Partners

Abstract:
The paper presents a comprehensive investigation of the influence of the main process parameters of spark plasma sintering on the mechanical and microstructural properties of nickel-silicon carbide composites at various scales. Microstructure analysis performed by scanning and transmission electron microscopy revealed a significant interfacial reaction between nickel and silicon carbide due to the decomposition of silicon carbide. The chemical interaction of the matrix and reinforcement results in the formation of a multicomponent interphase zone formed by silicides (Ni31Si12 or/and Ni3Si) and graphite precipitates. Furthermore, several types of structure defects were observed (mainly nano/micropores at the phase boundaries). These significantly influenced the mechanical response of nickel-silicon carbide composites at different levels. At the macroscopic scale, uniaxial tensile tests confirmed that applying a 1000 oC sintering temperature ensured that the manufactured composite was characterised by satisfactory tensile strength, however, with a considerable reduction of material elongation compared to pure nickel. Moreover, the fractography study allowed us to identify a significant difference in the damage mode for certain nickel-silicon carbide samples. Secondly, the interface of the nickel matrix and silicate interphase was tested by bending with microcantilevers to evaluate its deformation behaviour, strength, and fracture characteristics. It was confirmed that a diffusive kind of interface, such as Ni-NiSi, demonstrates unexpected bonding properties with a relatively large range of plastic deformation. Finally, the nanoindentation of three main components of the nickel-silicon carbide composite was executed to evaluate the evolution of nanohardness, Young’s modulus, and elastic recovery due to the application of various spark plasma sintering conditions.

Keywords:
nickel-based composite, silicon carbide, spark plasma sintering, multiscale characterization, mechanical properties, nanoindentation, bending of microcantilevers

Abstract:
The development of powder metallurgy methods in recent years has caused traditional casting methods to be replaced in many industrial applications. Using such methods, it is possible to obtain parts having the required geometry after a process that saves both manufacturing costs and time. However, there are many material issues that decrease the functionality of these methods, including mechanical properties anisotropy and greater susceptibility to cracking due to chemical segregation. The main aim of the current article is to analyze these issues in depth for two powder metallurgy manufacturing processes: laser powder bed fusion (LPBF) and hot-pressing (HP) methods—selected for the experiment because they are in widespread use. Microstructure and mechanical tests were performed in the main manufacturing directions, X and Z. The results show that in both powder metallurgy methods, anisotropy was an issue, although it seems that the problem was more significant for the samples produced via LPBF SLM technique, which displayed only half the elongation in the building direction (18%) compared with the perpendicular direction (almost 38%). However, it should be noted that the fracture toughness of LPBF shows high values in the main directions, higher even than those of the HP and wrought samples. Additionally, the highest level of homogeneity even in comparison with wrought sample, was observed for the HP sintered samples with equiaxed grains with visible twin boundaries. The tensile properties, mainly strength and elongation, were the highest for HP material. Overall, from a practical standpoint, the results showed that HP sintering is the best method in terms of homogeneity based on microstructural and mechanical properties.

Keywords:
Haynes 282 nickel alloy, LPBF, HP, SEM , Static tensile test

Abstract:
Experiments were conducted to investigate, within the framework of a multiscale approach, the mechanical enhancement, deformation and damage behavior of copper–silicon carbide composites (Cu–SiC) fabricated by spark plasma sintering (SPS) and the combination of SPS with high-pressure torsion (HPT). The mechanical properties of the metal–matrix composites were determined at three different length scales corresponding to the macroscopic, micro- and nanoscale. Small punch testing was employed to evaluate the strength of composites at the macroscopic scale. Detailed analysis of microstructure evolution related to SPS and HPT, sample deformation and failure of fractured specimens was conducted using scanning and transmission electron microscopy. A microstructural study revealed changes in the damage behavior for samples processed by HPT and an explanation for this behavior was provided by mechanical testing performed at the micro- and nanoscale. The strength of copper samples and the metal–ceramic interface was determined by microtensile testing and the hardness of each composite component, corresponding to the metal matrix, metal–ceramic interface, and ceramic reinforcement, was measured using nano-indentation. The results confirm the advantageous effect of large plastic deformation on the mechanical properties of Cu–SiC composites and demonstrate the impact on these separate components on the deformation and damage type.

Keywords:
copper–silicon carbide composite, high-pressure torsion, metal–matrix composites, multiscale analysis, nano-indentation, small punch test

Abstract:
The presented research investigation comprises the study of the mechanical properties of modified copper–silicon carbide composites at the micro- and macroscopic scale. The improvement of a copper–silicon carbide composite refers to the addition of a protective layer at the ceramic reinforcement in order to prevent the dissolution of silicon in the copper matrix. The macromechanical behaviour has been evaluated by the performance in a small punch test. The investigation has been carried out with samples with varying volume content of ceramic reinforcement and different protective layers of the silicon carbide particles. Moreover, the influence of temperature during the strength test has been studied. Next, the results have been referred to the interfacial bonding strength of Cu and SiC particles. SEM characterization of samples has been performed to link the composites' microstructure with the mechanical behaviour. Finally, the experimental results of the small punch test have been predicted via a numerical approach. Finite element analysis has been employed to reproduce the response of the composite specimen during the test. Satisfactory agreement with the experimental curve has been obtained.

