC-phase) after forging to a mixture of FCC-phase and HCP-phase after RSR during processing. The structure gradient characteristic of RSR decreased as the total elongation ratio increased, maintaining a tendency towards a finer-grained structure near the surface of the bars and a coarser one in the center. This tendency was reflected in the average grain size and the level of mechanical properties. Combined thermomechanical treatment, including the RSR process, made it possible to achieve a unique formation of microstructure and phase composition in the Co-Cr-Mo alloy, ensuring high strength while maintaining ductility.

1. Introduction

Modern methods of thermomechanical treatment make it possible to achieve unique physical, mechanical, and operational properties in both existing and new materials [1,2]. In addition, the combination of metal forming processes can lead to various effects, including enhanced material characteristics, economic benefits, reduced production labor costs, etc. [3,4,5]. Co-Cr-Mo alloys have a long-standing presence in the field of medicine and are superior to other modern biomedical materials in strength, corrosion resistance, and wear resistance [6,7,8,9]. However, hot processing of these alloys is quite difficult due to the need to maintain a narrow temperature range of deformation to achieve required mechanical properties and structure of the products. Studies presented by different scientific groups demonstrate that the final properties of products made from Co-Cr-Mo alloys are significantly influenced by the chemical composition, phase composition, and processing method [10,11,12]. Hot forging is the most used method of deformation treatment of these alloys; it provides the primary processing of the cast structure of the ingot [13]. Moreover, forging often results in a significant increase in strength characteristics with a slight improvement in ductility. Thus, Yamanaka et al. [14] have demonstrated that conventional forging of the Co-29Cr-6Mo alloy makes it possible to obtain an ultrafine-grained structure (UFG) of the alloy with an average grain size of 0.8 μm, which, in turn, leads to a significant increase in yield strength up to 1330 MPa. At the same time, the relative elongation after reaching grain size values of 5–10 μm sharply decreases. In addition, it has been shown that the level of ductility is influenced by the ε-phase formed during the quenching process. However, the capabilities of hot forging are also limited, and it is difficult to achieve such properties on an industrial scale, especially for long semi-finished products with small and medium cross-sections.

Mori et al. [15] have shown that thermomechanical treatment of Co-28Cr-Mo-0.13N as multi-pass rolling with low strain per pass at high temperatures allows a significant increase in strength (in particular, YS up to 1400 MPa) with a total deformation of 90% and without loss of ductility. However, these values were obtained on experimental samples at an initial strain rate of 1.6 × 10⁻⁴ s⁻¹.

Co-Cr-Mo alloys are characterized by polymorphic transformation in the temperature range of ~900–1000 °C, the boundaries of which also depend to some extent on the chemical composition [9,16]. However, in various metal forming processes, the deformation starting in the high-temperature region ends in this temperature range due to cooling of the processed metal, which can significantly affect the structure and phase composition. Furthermore, the alternating “heating–deformation–cooling” processes during the production of Co-Cr-Mo alloy products are likely to contribute significantly to the final property level. Thus, studying the behavior of this alloy at all stages of processing seems to be an important task.

Earlier authors have carried out a series of studies on the potential application of hot forging and multi-pass radial shear rolling processes for the processing of Co-Cr-Mo alloy. These studies have involved the estimation of microstructure formation and properties during these processes [17,18]. However, the study of the proposed processing methods as a complete technological production process of a deformed semi-finished product from Co-Cr-Mo alloy has not yet been carried out. The proposed method of thermomechanical treatment, including RSR, can be a good production technology for obtaining high-quality semi-finished products of various diameters without significant costs for tools and equipment setup time. This requires a study of the possibility of achieving the properties and an understanding of the microstructure formation at each stage of processing.

The aim of this paper is the study of the evolution of the microstructure, phase composition and mechanical properties of semi-finished products in the form of bars of small cross-section from Co-Cr-Mo alloy during each stage of thermomechanical treatment from initial ingot to finished operation of radial shear rolling. The contribution of each stage and the possibility of improving the structure and mechanical properties were studied. In addition, the possibility of using the proposed thermomechanical processing scheme as an effective method for processing the alloy of the Co-Cr-Mo system was considered.

2. Materials and Methods

2.1. Experimental Procedure

In the present study, the initial workpiece was an ingot manufactured by the vacuum induction melting method.

The chemical composition of the ingot was determined using X-ray fluorescence and gas analysis (

Table 1. Chemical composition of ingot from Co-Cr-Mo.

