**7. Case study: Developing a Metal Matrix Composite (MMC) — Eurofer 97 steel reinforced with tantalum carbides — TaCs**

EUROFER 97 steel is a promising alloy for use in nuclear reactors or in applications where the material is submitted to working temperatures up to 550°C due to its lower strength under fluency. Factors that influence the slip of boundaries are the grain morphology, angle and speed of grain boundaries. The speed can be reduced with the presence of a dispersed phase in the material, provided it is thin and evenly distributed.

The state of Rio Grande do Norte is a major producer of refractory metals (W, Ta, Nb), ceramic minerals (diatomite, kaolin, feldspar, mica, barite, clays, etc.) and other minerals containing rare earths and semi-precious stones. However, this natural wealth that places Rio Grande do Norte among the top five producers of minerals in the country has not reversed in progress and development for the region, mainly due to lack of technology to aggregate these resources to local raw materials.

This study presents the development of a new metal matrix composite (MMC), which has the following starting materials: ferritic / martensitic EUROFER 97 stainless steel and tantalum carbides – TaC as reinforcement, one of them synthesized in laboratory (UFRN) and the other supplied by Aldrich, the first with average crystallite size of 13.78 nm and the second with crystallite size of 40.66. TaC nanometric particles were inserted into the metallic matrix through the processing steps of powder metallurgy seeking to improve the properties of the final product.

Initial sintering studies with EUROFER 97 steel reinforced by TaC nanosized particles dispersed into the matrix of MMC composites showed satisfactory values with respect to the improvement of the mechanical properties regardless of processing, as sintered materials with similar microhardness values and even greater that 100% the value of 333.2 HV for bar-shaped pure steel was obtained (Oliveira, 2013).

### **7.1. Experimental procedure**

1998). This method uses high-energy milling to form composite powders typically for long

In high-energy milling, the constant ball-powder-ball collisions result in deformations and fractures that define the dispersion of components, homogenization, phases and the final powder microstructure. The nature of these processes depends on the mechanical behavior of the powder components, their equilibrium phase and stress state during milling (Suryanar‐ ayana, 1998 and 2001). High-energy milling can be performed with three different categories of metal powders or alloys, namely: ductile-ductile components, ductile-brittle components

The influence of particle dispersion with the formation of a second phase, slowing the metal surface movement and its sintering, has been proposed by Kuczynsky and Lavendell, who considered that moving particles act as barriers to the surface advancement. Later, it was discussed that particles are swallowed by the surface movement exerting force and preventing

The dispersion of large amounts of ceramic particles by powder metallurgy aims to improve their tribological properties and thus their mechanical properties; therefore, 12% SiC powder with average size of 3 μm was added to 316L steel powders of 5 μm, mixing for 8 hours, uniaxially compressed at 100 MPa and sintered at temperatures of 1100°C for one hour in inert

Another study that used 3% W TaC dispersed in the metal matrix of atomized 316L austenitic stainless steel and processed by powder metallurgy showed higher density values and a significant increase in hardness compared to material without reinforcement (Oliveira, 2008).

Sintering is a non-equilibrium thermodynamic process in which a system of particles (powder or compacted aggregate) acquires a coherent solid structure by reducing the specific surface area. The result is the formation of grain boundaries and growth of necks and inter-particle bond, typically leading the system to densification and volumetric shrinkage (Gomes, 1995). Solid-phase sintering occurs at temperature where none of the system elements reaches their melting point. This is accomplished with material transport (atomic diffusion, vapor transport,

In the second case, liquid-phase sintering leads to the formation of a liquid phase capable of dissolving a percentage of particles. This yields a diffusion pathway analogous to the grain boundary in the solid-phase sintering, causing a rapid initial density increase and then, dissolution of solid particles in the liquid and precipitation on the neck region occur (Gomes,

The sintering of metal matrix composites can be divided into two categories: a) solid-phase sintering (including powder metallurgy) and b) liquid-phase sintering (Kocjak, *et al*, 1993).

atmosphere, resulting in complete fusion of samples (Patankar, *et al*, 2000).

milling times (Gomes *et al*, 2001; Costa *el al*, 2002.).

and brittle-brittle components.

112 Sintering Techniques of Materials

**6. Sintering**

1995).

viscous flow, etc) (Costa, 2004).

its movement (Sbrockey and Johnson, 1980).

