Ceramic Extrusion Technologies for Fine Grained TRIP Matrix Composite Materials
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Metal-Matrix-Composites (MMCs) based on steel with certain ceramic additions offer a wide range of applications in automotive, construction, and mechanical engineering. These MMCs combine the specific properties of steels such as their room temperature deformation behavior with the advantageous hard but brittle ceramic reinforcements which makes them favorable in crash-absorbing or strengthening components. However, common technologies such as casting or infiltration of ceramic preforms by metal melts suffer from the differences between metal and ceramic material characteristics involved during material processing and from geometrical restrictions. The adaption of the ceramics-derived extrusion technology at ambient temperature on materials established in the powder metallurgy (PM) enables an efficient manufacturing process of advanced fine-grained materials with particularly cellular (lightweight) structures as well as bulk specimens. Using powder raw materials enables a wide range of material combinations between TRIP/TWIP steels and various ceramic components considering the pronounced material characteristics of the composite. Knowing the influence of the raw materials, the processing parameters for shaping and the indispensable thermal processing transforming the formed powders into a solid material is crucial for proper manufacturing of MMCs with tailored characteristics. The joining process for these components extends the applicability of the investigated PM-MMCs.
Zirconia (ZrO2) ceramics offer enhanced mechanical properties at ambient temperature due to its polymorphism. The polymorphism is associated with the appearance of room temperature stable monoclinic (m-ZrO2, space group P21/c) and high temperature stable tetragonal/cubic (t-ZrO2, space group P42/nmc, and c-ZrO2, space group Fm-3 m, respectively) crystal structures at ambient pressure. The martensitic phase transformation from the tetragonal to the highly distorted monoclinic modification during cooling below 1170 °C occurs simultaneously with a volume expansion of roughly 3% and which generates stress in the surrounding material resulting in cracking and failure of pure zirconia components. By the addition of stabilizing agents, such as MgO (forming magnesia partially stabilized zirconia—Mg-PSZ), the high temperature ZrO2-modifications can be metastably retained at ambient temperature. These polymorphs are capable to undergo a delayed tetragonal to monoclinic phase transformation, providing the well-known stress-assisted transformation-toughening mechanism and which results in the specific characteristics of commercial zirconia applications. The distinctive phase transformation triggered by external loading cannot only improve the mechanical properties of ceramics but also reinforce metal-matrix materials.
Composite materials based on austenitic stainless steel with particulate reinforcement prepared via cold extrusion are in focus of the present work. The correlation of the initial materials, the processing and the resulting properties of the materials are presented. A very important part of this work deals with chemical and microstructural interactions between the components steel and the reinforcing ceramic particles during sintering. Hereto, no studies on magnesia partially stabilized zirconia in conjunction with high manganese steels have been reported.
The addition of ceramic particles to a ductile matrix increases the strength, but lowers the material’s deformability by inducing typical damage events like debonding, particle fracture or crack coalescence even at moderate fractions of ceramic particles. Thus, improving the interface bonding between the metal matrix and the zirconia particles is of major importance. Wittig et al.  reported the pressureless infiltration of zirconia preforms with a CastX38CrMoV5-1 (AISI H11) steel by adding 20 wt% elemental titanium. Previous investigations revealed similar results with alumina preforms . The high diffusibility of Ti, the formation of TiOx interlayers, and the improved wetting behavior promote sufficient infiltration and the formation of reliable interface boundaries. Titanium is also used as an active component in the brazing of zirconia/zirconia or zirconia/alloy joints [17, 18]. Beyond this, titanium is known for its partial stabilization efficiency in zirconia ceramics by sintering in inert gas atmosphere [19, 20]. Variants of steel and magnesia partially stabilized zirconia were admixed with minor fractions of titanium in order to improve the interface formation during sintering in the frame of this work. The influence of the active interface agent on the mechanical properties has been tested by quasistatic compression tests. The microstructure and the phase compositions were investigated in order to correlate the mechanical response with the desired phase transformations of TRIP steel and zirconia.