Chemical Element wt. %
Co	Cr	Mo	Ni	Fe	Si	Mn	C	N	S
Balance	28.30	5.97	0.24	0.12	0.39	0.33	0.09	0.004	0.005

). The results met the ISO 5832-12:2019 requirements [19].

The ingot, 80 mm in diameter, was homogenized in a vacuum furnace at the temperature of 1250 °C with a holding time of 9 h with subsequent water quenching to equalize chemical composition and dissolution of the brittle σ-phase formed during ingot crystallization and specific to these alloys’ group [20,21]. The temperature–time holding parameters were determined based on previously performed calculations, ensuring the equalization of the chemical composition and the complete dissolution of the excess phases [17]. After the homogenization process, the multiaxial forging of the initial ingot up to a diameter of 55 mm was carried out using a hydraulic press (model PA2642, nominal force 16 MN) with dies heated to a temperature of 950 °C. Before forging, the ingot was subjected to heating at a temperature of 1200 °C to ensure the deformation process of the alloy in the single-phase austenitic region. The forged workpiece was cooled in the air after the deformation process.

Next, the forged workpiece was rolled in several stages using mini-mills of radial shear rolling (RSR) [22,23,24] to obtain bars with various diameters. The heating temperature before RSR was in the range of 1195–1180 °C. This ensured the onset of alloy deformation in the single-phase region of austenite, which has better plasticity. Before each series of RSR, the workpieces were heated in a camber-type furnace for 1–2 h depending on the size. The heating time was calculated based on the size of the workpiece and a specific heating rate of 1.5 min/mm (taking into account the thermostatting time). The series of RSR was carried out according to the following schemes:

-

Ø 56.5 → Ø 40 mm (total elongation ratio µ_Σ = 2.0);

-

Ø 40 → Ø 37.8 → Ø 33.9 → Ø 30 mm (total elongation ratio µ_Σ = 3.5);

-

Ø 30 → Ø 27 → Ø 21.5 → Ø 18 mm (total elongation ratio µ_Σ = 9.7).

Total elongation ratio µ_Σ is calculated according to Equation (1):

$${\mu }_{\mathsf{\Sigma }}={\left({\displaystyle \frac{D_0}{D_f}}\right)}^2,$$

(1)

where D₀ is a diameter of the initial workpiece, mm; D_f is the diameter of the final bar after RSR, mm.

The deformation modes are selected based on the level of alloy resistance to deformation and the capabilities of existing rolling equipment.

After the rolling on each stage, the bars, with diameters of 40 mm, 30 mm, and 18 mm, were subjected to water quenching. The general scheme of thermomechanical treatment of the Co-Cr-Mo alloy is shown in Figure 1.

2.2. Analysis of Microstructure and Phase Composition

At each stage of thermomechanical treatment, including homogenization, forging and RSR, specimens were selected for metallographic analysis using optical microscopy (Carl Zeiss Axio Observer.D1m, Carl Zeiss Microscopy GmbH, Oberkochen, Germany). The surface of the specimens was pre-ground using abrasive discs with different grain sizes and then polished. Subsequently, the specimens were exposed to electrolytic etching in a solution composed of methanol and sulfuric acid at a volumetric ratio of 9:1. The etching was conducted at an operating voltage of 9 V for a duration of 1–2 min. To calculate the average grain size, the measurements were performed using the ImageJ v1.54g program with subsequent statistical analysis of the data obtained. The microstructure analysis of the specimens after forging and RSR was performed in the central and near-surface areas.

The Rigaku PSF-3M universal X-ray diffractometer (Rigaku Corporation, The Woodlands, TX, USA) was used to carry out X-ray diffraction (XRD) analysis of specimens after forging and RSR processes. The study was performed under the following operating parameters: Cu-Kα radiation in the range of angles (2θ) from 20 to 110° with a step size of 0.05° and an exposure duration of 1 s per point. According to the date obtained, the volume fractions of identified phases were calculated in the central and near-surface areas of bars. The procedure for calculating the volume fraction of phases is described in detail in [25].