EUROFER steel was received in the form of bar and has ferritic / martensitic microstructure as can be seen in Figure 2, as well as the presence of grain boundaries with considerable sizes, unable to be viewed in full due to their size indicated by arrows. Microhardness measurements were made and the average value was 333.2 HV and according to the literature, the theoretical density ranges from 8.0 to 8.1 g/cm3 .

**Figure 2:** MO micrograph (500x) of EUROFER steel as received, attacked with 2% Vilela. **Figure 2.** MO micrograph (500x) of EUROFER steel as received, attacked with 2% Vilela.

 After characterization of the bar-shaped starting material, machining was performed, where chip was removed, and from this, the grinding process began. Firstly, pure steel was performed, and then steel with the addition of 3% UFRN TaC and 3% commercial TaC. The powders were milled for 5 hours in highenergy mill. Each of the resultant particulate samples were characterized by x-ray diffraction and SEM and then cold compacted under uniaxial pressure of 600MPa in a cylindrical array of 5 mm in diameter. Subsequently, the compressed powders were sintered in a vacuum oven at temperature of 1250°C isotherm for 60 minutes. Sintered samples were cooled to room temperature. The structure of sintered After characterization of the bar-shaped starting material, machining was performed, where chip was removed, and from this, the grinding process began. Firstly, pure steel was per‐ formed, and then steel with the addition of 3% UFRN TaC and 3% commercial TaC. The powders were milled for 5 hours in high-energy mill. Each of the resultant particulate samples were characterized by x-ray diffraction and SEM and then cold compacted under uniaxial pressure of 600MPa in a cylindrical array of 5 mm in diameter. Subsequently, the compressed powders were sintered in a vacuum oven at temperature of 1250°C isotherm for 60 minutes. Sintered samples were cooled to room temperature. The structure of sintered samples wasobserved by optical and scanning electron microscopy and analyzed by microhardness tests.

#### samples was observed by optical and scanning electron microscopy and analyzed by microhardness tests. **7.2. Results**

**7.2 Results**  Figure 3 shows the microstructure and the X-ray pattern of pure EUROFER steel milled for 5 hours. The electron micrograph of EUROFER 97 shows uneven particles with rough surface, which can be compared to the morphology of powder atomized with water, Fig (3a). The X-ray diffraction pattern, Figure (3b) shows only peaks associated to Fe with CCC structure, which is the same structure of the ferrite phase. This is a strong indication that the milling conditions used significantly influenced the phase Figure 3 shows the microstructure and the X-ray pattern of pure EUROFER steel milled for 5 hours. The electron micrograph of EUROFER 97 shows uneven particles with rough surface, which can be compared to the morphology of powder atomized with water, Fig (3a). The Xray diffraction pattern, Figure (3b) shows only peaks associated to Fe with CCC structure, which is the same structure of the ferrite phase. This is a strong indication that the milling conditions used significantly influenced the phase transformation occurred during sintering of EUROFER steel. During milling, the starting percentage of martensite was transformed into ferrite from the effects of powder processing.

transformation occurred during sintering of EUROFER steel. During milling, the starting percentage of martensite was transformed into ferrite from the effects of powder processing. Figure (4a) shows the electron micrograph of EUROFER 97 steel composite powder with addition of elemental TaC (UFRN) after 5 hours of milling. It was observed that there is a similarity in images-SEM of pure steel powder and composite powder, Figure (4a) and Figure (4a). The presence of carbides (light spots) is noticeable, which are distributed and inserted in a dispersed form on the surface of the metal matrix (dark gray surface). Figure (4b) shows the XRD pattern of X-ray of EUROFER 97 steel composite powder with elemental TaC (UFRN)

(light spots) is noticeable, which are distributed and inserted in a dispersed form on the surface of the **Figure 3.** Pure EUROFER 97 steel powder milled for 5 hours, (a) SEM-magnification 3000x and (b) XRD.