Keywords:
metal matrix composites, silicon carbide, metallic layers deposition, small punch, interface strength, finite element method

Abstract:
In this paper, small punch test (SPT) which is one of miniaturized samples technique, was employed to characterize the mechanical properties of carbon steel P110. The tests were carried out in the range of –175°C to RT. Results obtained for SPT were compared to those calculated for tensile and Charpy impact test. Based on tensile and SPT parameters numerical model was prepared. 8 mm in diameter and 0.8 mm in height (t) discs with and without notch were employed in this research. The specimens had different depth notch (a) in the range of 0.1 to 0.4 mm. It was estimated that α factor for comparison of Tsp and DBTT for carbon steel P110 is 0.55 and the linear relation is DBTT = 0.55TSPT. The numerical model fit with force - deflection curve of SPT. If the factor of notch depth and samples thickness is higher than 0.3 the fracture mode is transformed from ductile to brittle at -150°C.

Keywords:
small punch test, carbon steel P110, small samples, low temperature, mechanical properties

Abstract:
This work presents the results of fatigue tests of MAR 247 alloy flat specimens with aluminides layers of 20 or 40 µm thickness obtained in CVD process. Fatigue test was conducted at amplitude equal to half of maximum load and ranging between 300 and 650 MPa (stress asymmetry ratio R = 0, frequency f = 20 Hz). Additionally, 4 of the tests, characterized by the highest amplitude, were accompanied with non-contact strain field measurements by means of electronic speckle pattern interferometry and digital image correlation. Results of these measurements allowed to localize the areas of deformation concentration identified as the damage points of the surface layer or advanced crack presence in core material. Identification and observation of the development of deformation in localization areas allowed to assess fatigue-related phenomena in both layer and substrate materials.

Keywords:
aluminide layer, fatigue testing, full-field optical strain measurements, super nickel alloy

Abstract:
The presented work is devoted to experimental studies of the energy storage process in the tensile test of the ultrafine-grained (UFG) titanium in comparison with the coarse-grained one. The UFG titanium was obtained using severe plastic deformation method (SPD) called twist extrusion (TE) that is briefly presented.
The experiments were performed on three groups of titanium specimens. Two of them (T1 and T2) were cut from the materials obtained by TE method. The T1 titanium was processed by 4 passes through the left twist die, whereas for the T2 titanium the twist direction was changed after the first pass. The last group (T0) was prepared from the annealed sheet of coarse-grained titanium. It was noticed that mechanical properties of the material underwent TE differs considerably from properties of te coarsed-grained one. It was observed that yield point obtained for specimens after TE is about 30% higher then that for coarsed-grained material. However, the elongation decrease was observed for both groups of specimens after TE (T1~60%, T2~25%) with respect to T0 ones.
The energy storage investigations show the differences in the energy storage rate for T1 and T2 specimens. In the case of T1 specimens the energy storage rate decreases rapidly with strain whereas for T2 specimens (where twist direction was changed) the energy storage rate remains constant at the homogeneous deformation range. The experimental results show that the change of the twist direction during TE may improve the mechanical properties of the material. The constant rate of energy storage in specimens after twist direction change may be macroscopic manifestation of homogeneous and more stable structure of the material.

Keywords:
energy storage rate, ultrafine-grained titanium, severe plastic deformation, twist extrusion

Abstract:
In the case of the sintering of composite materials exhibiting mutual solubility, intermediate phases with varying concentrations of elements may appear during the densification process. Microstructural and structural changes, especially in the area of the interface, strongly influence mechanical or thermal properties [1]. A good example of such materials is nickel – silicon carbide composites. At elevated temperatures nickel reacts with silicon carbide, which causes total SiC decomposition, and as a result, new Ni-Si phases are formed and free carbon is precipitated within the reaction zone. In this work, nickel-silicon carbide composites were obtained via the Spark Plasma Sintering method. The detailed microstructural analyses using X-ray diffraction, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy revealed the material’s evolution during sintering. To investigate the correlation between microstructure and properties of obtained materials, the mechanical test at three different length scales (in macro-, micro- and nanoscale) was conducted. To evaluate the strength of Ni-SiC composites at a macroscopic scale the uniaxial tensile and compression tests were employed. The sample deformation and failure mechanism for different stages of sintering were analyzed. The strength of the nickel-silicon carbide interface was determined by bending tests of micro-cantilever beams. Nanoindentation was used to evaluate the hardness of each composite component. The conducted research revealed a strong relation between mechanical strength and sintering conditions.