2.3. Tensile Test

To determine mechanical properties at each stage of thermomechanical treatment, tensile tests (INSTRON 150LX, Instron, Norwood, MA, USA) at room temperature were carried out using flat specimens with a cross-section of 1.6 × 1.0 mm and work length of 11.5 mm cut from the ingot after homogenization, forged workpiece and RSR bars. The strain rate during tensile tests was 2 × 10⁻³ s⁻¹. The specimens after RSR were cut from central and near-surface areas of bars (2 specimens in each area, respectively) at each stage of rolling (Figure 2). Since the RSR process is characterized by the formation of a gradient structure over the cross-section of the workpiece, the specimens were cut in different areas to evaluate the level of properties.

3. Results

3.1. Microstructure

Metallographic analysis of specimens, which was carried out at all stages of thermomechanical treatment, revealed significant grain refinement of deformed metal compared to its initial state. It should be noted that no precipitation of the embrittling σ-phase was recorded during specimen analysis using optical microscopy.

The ingot microstructure after homogenization annealing is defined by the presence of coarse grains with average grain size Davg = 334.1 ± 113.3 μm, formed as a result of prolonged holding time at high temperature (Figure 3a,b). Some grains, both on the near-surface and in the axial zone of the homogenized ingot, contained annealing twins. Following multi-axial isothermal forging resulted in the significant refinement of the grain structure of the alloy, with a slight gradient of average grain size across the section of the forged workpiece (Figure 3c,d). Therefore, at the near-surface area, Davg was 6.1 ± 2.0 µm, while at the center, it was 12.4 ± 4.5 µm. The grains of the forged workpiece contain a large number of twins (Figure 3c).

After further deformation of the forged workpiece by the RSR process into a bar with a diameter of 40 mm in a single pass, an increase in the grain size occurred due to recrystallization processes. It was observed that the alloy structure had a noticeable gradient, from fine-grained in the near-surface layers to coarse-grained in the center. The average grain size was 24.1 ± 2.4 μm and 65.4 ± 5.1 μm, respectively (Figure 3e,f). Such significant grain growth is primarily associated with the heating operation and holding the workpiece in the furnace at a high temperature prior to RSR. However, at the subsequent stage of deformation in 3 passes with brief intermediate heatings to a bar diameter of 30 mm, a decline in both grain size and the gradient across the section was evident (Figure 3g,h). The average grain size in the near-surface and central areas was 16.5 ± 1.7 μm and 31.0 ± 3.8 μm, respectively. When rolling a bar with a diameter of 30 mm to a final diameter of 18 mm, also in 3 passes, in the central zone of the bar, the average grain size was 18.2 ± 2.8 µm, while at the near-surface area, it was 6.5 ± 1.8 µm (Figure 3i,j).

Figure 4 shows a change in the average grain size during thermomechanical treatment. When the total elongation ratio at the RSR process increased, the grain refinement was generally observed both in the central and the near-surface areas. However, at the total elongation ratio µ_∑ = 2.0 (bar diameter 40 mm), insufficient processing of the alloy structure was identified (pronounced coarse grain in the central area of the bar), whereas significant grain refinement was obtained at µ_∑ = 9.7 (final bar diameter 18 mm).

3.2. Phase Composition

Earlier authors have demonstrated that the phase composition of alloys in a homogenized state is characterized by the mixture of γ-FCC and ε-HCP phases [17]. Thus, in this study, the phase composition of the deformed semi-finished products is considered to a greater extent. The results of the XRD analysis are shown in Figure 5.

Due to the isothermal forging in a single-phase γ-region and subsequent slow cooling in air, the alloy phase composition of the forged workpiece was completely represented by FCC-phase (Figure 5). During further RSR and post-deformation quenching, a duplex structure was identified in the bars at each stage, represented by a mixture of FCC and HCP phases in an almost equal ratio in both the central and near-surface areas of the bars. The results of the XRD analysis, as well as the microstructure analysis using an optical microscope, did not show the precipitation of the σ-phase in the alloy even with slow cooling after forging. Also, when the total elongation ratio during RSR increased, a growth of the intensity of diffraction peaks and their increased width was observed. This fact is commonly associated with an increase in the number of crystal lattice defects (vacancies, dislocations, and stacking faults) that arise during the deformation process.

3.3. Mechanical Properties

Figure 6 presents the generalized average values of the mechanical properties of the Co-28Cr-6Mo alloy for the center and near-surface zones at each stage of thermomechanical treatment. Analysis of the changes in mechanical properties demonstrated that the processing, which combines isothermal forging and RSR, results in a significant improvement in both the strength and plastic properties of the alloy in general. Thus, after isothermal forging, the strength characteristics UTS = 905.3 MPa and YS = 649.4 MPa were found to be 1.7 and 2.75 times higher, respectively, than the same characteristics in the homogenized state. At the same time, the relative elongation remained at the same level (19.5–20.0%).

