3.1. Composition and Phase Component
The chemical composition of CoCr
13Pt
16B
10 alloy is listed in
Table 1. The analyzed results accord well with the nominal composition.
Figure 2 shows the XRD patterns of CoCr
13Pt
16B
10 alloys for the as-cast and as-hot-pressed samples.
It can be seen that both as-cast and as-hot pressed alloys have a face-centered cubic (fcc) structure (α-Co solution phase) and hexagonal close-packed (hcp) structure (ε-Co solution phase); no inter-metallic compound or single elemental phase was detected; Cr, Pt, and B are dissolved in Co solution [
18]. The I
(002) peak of the ε-Co phase overlaps the I(111) peak of the α-Co phase. I
(200) and I
(101) are characteristic peaks that can be used to distinguish different Co phases [
19]. Take I
(200) for α-Co, and I
(101) for ε-Co, the intensity ratio of I
fcc(200)/I
hcp(101) = 1.88 for the as-cast ingot and 2.69 for the as-hot-pressed billet; in turn, the calculated percentage of the ε-Co phase from XRD patterns is 34.72% in the as-cast ingot and 27.10% after hot pressing. The hot pressing process decreases the fraction of ε-Co phase in the alloy.
Figure 3 shows the OM and SEM micrographs of as-cast and as-hot-pressed CoCr
13Pt
16B
10 samples.
Figure 3 shows the OM and SEM micrographs of the as-cast and as-hot-pressed CoCr
13Pt
16B
10 samples. As depicted in
Figure 3a,b, the as-cast specimen exhibits a dendritic morphology characterized by the white–gray regions corresponding to dendrite arms (primary and secondary dendrites). The darker gray areas and certain adjacent small white–gray regions represent the inter-dendrite phase. Notably, the dendrite arm structure constitutes a substantial proportion of the microstructure.
A quantitative assessment was carried out, involving the determination of the proportion of area occupied by these distinct regions within three separate fields of view in the same sample. The statistical analysis revealed an average proportion of 72.19% for the dendrite arm area. By comparing the XRD result and the statistical fraction of the dendrite arm area, the latter is close to the fraction calculated with I
fcc(200)/I
hcp(101) in
Figure 2. Additionally, the average SDAS value for the as-cast sample (sampling near the hot top area) was 12.05 μm. Conversely, the as-hot-pressed sample, depicted in
Figure 3c,d, maintains its dendrite structural morphology. Notably, the proportion of dendrite arm structures further increases, averaging 80.01%, while the SDAS decreases to 8.51 μm.
Table 2 shows the EDS result regarding the distribution proportions of the major elements within the dendrite core (point 1, as indicated in
Figure 3a,c) and the inter-dendrite area (point 2, as delineated in
Figure 3b,d) following the casting and hot pressing processes.
The degree of segregation can be estimated using the subsequent equation [
20]:
where
CDC and
CID are element content of the dendrite core and inter-dendrite region. When
κ < 1, the element exhibits positive segregation between dendrites. The smaller the
κ, the more severe the segregation. When
κ > 1, the element exhibits negative segregation in the dendrite core; the larger the
κ, the more severe the segregation. When
κ = 1, there is no segregation of the element. From
Table 2, it can be seen that the
κ value of element Pt in the as-cast sample is 3.21, which is larger than 1, indicating negative segregation of the element in the dendrite core. The value of 0.99 for Cr is ~1, indicating relatively no Cr segregation. Compared with the as-cast results, the condition for the as-hot-pressed dendrite segregation is significantly improved, indicating that the hot-pressing parameters are insufficient to promote adequate diffusion of solute atoms, however, the average SDAS decreased. The measurement of SDAS serves as an indicator of the level of refinement within the dendrite structure. A reduced inter-dendrite spacing corresponds to a finer and more densely packed dendrite arrangement. Moreover, this reduction results in a narrower local distribution range of segregated elements. Consequently, following the hot pressing process, the extent of dendrite segregation within the CoCr
13Pt
16B
10 alloy was ameliorated, positively influencing the material properties of the alloy ingot. Numerous research studies have substantiated the significant influence of SDAS on the mechanical characteristics of alloys [
21]. It has been consistently observed that a finer SDAS correlates with enhanced processing performance in alloy materials.
