The implementation of hollow S60HS glass microspheres and Fillite 106 cenospheres in a martensitically transformable AISI 304L stainless steel matrix was realized by means of metal injection molding of feedstock with varying fractions of the filler material. The so-called TRIP-steel syntactic foams were studied with respect to their behavior under quasi-static compression and dynamic impact loading. The interplay between matrix material behavior and foam structure was discussed in relation to the findings of micro-structural investigations, electron back scatter diffraction EBSD phase analyses and magnetic measurements. During processing, the cenospheres remained relatively stable retaining their shape while the glass microspheres underwent disintegration associated with the formation of pre-cracked irregular inclusions. Consequently, the AISI 304L/Fillite 106 syntactic foams exhibited a higher compression stress level and energy absorption capability as compared to the S60HS-containing variants. The α ′ -martensite kinetic of the steel matrix was significantly influenced by material composition, strain rate and arising deformation temperature. The highest ferromagnetic α ′ -martensite phase fraction was detected for the AISI 304L/S60HS batches and the lowest for the TRIP-steel bulk material. Quasi-adiabatic sample heating, a gradual decrease in strain rate and an enhanced degree of damage controlled the mechanical deformation response of the studied syntactic foams under dynamic impact loading.

The mechanical properties and the related microstructure of metal-matrix composites (MMC) based on a high-alloyed CrMnNi steel with varying particle reinforcements (5% or 10 vol%) of magnesia partially stabilized zirconia and/or aluminium titanate were investigated. The powder metallurgical processing comprised the cold extrusion and conventional solid-state sintering at 1350 degrees C. The mechanical properties were examined by quasi-static compressive and tensile loading tests at ambient temperature. The microstructure characteristics contributing to significant changes in strength and ductility and affecting the failure mechanisms during deformation were characterised by scanning electron microscopy including energy-dispersive X-ray spectroscopy and electron backscatter diffraction, and by X-ray diffraction. For all compositions the stress-strain and the deformation behaviour were mainly controlled by dislocation hardening and alpha'-martensite formation in the steel matrix, but it was further improved by the particle strengthening of the ceramic reinforcement. Thus, the composite materials showed higher strength and work hardening than unreinforced steel specimens over a wide strain range. The pressureless sintering triggers several reactions at the metal/ceramic interface, which leads to pronounced destabilisation of the initial metastable zirconia particles with a lack of transformation toughening capability, and the formation of invalid olivine. These MMC suffer from early particle/matrix displacement and particle fracture under loading. A more positive effect of interfacial reactions was observed in the composites with addition of aluminium titanate. Here, the formation of a dense spinel structure and the reliable matrix/particle interface bonding during firing provides a significant particle strengthening effect. The combination of zirconia and aluminium titanate provides mechanical and microstructural benefits as well. Thus, aluminium titanate facilitates the consolidation of powder metallurgical processed steel-matrix MMCs via the conventional sintering for advanced load applications. The results of the study help to understand essential hardening mechanisms in composite materials and provide potential for future cellular MMC structures. (C) 2016 Published by Elsevier B.V.

The mechanical behavior of a high density square celled composite honeycomb structure is examined with respect to the influence of different matrix materials and ceramic particle additions. For this purpose two high alloyed metastable austenitic CrMnNi-steels with varying Ni contents and reinforcements of 0, 5 and 10vol.% magnesia partially stabilized zirconia (Mg-PSZ) were chosen as cell wall material to evaluate the contribution of the TRansformation Induced Plasticity (TRIP)- and TWinning Induced Plasticity (TWIP)-effect on the strength, deformation and damage characteristics of the honeycombs. Out-of-plane compression tests were conducted in the strain rate range of 0.001–180s−1 and at temperatures of −60°C and 20°C having a great effect on the driving force and extent of both material effects. The results reveal a considerably higher strength level for the TRIP-assisted structures which can be improved by the addition of Mg-PSZ at certain deformation degrees whereas the TWIP-matrices are more damage-tolerant.

