Cold Crucible Induction Melting for the Fabrication of Fe–xTiC In Situ Metal Matrix Composites: Alloying Efficiency and Microstructural Analysis

Fe–xTiC in situ metal matrix composites (MMCs) are fabricated utilizing a cold crucible inductive melting (CCIM) technique with varying TiC amounts (2, 3.5, and 5 wt%). Steel blocks and pure Ti plates are remelted in a water‐cooled copper crucible. The subsequent solidification of [Ti]‐ and [C]‐rich melt yields TiC reinforcement particles in an ingot. The holding time varies between 5 and 30 min, whereas the holding temperature alters depending on the amount of TiC, ranging from 1380 to 1620 °C for Fe‐5TiC and Fe‐2TiC, respectively. Alloying efficiency for TiC‐forming elements is estimated for all fabricated Fe–xTiC ingots by comparing [Ti] and [C] target values with measured ones. The lower TiC amounts and shorter holding times result in decreased [Ti] and [C] losses. SEM analysis of three cross‐section samples representing different TiC amounts reveals two distinct morphologies of TiC in situ reinforcements: primary blocky/cubic and eutectic plate‐like precipitates. The blocky precipitates appear slightly finer with a decrease in TiC amount (4.4 ± 1.2 μm for Fe‐5TiC and 3.8 ± 0.7 μm for Fe‐2TiC), whereas the length of plate‐like precipitates noticeably decreases in the samples with lower TiC amounts (11.0 ± 7.0 μm for Fe‐5TiC and 6.8 ± 3.6 μm for Fe‐2TiC).

MMCs: 1) induction melting of carbonyl iron titanium yielding a Fe-5.2 wt% Ti alloy; then, remelting of this alloy in an alumina crucible and adding carbon in a form of coal to form TiC; after a homogenization period of 20 min at 1550 °C, the resulting melt was cooled down in the crucible.The obtained Fe-TiC MMC showed well-dispersed micrometer-size precipitates.2) A Fe-24 wt% Ti alloy was prepared in an induction furnace by melting a mixture of pure iron and titanium at 1700 °C for 20 min.The obtained metal was extremely brittle due to the presence of the intermetallic Fe 2 Ti phase.Then, this alloy was mixed with an appropriate amount of graphite (33 and 4 g, respectively) and the resultant mixture was remelted and homogenized for 15 min at 1600 °C in a graphite crucible by induction heating.Consequently, well-dispersed rounded TiC particles of high volume fraction (up to 40 vol%) were observed in a ferrite matrix.Lee et al. [7] alloyed SA106 carbon steel with pure Ti, resulting in 0.05 wt% [Ti], using a VIM under Ar protective atmosphere.The resulting melt was homogenized for 5 min and cast into a metallic mold (i.e., rapid cooling).As a result, uniform dispersion of nanosized spherical TiC particles (8.6 nm average size) was observed within the steel matrix.The authors suggested that the relatively low quantity of Ti in the melt was responsible for the nanoscale size of TiC precipitates.
Cold crucible induction melting (CCIM), also known as induction skull melting (ISM), is a process primarily utilized for the production of high-reactive materials, such as Ti, Ti-Al, and Ti-Ta alloys. [13]During this process, a melt is inductively heated in a water-cooled copper crucible while being contained on a solid skull layer.This unique feature of the process ensures that the melt does not have direct contact with the crucible being partially suspended by the electromagnetic force. [14]To the authors' knowledge, there are no published studies dedicated to fabrication of Fe-TiC MMCs via CCIM.[17] Niu et al. [18] fabricated TiAl-3 vol% TiB 2 via ISM utilizing a mixture of powders (Ti-20 wt% Al-10 wt% B) in a cold compressed form, Ti sponge, Al blocks, and master alloys were the initial materials.After achieving a fully molten state and subsequent homogenization period of 5 min, the melt was cast to an iron mold preheated at 400 °C.Wei et al. [19] applied ISM for production of Ti-6Al-4V-TiC in situ MMC varying the reinforcement fraction between 5, 10, and 15 vol%.According to the experimental procedure, sponge Ti, pure Al, Al-V master alloy, and preformed blocks of Ti and C powders were melted in a cold crucible under vacuum (<10 À1 Pa).Then, the melt was homogenized for 5 min and cast to a graphite mold.Consequently, the obtained Ti-6Al-4V-5 vol% TiC ingot revealed bar-like or small globular eutectic TiC, whereas the ones with 10 and 15 vol% exposed primary equiaxed and dendritic TiC particles as the main reinforcements.
Based on the authors' recent studies, [20,21] utilization of an alumina crucible for processing of Fe-5 wt% TiC melt can induce deviations in the resulting reinforcement amount due to losses of Ti and C. Therefore, the current study applies CCIM for production of in situ Fe-TiC MMC expecting significant improvement of the alloying efficiency.For that, a Cr-alloyed steel (similar to AISI 420) was utilized as a matrix, while the amount of reinforcement varied between 2, 3.5, and 5 wt%.Consequently, the alloying efficiency was thoroughly investigated considering the TiC amount, matrix chemical composition, as well as homogenization temperature and time.In addition, the microstructure changes depending on the TiC amount were assessed, focusing primarily on the morphology and size of the reinforcement particles.