Furthermore, the findings derived from XRD, OM, and SEM collectively reveal a noticeable shift in the microstructural composition post-hot-pressing. Specifically, there is an increase in the α-Co phase content while the ε-Co phase proportion diminishes. Broadly speaking, metals or alloys characterized by a face-centered cubic (fcc) crystalline structure, which boasts a total of 12 primary slip systems ({111}<110>), tend to exhibit superior processing characteristics compared to those with a hexagonal close-packed (hcp) structure, which is characterized by only three main slip systems ({0001}<11
0>). A greater number of slip systems directly corresponds to an enhanced deformation coordination within the metal. This, in turn, results in reduced resistance to plastic deformation, rendering the material more amenable to processing. Consequently, the observed increase in the α-Co phase content after hot-pressing is conducive to the subsequent hot-rolling process.
In conjunction with the beneficial effects of adding Pt, Cr, and B to the CoCr13Pt16B10 alloy from an application perspective, this alloy modification elicits an augmentation in material strength and hardness, concomitant with reduced plasticity, ultimately leading to a detriment in its processing performance. The investigation revealed a proclivity for crack formation during and after the hot rolling process. Consequently, in order to enhance the effectiveness of the standard subsequent hot-rolling experiments, a more rigorous and systematic approach was employed. This involved extensive sampling and comprehensive analysis of the microstructural and property homogeneity of as-hot-pressed CoCr13Pt16B10 alloy ingot.
As described in
Figure 1, a detailed assessment of the microstructure and property homogeneity within the as-hot-pressed billet was conducted with the exclusion of the hot top portion. To initiate this analysis, the microstructure and morphology of CoCr
13Pt
16B
10 alloy after hot pressing were examined at fourteen distinct positions, denoted as
to
(the row along the margin) and
to
(the row along the middle) along the cross-section. These positions spanned the width, starting from the side and progressing toward the central region, with seven samples examined for each row. Simultaneously, the microstructure and morphology at locations
to
(the column along the margin) and
to
(the column along the middle) on the longitudinal surface, extending from the top to the bottom, were also scrutinized. This comprehensive examination encompassed an evaluation of the microstructure and property uniformity throughout the cross-sectional and longitudinal aspects of the billet. The hardness and magnetic sampling methods are also shown in
Figure 1.
3.2. Microstructure Uniformity
Figure 4 and
Figure 5 present optical micrographs obtained from various sampling locations positioned in the proximity of the hot top area along the width of the CoCr
13Pt
16B
10 alloy on the cross-sectional plane following hot pressing.
Figure 4 shows the optical micrographs, the SDAS and fraction of dendrite phase variation of different sampling locations on the margin of the cross-section after hot pressing for CoCr
13Pt
16B
10 alloy in
Figure 1.
Figure 5 shows the same characteristics in the middle of the cross-section for CoCr
13Pt
16B
10 alloy in
Figure 1.
It is evident on the comprehensive view of
Figure 4 and
Figure 5 that the dendrites at positions
and
are finer and the SDAS are smaller, while at positions
and
, the dendrites are coarser and the SDAS are larger. The size and SDAS of the dendritic structure at positions
to
and
to
show an increasing trend; from the principles of solidification, it can be inferred that during the melting and casting processes, positions
and
are close to the mold wall and their heat diffuses rapidly outward, promoting a corresponding faster cooling rate. In addition, the mold wall can serve as the substrate for heterogeneous nucleation, forming more crystal nuclei and rapidly growing, which can readily form small micro-structures, known as surface fine grain areas in the solidification structure. At positions
and
, due to the longer distance of the molten liquid front from the mold wall, the heat dissipation is slow, resulting in the formation of coarse dendritic structures. Simultaneously comparing the microstructure of the same row from
to
and
to
along the width, it was also found that the dendrite size and SDAS of the center layer through thickness namely, sample
, with an analytical mean value of 9.63 μm increases to sample
with a mean value of 18.52 μm and are coarser than the outside layer, namely, sample
to
, with an SDAS of 8.51 μm to 17.23 μm. The larger dendrite size and SDAS value of samples from
to
is also due to the comparatively long distance to samples from
to
, and slower heat dissipation in the center layer of the ingot during solidification, making the probability of heterogeneous nucleation low, resulting in the formation of a coarser dendritic structure. Hence, it is discernible that the principal factor contributing to variations in dendrite size and SDAS values, both along the width and through the thickness direction, is the distinct cooling rates experienced during the casting process. It is noteworthy that, even after the hot pressing procedure, eradicating microstructural heterogeneity along the width direction remains an ongoing challenge.