The mechanical and structural responses of high-density TRIP steel and TRIP-steel/zirconia composite honeycomb structures were studied under uniaxial compression in the out-of-plane loading direction over a wide range of strain rates. Their mechanical response, buckling, and failure mechanisms differ considerably from those of conventional thin-walled, low-density cellular structures. Following the linear-elastic regime and the yield limit of the bulk material, the high-density square honeycombs exhibited a uniform increase in compression stress over an extended range of (stable) plastic deformation. This plastic pre-buckling stage with axial crushing of cell walls correlates with the uniaxial compressive response of the bulk specimens tested. The dominating material effects were the pronounced strain hardening of the austenitic steel matrix accompanied by a strain-induced alpha'-martensite nucleation (TRIP effect) and the strengthening effect due to the zirconia particle reinforcement. The onset of critical plastic bifurcation was initiated at high compressive loads governed by local or global cell wall deflections. After exceeding the compressive peak stress (maximum loading limit), the honeycombs underwent either a continuous post-buckling mode with a folding collapse (lower relative density) or a symmetric extensional collapse mode of the entire frame (high relative density). The densification strain and the post-buckling or plateau stress were determined by the energy efficiency method. Apart from relative density, the crush resistance and deformability of the honeycombs were highly influenced by the microstructure and damage evolution in the cell walls as well as the bulk material's strain-rate sensitivity. A significant increase in strain rate against quasi-static loading resulted in a measured enhancement of deformation temperature associated with material softening. As a consequence, the compressive peak stress and the plastic failure strain at the beginning of post-buckling showed an anomaly with respect to strain rate indicated by minimum values under medium loading-rate conditions. The development of the temperature gradient in the stable pre-buckling stage could be predicted well by a known constitutive model for quasi-adiabatic heating. (C) 2015 The Authors. Published by Elsevier Ltd.

Metal-matrix composite materials composed of an austenitic stainless steel with different ceramic particle reinforcements were investigated in this study. The test specimens were prepared via a powder metallurgical processing route with extrusion at room temperature. As reinforcement phase, either magnesia partially stabilized zirconia or aluminium titanate with a volume content of 5% or 10% was used. The mechanical properties were determined by quasi-static compressive and tensile loading tests at ambient temperature. The microstructure characteristics and failure mechanisms during deformation contributing to significant changes in strength and ductility were characterized by scanning electron microscopy including energy dispersive X-ray spectroscopy and electron back-scatter diffraction, and by X-ray diffraction. The composite materials showed higher stress over a wide range of strain. Essentially, the deformation-induced formation of α′-martensite in the steel matrices is responsible for the pronounced strain hardening. At higher degrees of deformation, the material behavior of the composites was controlled by arising damage evolution initiated by particle/matrix interface debonding and particle fracture. The particle reinforcement effects of zirconia and aluminium titanate were mainly controlled by their influences on martensitic phase transformations and the metal/ceramic interfacial reactions, respectively. Thereby, the intergranular bonding strength and the toughness of the steel/ceramic interfaces were apparently higher in composite variants with aluminium titanate than in composites with magnesia partially stabilized zirconia particles.

304L stainless steel syntactic foam test specimens were produced by means of injection moulding of feedstock made from pre-alloyed 304L powder, binder and additions of S60HS micro hollow glass spheres and Fillite 106 cenospheres. After de-binding and sintering the foam pore structure and matrix micro-structure were analysed. Mechanical characterisation was done using tensile and compression tests. The results of the mechanical tests are discussed in terms of the observed foam and matrix microstructures.

The interactions between microstructure evolution and mechanical properties of square‐celled TRIP steel and particle‐reinforced TRIP steel/zirconia honeycombs are investigated over a wide range of test temperatures. Based on a powder metallurgical route of a ceramic extrusion technology, two‐dimensional channeled structures are made of an austenitic AISI 304 CrNi‐steel or a TRIP steel matrix composite with up to 10 vol.% of a MgO partially stabilized zirconia (Mg‐PSZ) ceramic. Honeycomb specimens with a cell frame of 196 cpsi and a relative density of 0.37 (viz. 2.9 g cm−3) are tested under quasi‐static compression in out‐of‐plane loading direction at temperatures between ‐196 °C and 150 °C. During compression, the cellular materials deform in a strain‐dominated buckling mode initiating a plastic hinge formation in the cell walls at higher strain levels. Their crush resistance is intensively controlled by the material design including the plastic yield and strengthening mechanisms of the TRIP steel matrix and the constraint or particle reinforcement effects of the embedded zirconia ceramic. Since the thermodynamic driving force for the α′‐martensite nucleation and the intrinsic stacking fault energy are strongly influenced by temperature, different material strengthening and softening processes and a varied microstructure evolution are detected. A sigmoidal initial stress‐strain response as well as the highest compression stress are measured at test temperatures below room temperature due to a pronounced α′‐martensite volume fraction. However, at higher test temperatures, mechanical twinning and dislocation glide are the dominant deformation mechanisms. In summary, strength, ductility and energy absorption which identify the crashworthiness of the honeycombs, are significantly affected by the temperature‐sensitive macro‐ and microstructure phenomena.