CCIM Fabrication
Production of Fe-TiC in situ MMC in the current study was accomplished on the CCIM unit (Linn High Therm GmbH) operating under a power supply of 100 kW from a mediumfrequency generator.As the initial material, the prefabricated Fe-Cr and Fe-Cr-C blocks of chemical composition presented in Table 1 and pure Ti plates were utilized.For premelting of these blocks, the VIM 12 (ALD Vacuum Technologies GmbH) was applied.According to the experimental procedure (see Figure 1), an appropriate amount of initial materials with respect to the target composition was placed in the cold crucible; herewith, the Ti plates were positioned on top of the steel blocks.The working vessel was vacuumed and filled with argon gas (Ar5, 99.999 vol% Ar) to a pressure of 970 mbar three times before heating to insure a low O and N content in the protective atmosphere.During the heating sequence, the power was increased stepwise so that a relatively high heating rate of %300 K min À1 was maintained.For the temperature control and visual observation of the melting process, a pyrometer camera (SensorTherm MQ11 0900 2500) was used.After the melt formation, a holding period at the maximum achievable temperature was applied to insure sufficient dissolution and homogenization of pure Ti in the Fe-based melt.Consequently, free cooling (generator turn-off ) was implemented for the melt in the cold crucible, resulting in a cooling rate of %7-9 K s À1 .
In total, 12 experiments were conducted in the current study: 4 for each variation of TiC amount (i.e., 2, 3.5, and 5 wt%).Table 2 summarizes key aspects of the accomplished CCIT experiments, including variations in the alloying strategy, initial material, holding time, and holding temperature.The holding time varied between 5 and 30 min inside each series to estimate its impact on the alloying efficiency.Meanwhile, the holding temperature fluctuated depending on the amount of TiC due to a technical issue.Specifically, the efficiency of inductive heating decreased as the Ti content in the melt increased.This, combined with high heat losses in the water-cooled copper crucible, limited the maximum achievable temperature.Moreover, formation of a solid layer on top of the melt likely caused some error in temperature measurement with the pyrometer, resulting in slightly lower measured values than the actual ones.This phenomenon was more pronounced in experimental series with higher TiC

Estimation of Chemical Composition
The chemical composition of the initial material used in this study was estimated using the spark spectrometer Foundry-Master UV (Oxford Instruments) for general analysis, with three measurements taken to obtain a mean value (see Table 1).In addition, the Bruker G4 Ikarus and Bruker G8 Galileo combustion analyzers were used to determine [C]/[S] and [O]/[N] values, respectively (three measurements for a mean value).
For the chemical analysis after CCIM, all ingots were subjected to the following procedure: two horizontal cuts to separate the top and bottom parts, taking care to avoid areas of positive and negative segregation of TiC precipitates.Then, a vertical cut was made in the middle of the remaining ingot.The obtained ingot half was subjected to spark spectrometry on its vertical surface, while a thin plate from the other half was used for estimation of [C], [O], and [N] values with higher accuracy via the combustion analyzers mentioned above.Additionally, the vertical surface of the ingot half was milled to %1-2 mm in depth to obtain microshavings.These shavings were then utilized for a more precise estimation of the [Ti] value via inductively coupled plasma-optical emission spectroscopy (Agilent 5100 ICP-OES VDV) (two measurements for a mean value).