**Figure 2:** MO micrograph (500x) of EUROFER steel as received, attacked with 2% Vilela.

**Figure 2.** MO micrograph (500x) of EUROFER steel as received, attacked with 2% Vilela.

**7.2 Results** 

**7.2. Results**

114 Sintering Techniques of Materials

After characterization of the bar-shaped starting material, machining was performed, where chip

Figure 3 shows the microstructure and the X-ray pattern of pure EUROFER steel milled for 5

hours. The electron micrograph of EUROFER 97 shows uneven particles with rough surface, which can be compared to the morphology of powder atomized with water, Fig (3a). The X-ray diffraction pattern, Figure (3b) shows only peaks associated to Fe with CCC structure, which is the same structure of the ferrite phase. This is a strong indication that the milling conditions used significantly influenced the phase transformation occurred during sintering of EUROFER steel. During milling, the starting percentage of

Figure (4a) shows the electron micrograph of EUROFER 97 steel composite powder with addition of elemental TaC (UFRN) after 5 hours of milling. It was observed that there is a similarity in images-SEM of pure steel powder and composite powder, Figure (4a) and Figure (4a). The presence of carbides (light spots) is noticeable, which are distributed and inserted in a dispersed form on the surface of the metal matrix (dark gray surface). Figure (4b) shows the XRD pattern of X-ray of EUROFER 97 steel composite powder with elemental TaC (UFRN)

Figure 3 shows the microstructure and the X-ray pattern of pure EUROFER steel milled for 5 hours. The electron micrograph of EUROFER 97 shows uneven particles with rough surface, which can be compared to the morphology of powder atomized with water, Fig (3a). The Xray diffraction pattern, Figure (3b) shows only peaks associated to Fe with CCC structure, which is the same structure of the ferrite phase. This is a strong indication that the milling conditions used significantly influenced the phase transformation occurred during sintering of EUROFER steel. During milling, the starting percentage of martensite was transformed into

martensite was transformed into ferrite from the effects of powder processing.

ferrite from the effects of powder processing.

was removed, and from this, the grinding process began. Firstly, pure steel was performed, and then steel with the addition of 3% UFRN TaC and 3% commercial TaC. The powders were milled for 5 hours in highenergy mill. Each of the resultant particulate samples were characterized by x-ray diffraction and SEM and then cold compacted under uniaxial pressure of 600MPa in a cylindrical array of 5 mm in diameter. Subsequently, the compressed powders were sintered in a vacuum oven at temperature of 1250°C isotherm for 60 minutes. Sintered samples were cooled to room temperature. The structure of sintered samples was observed by optical and scanning electron microscopy and analyzed by microhardness tests.

After characterization of the bar-shaped starting material, machining was performed, where chip was removed, and from this, the grinding process began. Firstly, pure steel was per‐ formed, and then steel with the addition of 3% UFRN TaC and 3% commercial TaC. The powders were milled for 5 hours in high-energy mill. Each of the resultant particulate samples were characterized by x-ray diffraction and SEM and then cold compacted under uniaxial pressure of 600MPa in a cylindrical array of 5 mm in diameter. Subsequently, the compressed powders were sintered in a vacuum oven at temperature of 1250°C isotherm for 60 minutes. Sintered samples were cooled to room temperature. The structure of sintered samples was observed by optical and scanning electron microscopy and analyzed by microhardness tests.

milled for 5 hours, which presents peaks characteristics of steel and TaC, and their respective phases. The diffractogram shows only the ferrite phase, without martensite, for iron after 5 hours of milling. composite powder with elemental TaC (UFRN) milled for 5 hours, which presents peaks characteristics of steel and TaC, and their respective phases. The diffractogram shows only the ferrite phase, without martensite, for iron after 5 hours of milling.

metal matrix (dark gray surface). Figure (4b) shows the XRD pattern of X-ray of EUROFER 97 steel

**Figure 4:** EUROFER 97 steel powder with TaC UFRN milled for 5 hours, (a) SEM- magnification 3000x and **Figure 4.** EUROFER 97 steel powder with TaC UFRN milled for 5 hours, (a) SEM-magnification 3000x and (b) XRD.

(b) XRD. Figure (5a) shows the SEM image and figure (5b) shows the diffraction standard of X-ray of EUROFER 97 steel composite powder with commercial TaC milled for 5 hours in a high-energy planetary mill. Initially, the SEM microstructure shows heterogeneous particle size and shape and the non-uniform dispersion of carbides into the metal matrix compared to particulate composites of steel with UFRN TaC Figure (5a) shows the SEM image and figure (5b) shows the diffraction standard of X-ray of EUROFER 97 steel composite powder with commercial TaC milled for 5 hours in a high-energy planetary mill. Initially, the SEM microstructure shows heterogeneous particle size and shape and the non-uniform dispersion of carbides into the metal matrix compared to particulate composites of steel with UFRN TaC Figure (4a). XRD pattern shows peaks of Fe with TaC and CCC structure with greater intensity and height for carbide when compared with XRD pattern of the EUROFER 97 composite powder with UFRN TaC, Figure (4b).