Figure 4h and
Figure 5h illustrate the SDAS and fraction of the dendrite arm area (mainly α-Co) for distinct sampling positions along the width direction in the proximity of the hot top area of theCoCr
13Pt
16B
10 alloy after the hot pressing process. These figures distinctly reveal that the SDAS exhibits a gradual increase from the ingot’s edge toward its center. At the same time, there is a declining trend in the area occupied by the dendrite core phase. From the fitting results, it can be seen that both samples from the margin and middle area of the billet satisfy a relationship of parabolic law:
, with
= −0.19,
= 2.93, and
= 6.03 for samples from
to
and
= −0.25,
= 3.49, and
= 6.38 for samples from
to
. For the dendrite arm phase fraction, a line relationship
was obtained with
= −2.31 and
= 89.38 for the margin region samples and
= −2.99 and n = 89.20 for the samples in the middle.
Figure 6 and
Figure 7 show microstructure photographs of the longitudinal surfaces of different sampling locations along the height direction on the surface for CoCr
13Pt
16B
10 alloy after hot pressing.
From
Figure 6 and
Figure 7, it can be seen that the relationship of SDAS and the dendrite arm area fraction with sampling spots shows nearly the same trend for different columns, but the relative value of which varies from
to
and
to
. The positions from
to
are closer to the middle of the surface compared with
to
, with a larger SDAS and smaller dendrite phase area proportion. The observed variation in values can be attributed to differences in solidification rates, where areas closer to the center of the surface experience slower solidification, leading to a coarser microstructure. Conversely, as shown in
Figure 6h and
Figure 7h, a uniform trend in SDAS and the dendrite arm phase faction occurs along the height direction due to the equidistant proximity from the mold wall to the sampling locations, resulting in a consistent cooling rate. Consequently, the SDAS and the fraction of the dendrite arm area exhibit minimal variation along the height direction. In a parallel manner, the inherent microstructural heterogeneity persists on the longitudinal surface even after the hot pressing process.
3.3. Vickers Hardness
Table 3 provides a comprehensive overview of the Vickers hardness values for the CoCr
13Pt
16B
10 alloy before and after the hot pressing procedure.
To assess the disparities in hardness, hardness assessments were conducted on the dendrite arm and interdendritic region. For each sample near the hot top area, three distinct spots within each region were meticulously selected for analysis.
The Vickers hardness value is consistently lower in the dendrite core than in the interdendritic area for the as-cast and as-hot-pressed samples. This disparity can be attributed to element segregation and the inherent strengthening mechanisms within the alloy. Comparatively, following the hot pressing treatment, a reduction in hardness is observed in contrast to the values exhibited by the as-cast ingot alloy. Simultaneously, a marked enhancement in uniformity is noted. The hardness values in the dendrite arm area decrease by approximately 7%, while those in the interdendritic area exhibit a reduction of nearly 3%. Consequently, the hot pressing process contributes to a reduction in hardness, with a more pronounced effect on the dendrite arm region. It is evident that hot pressing significantly improves the deformation performance of the CoCr13Pt16B10 alloy.
Figure 8 shows the Vickers hardness of the CoCr
13Pt
16B
10 alloy for various positions along the cross-section, encompassing the dendrite core and interdendritic region, subsequent to the hot pressing process.
It can be seen from
Figure 8 that the hardness of the interdendritic region on the cross-section has a similar trend from positions
to
and
to
, with only slight variation, while the hardness of the dendrite arm area shows a decreasing trend. The change in hardness is closely related to the change in microstructure structure. At positions
and
, the microstructure is finer; the finer the structure, the more grain boundaries. This hinders the movement of dislocations, increases the deformation resistance of the material, and increases its strength, resulting in a higher hardness value. At positions
and
the microstructure is coarse with fewer grain boundaries, the deformation resistance of the material is smaller, and its strength is lower, resulting in a correspondingly lower hardness value. For CoCr
13Pt
16B
10 alloy, its dendrite structure is well-developed, and the SDAS is easy to measure. However, the complete grain boundaries are difficult to observe.
According to the Hall Petch relationship [
22,
23], the relationship between grain size and alloy strength can be expressed as
where
is the yield strength,
represents the frictional stress that hinders the movement of dislocations within the grain,
represents the influence of grain boundaries on deformation, which is related to the grain boundary structure, and
represents the grain size or the interlayer spacing between two-phase structures. From Equation (2), it can be seen that as the grain size increases, the strength of the alloy decreases. The empirical relationship between the yield strength and hardness of the alloy satisfies Equation (3) [
22]:
Hence, it can be deduced that the correlation between the hardness and grain size of the alloy adheres to the Hall-Petch relationship, where an increase in grain size within the alloy leads to a corresponding decrease in hardness. This relationship between hardness and grain size can be established by amalgamating Equations (2) and (3) into a unified expression, denoted as Equation (4):
Therefore, this paper uses the SDAS (λ
2) to replace the grain size (
d) and bring it into Equation (4) to analyze the variation of SDAS on hardness. This can be expressed as Equation (5):
The relationship between the reciprocal square root of SDAS and Vickers hardness is shown in
Figure 9.