SEM/EDX Analysis and Image Processing
The top surfaces of Fe-5TiC1 and Fe-2TiC1 (5 min holding time) were exposed to the scanning electron microscopy (SEM) and energy-dispersive X-Ray spectroscopy (EDX) (Ultra55, Zeiss NTS GmbH) for discussion of TiC alloying efficiency.
Three cross-section samples prepared from ingots of varied TiC amount were analyzed via SEM/EDX for discussion of size, morphology, and distribution of in situ-formed TiC precipitates.Furthermore, the cross section of Fe-2TiC was subjected to EDX mapping.The investigated cross-section samples were cut as a thin plate parallel to the vertical surface of the ingot half, embedded via hot mounting with Polyfast resin (carbon filler), ground, and polished.The open-source image processing software ImageJ v.1.52was utilized for estimation of the size and morphology of TiC precipitates depending on the applied reinforcement amount.Specifically, initial SEM grayscale images with a resolution of 1024 Â 695 pixels and a scale of 2.9 pixels -1 μm were "threshold" filtered and processed via "analyze particles" procedure.One should note that one SEM image was taken for each TiC amount, and only particles larger than 15 pixels were considered.The minimum number of pixels per particle was applied to insure correct estimation of "perimeter" and "circularity" factors by ImageJ.Consequently, the analyzed images revealed several typical morphologies after applying the "size" and "circularity" factors in "analyze particles" sittings.For each morphological category, at least 50 particles were considered.One should note that the "size" factor refers to the area (in μm2) that is occupied by the analyzed particle, "perimeter" is the length of the outside boundary, and "circularity" is calculated as a ratio of the area to the perimeter for the analyzed particle (see Equation ( 1)).
Figure 2 provides an example of such analysis for the Fe-2TiC SEM image, where blocky (size 7.068-1000 μm 2 ; circ.0.36-1) and plate-like (size 0-1000 μm 2 ; circ.0-0.35) precipitates were distinguished.Meanwhile, the rest of TiC particles (size 0-7.068 μm 2 ; circ.0.36-1), having spherical or spheroidal morphologies and extremely fine size of approx. 1 μm, were not considered for the further analysis.For more convenient presentation, the obtained area value of each blocky TiC particle was recalculated to a particle size, which is a diameter of a spherical particle (d, μm) that has the same area as the investigated particle.Alternatively, elongated particles (i.e., plate-like precipitates) were estimated with the length and width of a rectangle (a and b, μm) that has the same area as the investigated particle.For this calculation, the obtained area and perimeter of each plate-like particle was used.Consequently, mean values with standard deviations for both categories of precipitates were established for each Fe-xTiC cross section.

Thermodynamic Calculation
The discussion of estimated dependencies with respect to the reinforcement amount was supported by thermodynamic calculations.Consequently, the phase diagram of Matrix-TiC was calculated using the Thermo-Calc software, TCFE-7 Steels/Fe-alloys database.For this diagram (see Figure 3), six components were considered, Fe, TiC, C, Cr, Mn, Si; other parameters were P = 1.01325E5,N = 1, W(C) = 1E-5.In accordance with the utilized TiC amount, the liquidus and solidus temperatures (T L and T S ) were calculated (see Table 3).The chemical composition for these calculations is presented in Appendix (see Table A1).
In addition, an effect of [N] and [O] content on T L for the investigated compositions was analyzed.For that, two phase diagrams were calculated, adding extra 100 ppm of either [O] or [N] to the initial element system (see dashed lines in Figure 3).Full versions of these diagrams are stored in Appendix (see Figure A1).