Further multi-pass RSR of the forged workpiece with a total elongation ratio µ_∑ up to 3.5 provided an increase in ultimate strength (UTS) from ~905.3 MPa to 1105–1150 MPa, with some reduction in yield strength (YS) to 522.5–534.6 MPa. In this case, the relative elongation (RE) increased by 1.6–1.7 times (from 19.3 to 33.7%). A subsequent increase in the degree of deformation (with a total elongation ratio of µ_∑ = 9.7) resulted in enhanced strength characteristics (YS up to 987 MPa and UTS up to 1307 MPa) while maintaining the relative elongation (RE = 31.3%).

The results of tensile tests on specimens cut from the center and near-surface areas of the bars at various elongation ratios after RSR are shown in more detail in Figure 7. The characteristic features of the RSR process, evident in the resulting gradient structure, are also reflected in the formation of the mechanical properties of the alloy. Thus, in the central areas of the bars at all stages of the RSR, higher values of ductility (relative elongation 33.0–38.2%) and lower values of strength (UTS = 1047.1–1273.0 MPa, YS = 401.2–619.0 MPa) were observed in comparison with the near-surface areas, where RE = 25.1–29.7%, UTS = 1162.1–1341.5 MPa, and YS = 643.9–987.4 MPa.

4. Discussion

The dataset that was obtained allows for the analysis of the change and relationship of the microstructure, phase composition, and mechanical properties during the thermomechanical treatment of the Co-Cr-Mo alloy system. The results demonstrate that RSR makes it possible to achieve a significant increase in strength and ductility simultaneously due to the unique formation of the microstructure and phase composition.

In the homogenized state, the alloy with a coarse-grained structure has satisfactory ductility (RE~20%), but relatively low strength, which does not meet the requirements of ISO and ASTM standards. Forging of the initial ingot allows us to significantly reduce the grain size, resulting in an enhancement of the yield strength by 2.75 times and the ultimate tensile strength by 1.7 times. However, despite the significant grain refinement during isothermal forging and the increase in strength characteristics, the level of ductility remained practically unchanged relative to the initial state of the ingot (~19.5%). During the subsequent slow air-cooling process, athermal martensitic transformation does not occur, thereby preserving the single-phase structure of γ-austenite (FCC-phase). This transformation can be further facilitated by the presence of Ni (0.24% wt.), which is an austenite stabilizer. In addition, as noted in [26], deformation at a low strain rate of 0.1–1.0 s⁻¹ and the presence of fine grains also suppresses the growth of martensite in the Co-Cr-Mo alloy.

Of great interest is a more detailed analysis of the formation of the microstructure and mechanical properties at the stages of deformation using the RSR method on the cross-section of the bar. As can be seen in Figure 3 and Figure 7, there is a gradient in grain size, strength and ductility between the near-surface area and the center of the bar, which is due to the characteristics of the RSR process and the stress–strain state of the metal [27,28,29,30]. At all stages of processing, the strength properties are higher at the periphery of the bar, and the ductility, on the contrary, is lower than in the axial zone. Moreover, an increase in the total elongation ratio results in a reduction in the ductility difference between the near-surface area and the center due to an increase in values on the surface and a slight decrease in the center (Figure 7). After quenching from the rolling temperature, the alloy phase composition is a mixture of residual metastable austenite (FCC-phase) and martensite (HCP-phase) in approximately equal proportions. During quenching of Co-Cr-Mo alloys from the high-temperature austenite region, phase transformation occurs by the mechanism of athermal martensitic transformation [31,32]. Moreover, athermal martensitic transformation in Co-Cr-Mo alloys is influenced by not only processing temperature, but also cooling rate, composition, and microstructural features [33]. Song et al. [32] showed that athermal martensitic transformation occurred in the temperature range of 950–1250 °C. In addition, the volume fraction of athermal martensite decreased from 70 to 30% as the temperature decreased from 1250 to 950 °C. It is also noted that lower carbon content in the alloy favored the formation of athermal martensite. On the other hand, the presence of fine grains in the Co-Cr-Mo alloy suppressed athermal martensite [33]. Thus, these factors determine the final volume fraction and distribution of martensite in Co-Cr-Mo alloys. In this case, for bars with a diameter of 40 and 30 mm with a coarser grain, the proportion of athermal martensite is observed to be higher in both the near-surface and the central areas. Near the surface, the HCP-phase is greater by an average of 5%, which can be explained by a higher cooling rate compared to the axial zone of the bar. In the final bar, the proportion of ε-martensite in the center remains predominant (55% HCP/45% FCC), while for the near-surface area, on the contrary, it decreases to 45.5%. This change can be explained by two factors. Firstly, the grain size decreases significantly (~6.5 µm), and secondly, the quenching start temperature also becomes lower.