Mechanical in situ CT investigations on cruciform members of square‐celled Mg‐PSZ reinforced TRIP‐steel honeycomb structures are presented. Furthermore, test series on a pure TRIP‐steel and MMC bulk material with 10 vol% Mg‐PSZ as well as macro‐scale compression tests on honeycomb structures are performed under quasi‐static loading. The measured stress–strain curves of the cruciform columns are related to their buckling and failure behaviour. The mechanical and structural characteristics of the cell wall node elements are compared to the damage mechanisms of the complete honeycombs. Differences in the damage patterns between the pure TRIP‐steel and the MMC are demonstrated. The phase transformation effect in the Mg‐PSZ particles is pointed out based on EBSD and EDX measurements.

Two designs of square-celled metallic honeycomb structures fabricated by a modified extrusion technology based on a powder feedstock were investigated. The strength and ductility of these cellular materials are achieved by an austenitic CrNi (AISI 304) steel matrix particle reinforced by an MgO partially-stabilized zirconia building up their cell wall microstructure. Similar to the mechanical behaviour of the bulk materials, the strengthening mechanism and the martensitic phase transformations in the cell walls are affected by the deformation temperature and the nominal strain rate. The microstructure evolution during quasi-static and dynamic impact compression up to high strain rates of 103 1/s influences the buckling and failure behaviour of the honeycomb structures. In contrast to bending-dominated quasi-isotropic networks like open-celled metal foams, axial compressive loading to the honeycomb’s channels causes membrane stretching as well as crushing of the vertical cell node elements and cell walls. The presented honeycomb materials differ geometrically in their cell wall thickness-to-cell size-ratio. Therefore, the failure behaviour is predominantly controlled by global buckling and torsional-flexural buckling, respectively, accompanied by plastic matrix flow and strengthening of the cell wall microstructure.

The mechanical and structural response of powder metallurgical square‐celled honeycomb structures to quasi‐static and dynamic impact loads are described. By constructing the cellular lattice with a novel metal matrix composite material based on a metastable high‐alloyed austenitic TRIP‐steel particle‐reinforced by magnesia partially stabilized zirconia (Mg‐PSZ), high specific yield and ultimate collapse strengths as well as a high ductility and an enhanced specific energy absorption were gained. In order to prove the temperature sensitivity of the honeycomb structures, a selected low‐reinforced composite condition was investigated in a pre‐series of quasi‐static compression tests at temperatures in the range between −190 and 150°C. The present study shows that the deformation mechanisms of the TRIP‐matrix composite honeycomb structures can be classified with respect to strain rate and deformation temperature, including the failure characteristics and the strain‐induced α′‐martensite transformation in the austenitic steel matrices ensuring the TRIP effect. The evolution of the α′‐martensite phase content in the central crush zone of the TRIP steel and TRIP‐Matrix Composite honeycombs is demonstrated based on the results of magnetic balance measurements.

Composite materials have been in focus of scientific studies since decades. Metal-matrix composites have received extensive attention in the last years. The combination of a metastable austenitic TRIP-steel with magnesia partially stabilized zirconia is presented in this study. The stress induced martensitic phase transformation in both components leads to advantageous mechanical behavior. Raised compression strength as well as increased specific energy absorption on plastic deformation offers a range of structural and crash-absorption applications. Samples with zirconia additions are reinforced by volume increase during tetragonal-monoclinic phase transformation at compressive strains below 35%. The microstructure and phase evolution of partially stabilized zirconia as well as steel has been investigated by EBSD with purpose to correlate mechanical properties with phase evolution.

The aim of the study is to clarify how far it is possible to describe the mechanical behavior of novel TRIP-Steel/Mg-PSZ composite open-cell foam structures using beam networks generated from random tessellations. Conventional compression tests were performed with various foam samples. Furthermore, the deformation of open-cell composite foams was observed as well by X-ray computed tomography (XCT). Up to a compressive strain of 20% different stages of deformation could be observed. Respective bulk samples were manufactured by powder metallurgy and tested in order to determine the mechanical properties of the bulk material. Numerical simulations were employed based on the suitable modeling of foams exposed to mechanical loading. The predictions of the simulation are compared with the results of the deformation experiments.