CCIM Fabrication and Alloying Efficiency
During all the conducted CCIM experiments, the formation of a solid layer on the melt surface was observed.The dissolution of this layer varied depending on the applied holding temperature and amount of TiC.Specifically, higher temperatures and lower amounts of TiC favored the layer dissolution.However, no additional dissolution was observed with prolonged holding time (30 vs. 5 min), despite expectations.Accordingly, the ingot appearance varied depending on the TiC amount, ranging from a clearly visible slag layer for Fe-5TiC and Fe-3.5TiC to a blank shiny top surface for Fe-2TiC (see Figure 4).It should be noted that the observed layer is named "slag layer" in the discussion of ingot appearance, while it was in fact solid during melt processing in CCIM.
The layer surface of Fe-5TiC and Fe-2TiC was investigated via SEM/EDX for comparison.As a result, Fe-5TiC revealed predominance of TiO x forming elements, whereas C was detected at lower quantities (see Figure 5a,b).Meanwhile, most of the Fe-2TiC surface (see Figure 5c) expressed much lower intensity for Ti and O, while Fe peak was intensified.However, there were rare regions on the Fe-2TiC surface with higher concentrations of TiO x and TiC(N) forming elements (see Figure 5d).One should note that the EDX measurement of light elements (i.e., O, N, C) is normally associated with a high level of inaccuracy.
Chemical composition measurements were conducted for all fabricated ingots, and the summarized results are presented in Table 4.The [Ti] and [C] contents were found to be slightly lower than the target values, attributed to alloying efficiency issues.Consequently, [Ti] and [C] losses (in %) were estimated by comparing the target and actual values according to Equation (2).

SEM/EDX Analysis of Cross-Section Samples
Three vertical cross-section samples with varied TiC amounts were compared focusing on the size, morphology, and distribution of TiC precipitates.SEM images for cross sections from Fe-5TiC2, Fe-3.5TiC2, and Fe-2TiC1 are presented in Figure 7a,c,e, respectively, while Figure 7b,d,f shows these microstructures with higher magnification.Consequently, two distinct morphologies were distinguished for the investigated precipitates, blocky/cubic and plate-like TiC.A prevalence in the number of plate-like precipitates over blocky ones was observed, especially for the samples with lower amount of TiC.The blocky precipitates formed agglomeration clusters, while plate-like ones were organized in eutectic assemblies of matrix þ TiC.Furthermore, the presence of matrix cells/dendrites free of any precipitates was observed for all three cross-section samples, with an increase in this microstructure for lower TiC amounts.Even though blocky precipitates were barely recognizable in Figure 7e (i.e., 2 wt% TiC), areas with these  precipitates were observed in this cross section as well, especially in the upper part of the ingot.
Analyzing variations in morphology within the two distinct groups, the blocky precipitates can have either a cubic shape with sharp edges or an angular shape with rounded edges.Meanwhile, in addition to the plate-like precipitates, some of those present in the eutectic assemblies can have a spherical shape.Furthermore, some plate-like precipitates may appear spherical due to their orientation relationship to a cross section.
To confirm the presence of TiC precipitates in the Fe-based matrix, the cross-section sample from Fe-2TiC was subjected to SEM/EDX mapping (see Figure 8).As a result, an enhanced concentration of Ti and C elements was obtained for both blocky and plate-like precipitates.Moreover, Cr and Fe elements were homogeneously distributed in a matrix.

Image Processing of SEM-Micrographs
The SEM images of each TiC amount were subjected to image processing via ImageJ to analyze the size and morphology of TiC precipitates.Two typical morphologies were distinguished from the analyzed particles: blocky and plate-like.Herewith, the rest of precipitates, which have spherical or spheroidal morphology and size of %1 μm, were not considered for the further analysis.Consequently, the average size for both morphological categories was estimated (see Table 5).For that, blocky precipitates were characterized with a diameter (d, μm) of a spherical particle that takes the same area as the investigated particle, whereas platelike precipitates were characterized with dimensions (a and b, μm) of a rectangle that has the same area on a cross section as the investigated particle.
The average size of precipitates decreased with a decrease in TiC amount.Herewith, blocky and plate-like particles from Fe-5TiC were noticeably coarser than those from Fe-3.5TiC and Fe-2TiC, whereas the difference between latter two was negligible.Specifically, the average diameter of blocky precipitates was 4.4 AE 1.2 μm for Fe-5TiC, compared to 3.9 AE 0.8 μm and 3.8 AE 0.7 μm for Fe-3.5TiC and Fe-2TiC, respectively.For plate-like precipitates, the average length was 11.0 AE 7.0 μm for Fe-5TiC, compared to 7.2 AE 3.5 μm and 6.8 AE 3.6 μm for Fe-3.5TiC and Fe-2TiC, respectively, while the width was 0.7 AE 0.3 μm for Fe-5TiC, compared to 0.6 AE 0.2 μm and 0.5 AE 0.1 μm for Fe-3.5TiC and Fe-2TiC, respectively.