3000x and (b) XRD.

Figure (4b).

Figure (4a). XRD pattern shows peaks of Fe with TaC and CCC structure with greater intensity and height

**Figure 5:** EUROFER 97 steel powder with commercial TaC milled for 5 hours, (a) SEM- magnification **Figure 5.** EUROFER 97 steel powder with commercial TaC milled for 5 hours, (a) SEM-magnification 3000x and (b) XRD.

Figure 6 shows the micrographs (SEM) of samples sintered at temperature of 1250°C for 60 minutes. Figure (6a) shows sintered pure EUROFER steel with microstructure that indicates that it is in the final stage because pores are small and rounded, typical of this stage. Figures (6b) and (6c) show the dispersion of TaC particles in ETU and ETC composites, in which it is observed that TaC particles (white portion) are dispersed in the grain boundaries of the metal matrix (gray portion); in the case of ETU sample still with the presence of many pores, a large cluster of TaC particles randomly dispersed in the metal matrix between the boundaries and within the pores of the sintered sample was observed, hindering Figure 6 shows the micrographs (SEM) of samples sintered at temperature of 1250°C for 60 minutes. Figure (6a) shows sintered pure EUROFER steel with microstructure that indicates that it is in the final stage because pores are small and rounded, typical of this stage. Figures (6b) and (6c) show the dispersion of TaC particles in ETU and ETC composites, in which it is observed that TaC particles (white portion) are dispersed in the grain boundaries of the metal matrix (gray portion); in the case of ETU sample still with the presence of many pores, a large cluster of TaC particles randomly dispersed in the metal matrix between the boundaries and within the pores of the sintered sample was observed, hindering the closing of pores and the non densification of the material.

the closing of pores and the non densification of the material.

**Figure 6.** Micrographs (SEM - 1000x) of samples sintered at 1250°C for 60 minutes.; (a) Pure EUROFER **Figure 6.** Micrographs (SEM-1000x) of samples sintered at 1250°C for 60 minutes.; (a) Pure EUROFER (EP), (b) EURO‐ FER with UFRN TaC (ETU) and (c) EUROFER with commercial TaC (ETC).

(EP), (b) EUROFER with UFRN TaC (ETU) and (c) EUROFER with commercial TaC (ETC).

 Micrographs (OM) of Figure 7 show sintered pure steel (Fig. 7a), sintered steel with UFRN TaC Micrographs (OM) of Figure 7 show sintered pure steel (Fig. 7a), sintered steel with UFRN TaC (Fig7b) and sintered steel with commercial TaC (Fig. 7c). Regarding porosity, there are small and well rounded pores, indicating the final sintering stage. The idealized sinteriza‐

(Fig7b) and sintered steel with commercial TaC (Fig. 7c). Regarding porosity, there are small and well

tion models proposed by various authors are specific to each stage. Figure 7a shows in the micrograph of sintered EUROFER pure steel sample, the size of grain boundaries and the presence of two phases, ferrite (lighter portion) and martensitic (darker portion); Figure 7b shows sintered ETU still with the presence of many pores compared to the two other sintered samples under the same conditions; however, pores in the segmented form in particles and also rounded pores, indicating sintering at stage from intermediate to the final stage. Microstructure with diffuse phases, dark and clear portions, was observed. Figure 7c shows densified samples with grain boundaries with regular shapes and small sizes; however, the presence of a single phase characteristic of ferritic phase can influence the mechanical properties of the material.