Nevertheless the aspiration of improving the mechanical behavior, especially the increase of the specific energy absorption (SEA) capacity, leads to intensive investigations of the raw materials, the selection of materials, the adaption of process parameters and finally to the knowledge of the processing-characteristic-knowledge of a branch of composite materials. Not only the well-known zirconia reinforcing particles but also the introduction of alternative ceramics led to significant improvements of the material properties. In general, the steel/zirconia interface shows destabilization due to the diffusion of Mg out of the particles and the formation of precipitations at the metal/ceramic interface as a result of indispensable thermal treatment at temperatures inappropriate for zirconia sintering. These ZrO2 particles with high amounts of non-transformable monoclinic zirconia are loosely embedded in the metal matrix. Thus, the ability of the reinforcing phase transformation is restricted to the low remaining amount of tetragonal/cubic zirconia. The alloying elements of TRIP/TWIP steels are also prone to diffusional interchange which boosts the interface formation but also the shift of the steel composition. But the interfacial reactions can also improve the material characteristics when exploiting alternative reinforcing ceramics. By replacing zirconia with aluminium titanate (Al2TiO5) a new family of MMCs with advanced mechanical properties under compressive and under tensile deformation is generated.
Furthermore an important issue in this work was the joining of MMCs using a ceramic-derived technology. Bulk specimens from the pure CrMnNi steel, and MMC variants reinforced with 5 or 10 vol% magnesia partially stabilized zirconia were used as joint partners. The samples were prepared via the ceramics-derived extrusion technology and different material combinations were joined in their dry green state using an aqueous paste to form the bonding. The mechanical testing comprised quasistatic tensile deformation tests of the base materials and of the sinter-joint variants at room temperature. The microstructural analyses revealed considerable differences between the ceramic joining process and the conventional welding of steel materials which is beneficial in future practical applications [21, 22].
5.2 Experimental Details
5.2.1 Plastic Processing of Steel/Zirconia Composite Materials
Composition of the initial mixtures, in wt% 
d50 = 45 µm
ZrO2 with 3.5 wt% MgO
d50 = 2 µm
Castament FS 60
Plasticizer methyl cellulose
Chemical composition of steel powder variants, in wt%
Fe + Others
Composition of the zirconia powder variants, in wt%
ZrO2 + Others
Recipe 0Z consists of 100 vol% TRIP steel and recipes 5Z and 10Z are based on 95 or 90 vol% TRIP steel with 5 or 10 vol% zirconia, respectively. In a further step, the plasticizers and water have been added to the batch during mixing. Because of a piston extruder (ECT, Germany), honeycombs 25.5 mm × 25.5 mm with 164 channels (200 cpsi) and a wall thickness of 250 µm have been extruded. The applied pressure was between 10 and 11 MPa. After extrusion, cube-shaped samples (25.5 mm × 25.5 mm × 25.5 mm) have been cut before stepwise water-removal in an air-circulated dryer. After drying, the debinding process has been carried out at 350 °C for 90 min in air in the same aggregate. The binder-free samples have been placed in an alumina crucible and protected against O2 contamination by a Ti casketing. The samples have then been sintered at 1350 °C for 2 h in an electrical furnace with high purity flushing Argon atmosphere.
Microstructure characterization was performed using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS) and electron backscatter diffraction (EBSD). In addition, phase labeling was performed on sintered surfaces by X-ray diffraction (XRD) with Cu–Kα radiation. The relative phase content of zirconia was determined by the method described by Evans et al. . The open porosity, the average pore radius and the pore size distribution of the sintered samples were measured by mercury porosimetry. The macroscopic structure of the honeycomb samples after deformation was examined by optical microscopy of metallographic sections. The mechanical in-plane testing of the sintered specimens was carried out under compressive loading in a 500 kN servohydraulic universal testing machine type MTS 880 with a displacement rate of 0.016 mm/s at ambient temperature.