Alloying Efficiency
Melt processing was accompanied with the formation of a slag layer, which is the undesired phenomenon given related Ti and C losses.Furthermore, the presence of this layer brings a potential risk of clogging during casting of the investigated compositions.As established, the higher temperature and lower amount of TiC prompted the layer dissolution.According to thermodynamic calculations (see Figure 3), the precipitation of primary TiC can occur already in the melt following the exothermic reaction presented in Equation (3).
An increase in the melt temperature above a certain value should lead to the dissolution of these high-melting-point precipitates (see T L temperatures in Table 3).However, only for Fe-2TiC, the homogenization temperature was above T L (i.e., 1540-1620 °C vs. 1455 °C), which insured a successful dissolution of the slag layer and could significantly increase the alloying efficiency.However, Fe-5TiC and Fe-3.5TiC were completed under homogenization temperatures below T L (i.e., 1380 and 1420-1460 °C vs. 1628 and 1548 °C, respectively).Therefore, effective fabrication of in-situ Fe-MMCs via CCIM with appropriate alloying efficiency requires a sufficient superheat above T L in accordance with the chosen amount of TiC.This required superheat can be attained by optimizing the CCIT thermal efficiency, which involves appropriate inductive coil positioning, accurate selection of working power and frequency, and utilization of an additional DC coil. [13]Moreover, the formation of a solid layer within conducted CCIM experiments likely contributed to additional heat losses due to an enlarged contact area between the melt and cold crucible.These issues should be considered in future processing of Fe-TiC via CCIM.
Based on thermodynamic calculations (see Figure 3), the presence of dissolved N in a melt leads to an increase in T L , which is related to the higher thermodynamic stability of TiC(N) compared to TiC.Furthermore, the TiO x phase formed in the presence of dissolved O is expected to remain in an undissolved, solid state throughout the working temperature range of the CCIM unit.Hence, during fabrication, the formed slag layer consisted of a mixture of solid phases (i.e., TiC, TiC(N), TiO x ), which is in accordance with visual observations in conducted CCIM experiments and SEM/EDX investigations (see Figure 4 and  5).Accordingly, TiO x forming elements were measured for both Fe-5TiC and Fe-2TiC within investigations of the surface layer.In agreement with these results, Galgali et al. [12] related significant Ti losses during fabrication of Fe-TiC MMC to both oxidation of steel and direct oxidation of Ti.Therefore, using a protective atmosphere and low levels of oxygen in the initial material are necessary for improving the efficiency of Ti alloying, whereas additional contamination with O can be significantly reduced through the application of the CCIM process.
The initial intention of the prolonged holding time was to improve the alloying efficiency through slag layer dissolution.However, this dissolution was not observed within the extended holding time; instead, an additional loss of Ti was obtained, particularly for higher TiC amounts.This was most likely due to the presence of indissoluble phases (e.g., TiO x ) and an insufficient holding temperature (i.e., below T L ).