The results demonstrate that a two-phase structure consisting of γ-FCC and ε-HCP phases, with a close phase ratio, can be formed through thermomechanical treatment conditions and the chemical composition of the alloy.

It is well known that several mechanisms contribute to the strength of metals during deformation, including the Hall–Petch (grain boundary strengthening) and Bailey–Hirsch (dislocation strengthening) relationships [34,35]. Figure 8a shows the relationship between the yield stress and average grain size of the Co-28Cr-6Mo alloys after RSR at various elongation ratios (periphery and axial areas). For the near-surface area and the center of the bar, the contribution of grain boundary hardening is somewhat different (different slopes of the trend lines in the graphs), indicating more intense hardening for the surface zone. A comparative analysis of the structure of bars made of various materials obtained by the RSR method demonstrates similar results [33,34,35,36,37,38], which is explained by the features of the distribution of stresses and strains in the deformation zone [39]. However, it should be noted that for bars with a diameter of 30 mm (µ_Σ = 3.5), there is some deviation from the Hall–Petch relation (Figure 8a), expressed in a decrease in the yield strength and a slight increase in relative elongation for the near-surface zone (Figure 8b). The grain size for the bar with a diameter of 30 mm is reduced by 2 times for the center and by 1.5 times for the near-surface area (radius) compared to the bar with a diameter of 40 mm (Figure 4). Consequently, with an unchanged phase composition (Figure 5), at this stage, there is a decrease in the contribution of the dislocation strengthening mechanism.

The change in structure and mechanical properties during RSR can be represented as follows. The bar with a diameter of 40 mm is obtained in one pass with an elongation ratio of 2.0. Such deformation leads to the formation of a significant structural gradient. Long-term heating of the bar prior to rolling causes recrystallization and grain growth. During rolling, the axial zone is subjected to less deformation processing and has recrystallized grains, which corresponds to higher ductility. Furthermore, the formation of such a structure is facilitated by a higher temperature in the axial zone and additional deformation heating. A smaller grain size is formed in the near-surface zone, which results in an increase in strength relative to the center. Subsequent rolling of the bar to a diameter of 30 mm is carried out in 3 passes with intermediate short-term heating in the furnace. In this case, deformation processing contributes to the reduction in grain size. However, short-term holdings contribute not only to the equalization of the temperature field, but also to the development of recovery processes, such as polygonization of the structure, redistribution, and reduction in dislocation density. This results in a slight decrease in strength at this stage. Further deformation to a diameter of 18 mm leads to significant grain refinement both on the surface (D_avg ~ 6.5 ± 1.8 µm) and in the center (D_avg ~ 18.2 ± 2.8 µm), reaching a level comparable to that obtained after forging. It is obvious that in the final passes, the deformation temperature is significantly lower than at the initial stage, and the total elongation ratio increases significantly (from 3.5 to 9.7); therefore, the contribution of dislocation strengthening also increases. The average yield strength and ultimate tensile strength exceed similar values obtained after forging. However, it is also possible to maintain relative elongation at a level of 30–33%. An interesting fact is that the alloy after forging has a significantly lower ductility level compared to the final bar after RSR. This outcome may be associated with strain-induced martensitic transformation, which occurred during the tensile test, as well as a large number of twins in the alloy obtained after forging. This effect requires further detailed study, since different researchers have different opinions on this issue [31,40,41].

Table 2. Comparison of average grain size and mechanical properties of Co-Cr-Mo alloy obtained by different processing methods.