Composition of mixtures for Ti-activated interface formation, in vol% 
5.2.2 Composite Variants with Additions of Zirconia and/or Aluminium Titanate
Further work was conducted on the development of alternative reinforcing mechanisms since the zirconia particles are prone to undergo the tetragonal to monoclinic phase transformation during thermal processing of the composite materials. Alumina and yttria are well established as reinforcing additions for steel-matrix composites [28, 29]. However, the alumina particles show no phase transformation and insignificantly interface formation. A new approach is the introduction of aluminium titanate as a reinforcing material which is a promising refractory material due to its excellent thermal properties but not prestigious for its mechanical properties.
Composition of mixtures, in vol%
The plastic processing was similar to previous materials with certain modifications of the binder system and the water fraction necessary for shaping. The specimens were heated up to 450 °C in an oxidizing atmosphere, applying a heating gradient of 1 K/min and a dwell time of 30 min at maximum temperature to eliminate organics from the extrusion process. After cooling to room temperature (at a rate of 1 K/min), the specimens were reloaded into a second furnace with a graphite lining and graphite heating elements (Xerion, Germany). The sintering step was conducted in a flushing argon atmosphere using a heating rate of 5 K/min and a maximum temperature of 1350 °C and a dwell time of 2 h at the maximum temperature before cooling with a maximum cooling gradient of 5 K/min.
The fired specimens were processed to a cylindrical geometry (6 mm in diameter and height) for compressive loading tests, as well as to round tensile specimens with a gauge diameter of 5 mm and a gauge length of 30 mm according to DIN EN ISO 6892-1 for tensile loading tests. The tensile test series were performed with a 100 kN universal testing machine of type Zwick 1476. Compression tests utilised a 200 kN servo-hydraulic testing machine of type MTS 810. All tests were carried out at a quasistatic nominal strain rate of 0.001 s−1 at room temperature.
5.2.3 Innovative Joining of Powder Metallurgically Processed TRIP/TWIP Steel Materials
The manufacturing of a composite structure by extrusion is limited to a uniform cross section throughout its entire length, which limits later use. Thus, the availability of a reliable (mechanical) joining technique is a major task in the development of MMCs for commercial applications. Fundamental research was performed on the adaption of a ceramics-derived joining process by adding a slurry on the joining partners at room temperature with the subsequent thermal processing.
Pure steel specimens, the composite variants, and the joining paste were composed of the steel powder batch PMX5CrMnNi17-7-3 (see Table 5.2). The ceramic component was the commercially available magnesia partially stabilized zirconia of type PSZ2 (cf. Table 5.3) [22, 30]. The basic composition (0Z) corresponds to 100% steel without any ceramic additions. Composite mixtures were prepared with fractions of 5 vol% (5Z) or 10 vol% (10Z) zirconia in their total solids composition. The powders were mixed for 90 min in their dry state before adding the organic binder system and water. Rods of 11 mm in diameter were extruded using a single-screw extruder of type LK III 2A (Linden, Germany) at ambient temperature. The water was removed stepwise in an air circulated dryer applying a maximum temperature of 110 °C. Dried samples of each composition were cut to a length of 50 mm and the surfaces to be joined were then polished with a rotary grinding machine to obtain a smooth surface.
The aqueous joining paste was prepared from pure metal powder and water with a ratio of 9:1 by weight. In addition, an organic binder system composed of 1.0 wt% KM2000 (Zschimmer & Schwarz, Germany), 0.6 wt% KM1001 (Zschimmer & Schwarz, Germany) and 2 wt% Optapix G1457 (Zschimmer & Schwarz, Germany) was necessary in order to ensure a sticky paste with well dispersed steel particles.
The joining paste was brushed on the polished end faces of the partners to be joined immediately before these specimens were manually assembled which led to some leaking of the paste and, thus, complete filling of the gap between both parts. The joined specimens were placed on the firing auxiliaries and handled without any rigid clamping during further processing. Then the samples were again dried stepwise in an air circulated dryer starting at 40 °C and increasing to a maximum temperature of 110 °C. Debinding and sintering was performed as shown before. The fired specimens were machined to round tensile loading test geometry with a gauge diameter of 4 mm and a gauge length of 25 mm according to DIN EN ISO 6892-1. The test series with 5–9 samples of each variant were performed at room temperature with a 50 kN universal tensile testing machine of type Tiratest 2850 (Germany) at a quasistatic nominal strain rate of 0.001 s−1. Microstructural characterization was conducted using light optical microscopy and scanning electron microscopy (SEM, Philips XL30). Energy dispersive X-ray analysis (EDS, EDAX Inc., USA) complemented the microstructural study.