The experimental data does not agree well with the Hall-Petch relationship. The Vickers hardness of the interdendritic area shows greater dispersion. The variation in the calculated correlation constants
and
can be attributed to the nature of the secondary dendrite spacing, which represents a statistical average of measured values. For the dendrite arm region, both primary and secondary dendrites exist. Due to differences in equilibrium distribution coefficients, solute segregation would occur, resulting in the concentration difference between the primary and secondary dendrite areas and, in turn, the hardness variation. Meanwhile, for the interdendritic area, high-order dendrites and segregation coexist, enhancing the inhomogeneity of the composition and the hardness. Additionally, Equation (3) may not be readily maintained in the CoCrPtB alloy [
22]. As a result, fluctuations in the disparity of SDAS and hardness are observed.
Figure 10 shows the Vickers hardness values for the CoCr
13Pt
16B
10 alloy at various positions along the longitudinal surface, encompassing the dendrite core and interdendritic region, after the hot pressing process.
From positions to and to , the hardness of the dendrite arm and interdendritic structures tends to agree for the same column. In contrast, the hardness at to is lower than that at to . This is because the microstructure from to and to is consistent, and the microstructure of the column at positions to is finer than from to ; thus, the hardness at positions to is higher than that from to .
3.4. Magnetic Characteristics
Figure 11 illustrates the hysteresis loops for the as-cast and as-hot-pressed CoCr
13Pt
16B
10 alloy.
Those loops enable the derivation of relevant magnetic characteristic parameters, also summarized in
Figure 11. The coercivity of the as-cast sample is 2416 A/m, and the remanent magnetism B
r and maximum magnetic energy product (BH)
max are 0.047 T and 0.021 J/m
3, respectively, with a squareness ratio, R
s, of 0.29. After hot pressing, the magnetic performance parameter values decreased. The coercivity value decreased significantly by an amplitude of 526 A/m. Coercivity is a structurally sensitive magnetic parameter that can be significantly altered by grain size, grain arrangement, orientation, machining (such as deformation), and heat treatment.
The magnetic performance parameters exhibit a decline after hot pressing, signifying a reduction in the magnetic performance of the alloy due to microstructural alterations incurred during the hot pressing process. The X-ray diffraction (XRD) analysis revealed that the CoCr13Pt16B10 alloy comprises a face-centered cubic (fcc) structure α-Co phase and a hexagonal close-packed (hcp) structure ε-Co phase. For the ε-Co phase, [0001] direction is the easily magnetized axis with strong magnetic crystal anisotropy. The magnetic properties of the alloy are mainly composed of hcp structures provided by the ε-Co phase. From XRD results, it can be inferred that after hot pressing, the ε-Co phase content is decreased, resulting in decreased magnetic properties of CoCr13Pt16B10 alloy.
The magnetic parameters of different sampling locations for the CoCr
13Pt
16B
10 alloy after hot pressing are listed in
Table 4.
Note that samples
,
, and
are nearly at the same position as samples
,
, and
; samples
,
, and
are nearly at the same position as
,
, and
as measuring the SDAS shown in
Figure 4. It is apparent that, for the same row, from samples
to
and
to
, the B
r, (BH)
max and R
s are relatively the same with the coercivity decreasing along the width direction on the cross-section. The relationship between the SDAS and magnetic parameters of the CoCr
13Pt
16B
10 alloy after hot pressing is shown in
Figure 12.
When transitioning from samples denoted as to , and to an evident pattern emerges. As the mean SDAS increases, the magnetic parameters, including Br, (BH)max, and Rs, exhibit minimal variation, remaining relatively consistent. In contrast, coercivity demonstrates a marked reduction. This observation underscores the heightened sensitivity of coercivity to microstructural alterations compared to other magnetic parameters. The simultaneous augmentation of SDAS and ε-Co phase content across different sample positions along the width direction stands out as the underlying cause for the observed decline in coercivity.
Meanwhile, for samples along the longitudinal section, namely
,
, and
in the exterior regions and
,
, and
within the interior regions of the billet, the magnetic properties are shown at
Table 4. For the same longitudinal layers, the magnetic properties from
to
or
to
exhibit relatively no change, respectively. However, the coercivity of samples from
to
is lower than the other longitudinal layer from
to
. In a similar vein, it can be deduced that the SDAS has a discernible influence, leading to variations in the dendrite fraction from the exterior to the interior region.