	Condition/Method	Area	Average Grain Size (µm)	YS (MPa)	UTS (MPa)	RE (%)
Co-28Cr-6Mo (Present work)	Homogenized	-	334 ± 113.3	263.3	537.6	20.0
	Hot forging	Center	12.4 ± 4.5	649.4	905.3	19.5
		Radius	6.1 ± 2.0
	RSR (µ∑ = 2.0)	Center	65.4 ± 5.1	448.8	1129.2	38.2
		Radius	24.1 ± 2.4	620.5	1173.9	25.1
	RSR (µ∑ = 3.5)	Center	31.0 ± 3.8	401.2	1047.1	36.6
		Radius	16.4 ± 1.7	643.9	1162.1	30.7
	RSR (µ∑ = 9.7)	Center	18.2 ± 2.8	619.3	1273.2	33.0
		Radius	6.5 ± 1.8	987.4	1341.5	29.7
CCM [40]	initial	-	~100	570	570	16.8
	hot forging		2.29 ± 0.14	800	1320	14.5
	hot forging		1.34 ± 0.10	1050	1450	10.3
	hot forging		0.82 ± 0.07	1330	1450	2.5
Co-29Cr-6Mo [42]	hot forging	-	43	590	718	15.1
	hot forging		11	648	1050	22.6
	hot forging		3	890	1415	21.6
CCMN [40]	initial	-	~100	540	988	25.2
	hot forging		0.93 ± 0.18	1400	1620	21.8
Co-29Cr-6Mo-0.14N [43]	hot rolling	-	50.7 ± 5.3	1080	1415	20.6
Co-27Cr-5Mo-0.16N [44]	reverse trans.	-	20 to 25	700	1282	36
ISO 5832-12	Annealed	-	not be coarser grain size No 5 (62.5 µm)	517.0	897.0	20.0
	Hot worked	-		700.0	1000.0	12.0

presents a comparison of average grain size and mechanical properties of Co-Cr-Mo alloy obtained by different processing methods. As can be seen, the strength of the alloy increases with the decrease in average grain size. The strength properties obtained for the final bars in the present study are comparable to those for the material obtained by forging with average grain size of 2.3 µm [40], while the relative elongation after RSR is approximately 2 times higher. For hot forged workpieces obtained in [42] and bars after RSR processing, the average yield strength is comparable (with equal average grain size). However, UTS and RE after RSR are also significantly higher. It has also been shown that the addition of nitrogen to the alloy improves the plastic properties of the material [43,44]. The alloy without added nitrogen, processed by RSR, has increased ductility at all stages of processing, including final bars with a fine-grained microstructure at the near-surface area.

The process of hot radial shear rolling of Co-Cr-Mo alloys can be considered an effective strengthening method. This process allows for the improvement of the microstructure and the simultaneous achievement of high strength without significant loss of ductility (relative elongation). Also, post-deformation quenching allows the fine-grain structure to be retained and a mixed phase composition to be achieved, which contributes to a favorable combination of properties, similar to those of duplex stainless steels [45,46].

5. Conclusions

The present study analyzes the evolution and relationships among the microstructure, phase composition, and mechanical properties of the Co-Cr-Mo alloy during thermomechanical treatment. Based on the data obtained, the following conclusions can be formulated.

• The forging of the homogenized ingot allowed for significant refinement of the grain structure, resulting in a substantial increase in yield strength by 2.75 times and ultimate tensile strength by 1.7 times compared to the initial state. At the same time, the relative elongation remained at the same level.
• During the deformation processing, microstructure, phase composition, and mechanical properties changed significantly at each stage of treatment. Strength properties increase to a large extent after hot forging due to the grain refinement. However, at RSR, a significant increase in both strength and ductility was observed simultaneously due to the unique formation of the microstructure and phase composition.
• During the isothermal forging with subsequent air cooling, a single-phase FCC structure was obtained. Subsequent deformation treatment (RSR) in the high-temperature region and quenching in water resulted in the formation of a two-phase structure (FCC+HCP phases) in approximately equal proportions in both the central and surface zones.
• The gradient of the structure during RSR decreased as the total elongation ratio increased, which was confirmed by the values of the average grain size and the level of mechanical properties both in the center and in the surface zone of the bars.
• The thermomechanical treatment of the Co-28Cr-6Mo alloy, combining isothermal forging and RSR with various types of post-deformation cooling, provides a significant improvement in both the strength and plastic properties of the obtained bars due to the formation of a two-phase composition and fine-grained structure.
• The results obtained can be used as a basis for the implementation of the technology for the production of deformed semi-finished products from the Co-Cr-Mo alloy system on an industrial scale.