Hardness profiles were recorded using a LECO M400 micro hardness tester. Measurement of the profiles was carried out after Vickers procedure with HV0.3 (2.94 N, 10 s load duration) in a triple row with a total length of 6 mm and 100 µm increment. Furthermore, macro hardness was likewise measured on a conventional hardness testing device after Vickers with HV10 (98.04 N, 10 s load duration).
5.3 Results and Discussion
5.3.1 Characteristics of Materials Prepared via Plastic Processing
The interactions between the steel matrix and the zirconia particles were investigated in detail from model mixtures of the zirconia variants PSZ2 and PSZ3 with solely additions of the main alloying elements (Cr, Ni, Mn and Fe) . The addition of Cr leads to several chemical reactions associated with a change of phase composition in zirconia matrix. Up to 18% of the steel mass are delivered as chromium, which reflects the importance of PSZ-Cr interactions in the present composite materials. Sintering magnesia partially stabilized zirconia at 1300–1400 °C causes pronounced changes in the phase composition with a minor fraction of tetragonal and/or cubic ZrO2 after firing. Additions of Mn and Fe already start reacting with zirconia matrix when sintering at 1300 °C leading to either destabilization (Fe) or stabilization (Mn). Further heating to 1400 °C indicates the reaction of chromium with stabilizer magnesia to form magnesia chromite. Variations of the dwell time at maximum temperature have negligible effects on ZrO2 phase composition. In contrast Ni has no significant influence on the sintering progress and phase composition of the fired zirconia.
Properties of the as-fired specimens 
11.2 ± 0.5
11.4 ± 0.2
10.3 ± 0.3
10.6 ± 0.3
10.8 ± 0.4
Bulk density (g/cm3)
7.01 ± 0.02
6.79 ± 0.03
6.77 ± 0.04
7.06 ± 0.04
7.16 ± 0.01
5.3.2 Effect of Zirconia and Aluminium Titanate on the Mechanical Properties of Composite Materials
Physical characteristics of the material variants
15.9 ± 0.1
16.4 ± 0.1
15.9 ± 0.1
15.2 ± 0.2
12.5 ± 0.1
Bulk density (g/cm3)
0.1 ± 0.1
0.1 ± 0.0
0.4 ± 0.3
0.4 ± 0.4
14.1 ± 0.7
7.54 ± 0.05
7.34 ± 0.04
7.25 ± 0.05
7.26 ± 0.05
6.39 ± 0.12
It is obvious that the zirconia particles with a silicate intermediate layer and these silicate structures remained reliably bonded in the matrix material, whereas a large amount of zirconia particles was broken out after specimen preparation. The tensile deformed microstructure of MMCs with 5 vol% initial aluminium titanate powder is characterized by the spinel particles that maintained their pronounced interface bonding to the steel matrix despite the extended multiple-crack formation in the interior of the ceramic particles. In contrast, debonding was the dominant damage initiation mechanism in previous TRIP steel/Mg-PSZ composites [11, 38, 39]. The intact particle/matrix interfaces indicate a higher shear strength at the interface than the particle fracture strength which is a further indication for the improved interface-related performance of the MMCs with initial Al2TiO5 [37, 40]. Still, some of the small-grained spinel particles were nearly intact and exhibited neither cracking nor debonding. Although these composites underwent internal damage by crack initiation and the ceramic particles show no phase transformation or decomposition as compared with composite materials comprising metastable zirconia particles [3, 28], the addition of aluminium titanate results in a higher strain-induced α′-martensite formation and these MMCs sustain a higher stress level than the pure matrix material. The microstructure of the composite materials with concurrent additions of Mg-PSZ and aluminium titanate largely complied with the observations on 5Z and 5AT and the specific features for each type of ceramic particles. Here, the zirconia particles were subject to intense cracking but less to debonding due to the formation of the Ti-enriched spinel interlayer between the ZrO2 particles and the steel matrix. The mutual interference of the zirconia particles and the initial aluminium titanate particles was apparently more pronounced in the MMCs with the higher ceramic volume fraction due to the coincidental distribution of these particles within the steel matrix .