Figure (4a). XRD pattern shows peaks of Fe with TaC and CCC structure with greater intensity and height for carbide when compared with XRD pattern of the EUROFER 97 composite powder with UFRN TaC,

**Figure 5:** EUROFER 97 steel powder with commercial TaC milled for 5 hours, (a) SEM- magnification

**Figure 5.** EUROFER 97 steel powder with commercial TaC milled for 5 hours, (a) SEM-magnification 3000x and (b)

Figure 6 shows the micrographs (SEM) of samples sintered at temperature of 1250°C for 60 minutes. Figure (6a) shows sintered pure EUROFER steel with microstructure that indicates that it is in the final stage because pores are small and rounded, typical of this stage. Figures (6b) and (6c) show the dispersion of TaC particles in ETU and ETC composites, in which it is observed that TaC particles (white portion) are dispersed in the grain boundaries of the metal matrix (gray portion); in the case of ETU sample still with the presence of many pores, a large cluster of TaC particles randomly dispersed in the metal matrix between the boundaries and within the pores of the sintered sample was observed, hindering the closing of pores and the

(a) (b)

Intensidade(u.a.)

minutes. Figure (6a) shows sintered pure EUROFER steel with microstructure that indicates that it is in the final stage because pores are small and rounded, typical of this stage. Figures (6b) and (6c) show the dispersion of TaC particles in ETU and ETC composites, in which it is observed that TaC particles (white portion) are dispersed in the grain boundaries of the metal matrix (gray portion); in the case of ETU sample still with the presence of many pores, a large cluster of TaC particles randomly dispersed in the metal matrix between the boundaries and within the pores of the sintered sample was observed, hindering

**Figure 6.** Micrographs (SEM - 1000x) of samples sintered at 1250°C for 60 minutes.; (a) Pure EUROFER

**Figure 6.** Micrographs (SEM-1000x) of samples sintered at 1250°C for 60 minutes.; (a) Pure EUROFER (EP), (b) EURO‐

Micrographs (OM) of Figure 7 show sintered pure steel (Fig. 7a), sintered steel with UFRN TaC (Fig7b) and sintered steel with commercial TaC (Fig. 7c). Regarding porosity, there are small and well rounded pores, indicating the final sintering stage. The idealized sinteriza‐

(Fig7b) and sintered steel with commercial TaC (Fig. 7c). Regarding porosity, there are small and well

Micrographs (OM) of Figure 7 show sintered pure steel (Fig. 7a), sintered steel with UFRN TaC

(EP), (b) EUROFER with UFRN TaC (ETU) and (c) EUROFER with commercial TaC (ETC).

the closing of pores and the non densification of the material.

FER with UFRN TaC (ETU) and (c) EUROFER with commercial TaC (ETC).

Figure 6 shows the micrographs (SEM) of samples sintered at temperature of 1250°C for 60

20 25 30 35 40 45 50 55 60 65 70 75 80

Fe {110} - CCC

TaC {200} - CFC

TaC {111} - CFC

Fe {200} - CCC

TaC {220} - CFC

TaC {222} - CFC

TaC {311} - CFC

EUROFER - TaC (ETC) IEXP ICALC IEXP - ICALC

**(a) (b) (c)**

**(a) (b) (c)**

Figure (4b).

116 Sintering Techniques of Materials

3000x and (b) XRD.

non densification of the material.

XRD.

**Figure 7.** Micrographs (SEM-500x) of samples sintered at 1250°C for 60 minutes; (a) Pure EUROFER (EP), (b) EURO‐ FER with UFRN TaC (ETU) and (c) EUROFER with commercial TaC (ETC).

Table 1 shows the microhardness values obtained for pure Eurofer 97 and samples added of elemental TaC sintered at temperature of 1250°C and time of 60 min.

It was observed that the ETU sample obtained the highest hardness value compared to all other samples, which can be related to the size of very fine particle, described in experimental procedure, not even when dealing with a well-densified sample.

The large difference in the microhardness values among pure steel samples can be observed, and for this, the processing of powder metallurgy (milling and sintering) was sufficient to improve the mechanical properties of steel samples with UFRN TaC compared to steel samples with commercial TaC, both at the same sintering temperature and sintering time. These differences in the microhardness values can be probably associated with the dispersal of coarse and / or fine particles in the metal matrix, and this is due to the synthesis of carbides; even with low energy input, UFRN TaC produced particles with small crystal‐ lite size and more homogeneous compared to that provided by Aldrich, or may even be related to phase transformation, i.e., in sintered samples with commercial TaC, the prevailing phase is ferrite, which has low hardness, whereas in those with UFRN TaC, the presence of other phases, i.e., the metal matrix composite remains ferritic / martensitic as the initial steel and the presence of fine particles dispersed or embedded in the metal matrix of the EUROFER steel.


**Table 1.** Microhardness results of composites milled for 5 hours and sintered at temperature of 1250°C for 60