Microstructure Characterization
Based on a cross-section investigation of CCIM samples, two distinct morphologies of TiC precipitates were detected, blocky/cubic and plate-like precipitates.The presence of these precipitates is consistent with thermodynamic calculations, where the blocky/cubic precipitates are most likely primary, while the plate-like ones were formed during the eutectic reaction L ↔ bccðFeÞ þ TiC.
In accordance with the phase diagram (see Figure 3), Equation ( 4) and ( 5) outline the solidification sequences for Fe-5TiC and Fe-3.5TiC as well as for Fe-2TiC, respectively.
Equation ( 4) assumes the formation of primary TiC precipitates in the liquid; herewith, the amount of these precipitates and associated T L is greater for Fe-5TiC compared to Fe-3.5TiC.As for Fe-2TiC, the solidification initiates with the bcc(Fe)forming primary ferrite dendrites, after which the remaining liquid undergoes the eutectic reaction, yielding an assembly of bcc(Fe)þTiC.The presented solidification sequences reveal that in situ TiC reinforcement particles nucleate and grow from the melt via the dissolution-precipitation mechanism.
The presented thermodynamic observations describe the equilibrium solidification process, whereas nonequilibrium TiC precipitation can occur under certain cooling conditions.Du et al. [22] studied the influence of cooling rate on the microstructure of Fe-TiC MMC.For that, samples with 0.35 wt% C and 3 wt% Ti were fabricated under cooling rates of 0.2, 162, and 267 K s À1 .Herewith, based on thermodynamic calculations, the utilized chemical composition assumed the formation of eutectic carbides in a solid-liquid zone.As a result, these eutectic carbides of plate-like and spherical morphology were revealed for the low-cooling-rate sample.In contrast, the high-cooling-rate samples displayed primary cubic TiC precipitates in addition to eutectic ones.The authors explained this phenomenon through nonequilibrium TiC precipitation, where the melt can be significantly enriched with Ti and C at a solid/liquid interface as the solidification progresses.Furthermore, a higher cooling rate prompts higher enrichment, which enables the precipitation of primary TiC for the high-cooling-rate samples.With a measured cooling rate of 7-9 K s À1 for the CCIM samples, the obtained microstructure in the current study is similar to the low-cooling-rate sample from Du et al. [22] .However, Fe-2TiC appeared to have a limited number of primary blocky precipitates as well, which is contrary to the solidification sequence (see Equation ( 5)) and thus an indication of the nonequilibrium TiC precipitation.Furthermore, the observed primary TiC in CCIM experiments were organized in agglomeration clusters surrounded by precipitation-free zones, which supports the hypothesis of the nonequilibrium TiC precipitation.Similarly, the presence of primary matrix dendrites (i.e., bcc(Fe)) free of precipitates corresponds to a negative segregation of Ti and C. As was established, the area of these primary dendrites increased with a decrease in TiC wt%; however, even for Fe-5TiC, the primary matrix dendrites were observed.
The formation of Cr-rich M 23 C 6 carbides was predicted after thermodynamic calculations (see Figure 3).Nevertheless, these carbides were not observed within investigated cross sections of CCIM samples.Dogan et al. [23] reported a rare formation of Cr-rich carbides in Fe-TiC MMC having 12.6 wt% Cr, 4.8 Ti,  As demonstrated by the image processing of CCIM SEM micrographs, the average size of both primary cubic/blocky and eutectic plate-like precipitates slightly decreased with a decrease in the amount of TiC.Similar tendencies have been reported in the literature.Liu et al. [24] observed an increase in the maximum crystal size of TiC precipitates from 3 up to 10 μm as Ti content increased from 1 to 8 wt%.Dogan et al. [23] investigated the effect of cooling rate and amount of TiC on morphology and size of TiC precipitates.As a result, a higher solidification rate yielded finer precipitates; herewith, at the lowest cooling rate of 1 K min À1 , TiC was in the form of coarse plates within the eutectic cells and at the highest cooling rate of 85 K min À1 , well-dispersed fine TiC particles dominated the microstructure.However, a variation of the TiC amount (i.e., 2.6, 3.5, 4.5, and 4.8 wt% Ti) in samples with the cooling rate of 85 K min À1 did not induce significant changes in TiC particle size (3-4 μm size).Meanwhile, a sample with 4.8 wt % TiC demonstrated two distinct morphologies, cuboidal and elongated particles.In the current study, despite the relatively high cooling rates achieved within the CCIM fabrication, the in situ-formed TiC reinforcement particles remained within the micrometer (μm) range.Moreover, some of the plate-like precipitates revealed sizes exceeding 10 μm in length.Notably, a substantial shift of the in situ reinforcements to the nanometer (nm) size range for the investigated chemical compositions is possible through the utilization of rapid cooling processes, such as gas atomization and additive manufacturing.In the authors' recent investigations, [25,26] this approach has been successfully employed to fabricate Fe-5TiC composites, resulting in significantly finer reinforcement particles and, consequently, improved mechanical properties of the MMC.