5.3.3 Joining of Zirconia Reinforced MMCs
Properties of the fired base materials without joining 
16.0 ± 0.2
16.5 ± 0.1
15.9 ± 0.2
Total porosity (%)
4.1 ± 1.4
4.4 ± 0.6
4.5 ± 0.5
Bulk density (g/cm3)
7.46 ± 0.11
7.34 ± 0.04
7.24 ± 0.04
Since the surface preparation is of major interest in joining processes, the sample preparation and material derived variations have to be taken into consideration. Nevertheless all surfaces to be joined appeared smooth with some transverse stress marks from the rotating grinding wheel indicating the definitive inclusion of the steel particles in the binder matrix in their dry green state. Break-off of particles and spalling on the edges of the samples was not observed. The addition of 5 or 10 vol% Mg-PSZ did not significantly affect the properties of the surface to be joined, since the ceramic particles were well dispersed and reliably fixed in the composite variants with their metal/binder matrix. The average roughness value Ra of the machined joining surface of the material variants (21.7 µm—0Z; 20.2 µm—5Z; 18.1 µm—10Z) marginally deviates among all materials tested and, thus, a preparation-driven impact on the material characteristics is not to be expected [22, 30].
The formation of distinctive sections, e.g. heat-affected zone and welding seam as known from fusion welding, was neither detected by light optical microscopy nor scanning electron microscopy in the present sinter-joint samples. The joining interface is characterized by a smooth transitional area at a thickness of roughly 100–400 μm. The measurement of the width is only feasible in the material combination 5Z/10Z due to an enlarged zirconia-free intersection between the base materials (see Fig. 5.17b). Contaminations and nonmetallic inclusion are present in all material variants since the pressureless sintering promotes the formation of silicates and spinels at the steel/zirconia interface and even in the pure steel material [3, 41]. Firing the pure joining paste under equivalent thermal conditions results in the formation of a microstructure (e.g. grain size, porosity, and precipitations) similar to the fired 0Z base material .
The ceramics-derived extrusion technology has been successfully applied on the manufacturing of metallic matrix composites. The combination of austenitic stainless TRIP/TWIP steel matrix variants with certain ceramic additions at metal volume fractions ≥90% is feasible. Mixing solid powders with an aqueous organic binder system enables the shaping of bulk and cellular lightweight structures at room temperature. The particle size and shape of the starting powders, the metal/ceramic-ratio, and the binder system crucially affect the shaping process and the microstructure of the shaped specimens. Generally, the ceramic particles were homogeneously dispersed within the metal matrix with a certain degree of agglomeration. To improve the particle dispersion, the size distribution of the metal and the ceramic component should be narrow and compliant to avoid clustering of fine reinforcing particles in the interstices of the metal matrix. Set proper drying parameters (temperature, heating rate) prevents the material from detrimental crack formation and damage of the materials. The subsequent thermal processing comprising the binder removal (debindering) and the consolidation (sintering) transfers the material into its final state. Pressureless sintering of the powder metallurgically processed materials requires temperatures slightly below the metal melting temperature with maximum temperatures of 1300–1400 °C and distinct dwell time of 1–2 h. The thermally induced diffusional interchange of the alloying elements of the steel and the ceramic fractions led to mutual alterations of the components. The crystal structure of zirconia in the MMCs was predominantly monoclinic due to diffusion of Mg out of the particles and the formation of spinel and/or silicate precipitations at the steel/zirconia interface during firing. Thus, the desired tetragonal to monoclinic transformation of the zirconia particles during mechanical testing is hampered.