Several composites, consisting of a metastable austenitic steel matrix and varying amounts of MgO partially stabilized zirconia particles (Mg-PSZ), were produced through spark plasma sintering (SPS). Compression tests were carried out at room temperature in a wide range of strain rate (4.10(-4) s(-1), 2.10(-3) s(-1), 10(-1) s(-1), 1s(-1), 10(2) s(-1)). In conjunction with subsequent microstructural investigations, the mechanical material behaviour was clarified. All composites showed a good ductility and a high strength. The strength increased with an increase of the ceramic content and with higher strain rates. Both, the martensitic transformation of the steel matrix and of the ceramic particles, could be proved at all strain rates. In this study no significant influence of the strain rate on the amount of transformed ceramic could be detected while the steel matrix showed less alpha'-martensite after compression at rising strain rates. Local material failure occurred around 0.3 true compressive strain depending on the applied strain rate and the amount of the Mg-PSZ powder. The main reason for the damage is the relatively weak ceramic-ceramic interface within the ceramic clusters.

A pure TRIP-steel alloy and a novel zirconia reinforced TRIP-steel matrix composite were implemented in a 2D square-celled honeycomb structure fabricated by a paste extrusion method, respectively. In terms of a series of compression tests in out-of-plane loading direction the buckling and the pronounced strain hardening behavior of the honeycomb structures are described with regard to different material compositions and varied nominal strain rates. Both the compressive flow behavior and the microstructure evolution in the crushed zones are controlled by the rate of formation of strain-induced martensite and the ceramic particle/steel matrix interactions. The insertion of magnesia partially-stabilized zirconia (Mg-PSZ) particles in the austenitic steel matrix cause an increased yield strength and higher compression stresses up to certain deformations degrees. The limited ductility of the composite materials is a consequence of the rearrangement and fracture of zirconia particles initiating cracks and shear bands during deformation. Consistently, the visible strain rate effects on the mechanical responses of the honeycomb structures are similar to AISI 304L austenitic stainless steel specimens in the form of compact rods. However, at high local strain rates generated in drop weight impact tests a micro-inertia factor support the failure behavior of the cellular structures.

steel research internationalVolume 82, Issue 9 p. 979-981 ContentsFree Access steel research int. 9/2011 First published: 05 September 2011 https://doi.org/10.1002/srin.201190017AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Volume82, Issue9September, 2011Pages 979-981 RelatedInformation

Novel composites based on austenitic stainless TRIP steel AISI 304 as a matrix with reinforcements of MgO partially stabilized zirconia (Mg-PSZ) were developed. The presented honeycomb materials were produced by a modified ceramic extrusion technology that is composed of mixing precursor powders with binders, paste preparation and plastic molding, finally debinding and sintering. After processing, sintered products have a global density in the range of 2.7 to 3.0 g cm(-3) and a wall thickness of 260 mu m. These square-celled honeycomb samples are characterized by optical and scanning electron microscopy before and after quasi-static or dynamic compressive deformation, indicating a noticeable deformation-induced martensite formation. The mechanical properties of samples with up to 10% Mg-PSZ are compared with zirconia-free samples in terms of compression tests at strain rates in the range of 10(-3) to 10(2) s(-1). The honeycomb composite materials exhibit an increased work hardening and also extraordinary high specific energy absorption per unit mass and unit volume, respectively. According to improved property-weight-ratio and excellent crashworthiness, such filigree cellular structures can be beneficial as crash absorbers or stiffened core materials in aerospace, railway or automotive applications.

Lightweight linear cellular composite materials on basis of austenite stainless TRIP- (TRansformation Induced Plasticity-) steel as matrix with reinforcements of MgO partially stabilized zirconia (Mg-PSZ) are described. Two-dimensional cellular materials for structural applications are conventionally produced by sheet expansion or corrugation processes. The presented composites are fabricated by a modified ceramic extrusion powder technology. Characterization of the microstructure in as-received and deformed conditions was carried out by optical and scanning electron microscopy. Magnetic balance measurements and electron backscatter diffraction (EBSD) were used to identify the deformation-induced martensite evolution in the cell wall material. The honeycomb composite samples exhibit an increased strain hardening up to a certain engineering compressive strain and an extraordinary high specific energy absorption per unit mass and unit volume, respectively. Based on improved property-to-weight ratio such linear cellular structures will be of interest as crash absorbers or stiffened core materials for aerospace, railway, or automotive applications.