Conclusion
During the CCIM experimental series, Fe-xTiC in situ MMCs were successfully fabricated via inductive melting with subsequent solidification in a cold crucible.The TiC amount varied for conducted experiments between 2, 3.5, and 5 wt%.As a matrix, the Cr-alloyed steel (similar to AISI 420) was applied.
The key idea was to remelt bulky initial materials, including steel blocks and pure Ti plates, so that the homogeneous [Ti]-and [C]rich melt would be obtained, and the subsequent solidification would yield TiC reinforcement particles.The major findings can be expressed as follows.
Metallurgical processing of the Fe-TiC melt within CCIM was accompanied by the formation of a slag layer.This layer is undesired due to associated Ti and C losses.The appearance of CCIM ingots varied depending on the TiC amount, ranging from the clearly visible slag layer for Fe-5TiC and Fe-3.5TiC to a blank shiny top surface for Fe-2TiC.SEM/EDX analysis of the slag layer revealed the presence of TiO x , TiC, and TiC(N) forming elements.
Alloying efficiency for TiC forming elements varied depending on the reinforcement amount and holding time.Specifically, [Ti] losses notably decreased with a decrease in the amount of TiC (6.5% vs. 2.1% deviation from a target value for 5-min holding time samples of Fe-5TiC and Fe-2TiC, respectively).Meanwhile, the prolonged holding time of 30 min led to increased [Ti] losses (especially for Fe-5TiC with 12.7%).Significantly higher [C] losses were observed only for Fe-5TiC (particularly for 30 min holding time samples, 15.9%), whereas, for lower TiC amounts, the fabricated ingots exhibited much lower [C] losses (e.g., 4.8% for 30 min Fe-2TiC), and their fluctuation depending on the reinforcement amount and holding time was negligibly low.
SEM investigation of CCIM cross-section samples revealed two distinct morphologies of TiC precipitates, primary blocky/cubic and eutectic plate-like precipitates.The blocky/cubic precipitates were organized in clusters surrounded by precipitation-free zones, whereas the plate-like ones were in eutectic assemblies with a matrix.
A decrease in TiC amount was accompanied with a decrease in the number of blocky precipitates.Furthermore, prevalence in the number of plate-like precipitates over blocky ones increased and an area of matrix cells/dendrites free of any precipitates increased with a decrease in TiC amount.
Size of precipitates varied depending on the amount of TiC.Specifically, the blocky precipitates appeared to be slightly finer with the decrease in TiC amount (4.4 AE 1.2 μm vs. 3.8 AE 0.7 μm for Fe-5TiC and Fe-2TiC, respectively), whereas the length of plate-like precipitates noticeably decreased in the samples with lower TiC amounts (11.0 AE 7.0 μm vs. 6.8AE 3.6 μm for Fe-5TiC and Fe-2TiC, respectively).
The obtained results can be of high interest for the further development of in situ Fe-TiC MMCs.Revealed dependencies for the alloying efficiency within an application of CCIM should be considered in the future work, addressing the metallurgical processing of the Fe-based [Ti]-and [C]-saturated melt.Additionally, obtained microstructures depending on the reinforcement amount, including morphology, size, and distribution of in situ-formed TiC precipitates, are valuable insights for the development of as-cast Fe-TiC MMCs.

Figure 3 .
Figure 3. Thermodynamic calculation of investigated MMCs.Matrix-TiC phase diagram for Fe-xTiC compositions with extended view of the high-temperature region.Diagram is calculated under condition 10 À5 wt% C. FCC_A1#2 represents TiC.Dashed and dashed-dotted lines represent changes in T L with addition of either [N] or [O].

Figure 5 .
Figure 5. SEM images with EDX measurement for slag after a,b) Fe-5TiC and c,d) Fe-2TiC.Measurements were conducted for the whole depicted area.

Figure 6 .
Figure 6.Analysis of alloying efficiency for TiC forming elements.a) [Ti]content and b) [C]-content.

Table 1 .
Chemical composition of initial material for CCIM experiments, wt%.

Table 2 .
Key aspects of conducted CCIM experiments.

Table 3 .
Calculated liquidus and solidus temperatures for various Fe-xTiC compositions.

Table 4 .
Chemical composition of fabricated blocks after CCIM experiments, wt%.

Table 5 .
Estimated average sizes of TiC particles depending on morphology.and 1.5 wt% C. Thus, a further increase in C-content compared to the values of the current study could induce the formation of these precipitates.It should be noted that the Cr-rich carbides are undesired given the associated reduction of corrosion resistance.