The reinforcing mechanism of ceramic particles dispersed in TRIP/TWIP steel matrices enables composite materials with superior mechanical properties in a certain range of deformation. The stress-strain and deformation behavior of the materials were mainly controlled by dislocation hardening and α′-martensite phase formation in the steel matrix further affected by the reinforcing mechanisms of the ceramic particles. Finally, all composite materials failed at a lower strain than the unreinforced matrix material by internal damage after cracks have initiated in the steel matrix surrounding the ceramic particles and which is more pronounced under compressive deformation than under tensile loading. Advanced properties of the composite variants have been observed in terms of higher compressive stresses as well as an improved energy absorption capability. The desired martensitic phase transformation was proven in both, the matrix material and the zirconia particles with pronounced phase evolution in an early stage of deformation. Upon exceeding a critical strain the materials without ceramic additions benefit from the lack of non-plastic particles leading to higher strength at equivalent deformation. The addition of 1–2 vol% titanium improves the densification of the matrix material and the firm incorporation of zirconia particles into steel matrix. The formation of a Ti–Zr–O solid solution was observed at the metal/ceramic interface that promotes reliable interface boundary.
Aluminium titanate was identified as a proper reinforcing material for TRIP/TWIP composite materials in the frame of the present study. The diffusional interchange of the alloying elements of the steel and the aluminium titanate particles led to the decomposition of the aluminium titanate and the formation of a dense (Mn, Mg)(Al, Ti, Cr)2O4 spinel structure during firing. The intergranular bonding strength and the toughness of the metal/ceramic interfaces seem to be higher than in composites with loosely embedded zirconia particles. The composite materials with aluminium titanate offered a significant strengthening effect and a distinctive strain hardening under quasistatic compressive and tensile loading as compared with the matrix material. The void-free interface between the steel and the spinel structures of the initial microstructure offered optimal conditions for the load transfer; the intergranular bonding strength and the toughness of the metal/ceramic interfaces are assumed to be higher than in composites with zirconia particles. The combination of Mg-PSZ and Al2TiO5 further improved the stress level of the MMCs during deformation.
An innovative technique for joining metal-matrix composite materials with minor fractions of zirconia reinforcing particles was studied. The bulk specimens were prepared with the powder metallurgical processing route via extrusion of plastic pastes at room temperature. Various combinations of the base materials with 0, 5 or 10 vol% zirconia were joined by applying an aqueous high-solids paste on the surfaces to be joined in their dry green state at room temperature. The preparation of the surfaces before joining comprised cutting and rotary grinding with low mechanical and thermal impact on the base materials. During consecutive thermal processing the material’s consolidation and the generation of the joint took place simultaneously. Thus, the formation of a consistent metal-matrix composite material and the formation of high bonding strength were initiated. Microstructural analysis revealed a homogeneous structure without inhomogeneities within the metal matrix nor segregation of the ceramic particles. Since the sintering and joining proceeded simultaneously the formation of a heat-affected influence zone as known from metal joints prepared by brazing or welding did not occur. The hardness of the pure steel specimens, the composite materials and the joining zone was measured in terms of Vickers’s indention method. Since the fired specimens showed certain porosity and precipitations, the addition of zirconia particles showed no excessive hardening in the composite materials. Also, the formation of the joining zone had a negligible effect on the micro hardness. Mechanical tests under quasistatic tension at room temperature revealed similar strength of the joint variants as compared with the base materials. Strength and plasticity decreased as soon as ceramic particles were present in the base materials. Therefore, the deformation behavior of the joint variants is closely related to the mechanical properties of the base materials. In general the failure is initiated within the joining partner with the higher zirconia volume fraction without fracture at the bonding interface. The sinter joining technology is applicable for a wide range of material combinations and geometries. Efforts to improve the mechanical properties of joint materials demand improved base materials, since the materials are prone to fail beyond the joint.
The authors gratefully acknowledge the financial support of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) for funding the research project A5 and transfer project T1 within the frame of the Collaborative Research Center 799—TRIP-Matrix-Composites—project number 54473466. Additionally, the authors thank all technical employees and students who were involved in processing, machining, and analyzing the materials.
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