Zbigniew Pakiela

Influence of long-range ordering on mechanical properties of nanocrystalline Ni3Al

Abstract The influence of a grain size and long-range ordering on mechanical properties of the intermetallic compound Ni 3 Al at room temperature was investigated. It has been found that the maximum values of strength and ductility are observed in the material in a completely disordered nanocrystalline state.

Use of micro tensile test samples in determining the remnant life of pressure vessel steels

The aim of this collaborative study was to measure mechanical properties of 14MoV67-3 steel taken from small sections of material machined in-situ from an operating high pressure collector pipe after different operating lifetimes (from 0h to 186 000h) at elevated temperatures (540°C). Conventional methods of measuring mechanical properties of materials, such as the uniaxial tensile test require relatively large test samples. This can create difficulties when the amount of material available for testing is limited. One way of measuring mechanical properties from small quantities of material is using micro tensile test samples. In this work, micro-samples with a total length of 7.22mm were used. Digital Image Correlation method (DIC) was employed for the strain measurements in a uniaxial tensile test. This paper shows that there is measurable difference in the yield, ultimate tensile strength and elongation to failure as a function of the plant operating conditions. This work demonstrates, therefore, a ‘semi-invasive’ method of determining uniaxial stress-strain behaviour from plant components.

Changes in distributions of grain boundary diffusion properties after grain growth in austenitic steel

Abstract The paper deals with changes in the grain boundary (GB) properties caused by grain growth in type 316L austenitic steel. Samples made from rods after 30% cold working were annealed at 900°C, 950°C, 1000°C and 1200°C, which resulted in mean grain sizes of 11 μm, 12 μm, 46 μm and 125 μm respectively. The annealed samples underwent tensile tests at different temperatures and, subsequently, the number of GBs free from dislocations was determined by means of transmission electron microscopy. It was found that the ability of GBs to annihilate dislocations decreases with increasing mean grain size. It was further found that, in materials with the same mean grain sizes obtained as a result of different recrystallization times and temperatures, the properties of the GBs did not differ.

Grain Refinement and Mechanical Properties of Nickel Subjected to Severe Plastic Deformation

The structure and mechanical properties of Ni subjected to sever e plastic deformation have been investigated. By combining ECAP and cold rolling, Ni of grain size about 300 nm was obtained. The material displayed an ultimate tensile stress of m ore than 1000 MPa and considerable ductility. Annealing at 200 C decreased the strength by 15% but increased the ductility. The a transition from brittle fracture of coarse-grained Ni to quasitough one in fine grained Ni at room temperature was observed. It is proposed that the high strength and pl asticity of the ultra fine grained nickel is related to a deformation mechanism involving grai n boundary sliding and grain rotation. High strength Ni with good ductility may have prospect iv practical applications, particularly for micro-systems. Introduction Recently the so-called paradox of high strength and ductility of metals subjected to severe plastic deformation (SPD) has been investigated extensively [1]. Such metals may alos display a high fatigue limit, so, potentially, that they may find applications i heavy duty situations such as micro systems. The purpose of the current work was to optimise the prepara tion method of ultra fine grained nickel to achieve the optimum combination of microstructural s tability, high strength and plasticity. Experimental procedures A pure Ni (99.99%) rod of diameter 20 mm was used as the as initial material. SPD at room temperature was achieved by equal channel angular pressing (E CAP) with a 90 inclination angle and 8 passes. The strain degree after this treatment was about e ECAP = 8.8 [2]. After ECAP the samples were subjected to cold rolling (CR) to a 90% reduction. The a dditional strain degree e CR was 2.7. So the accumulated strain degree was e = e ECAP + eCR = 13.5. This processing produced a bar 0.2 mm in thick, about 12 mm wide and 200 mm long. The microstructure of th material was investigated by means of a transmission electron microscope J EM-100 operating at 100 kV. The mechanical tests were performed with non-standard samples wi th a gauge section 0.2 1.5 2.5 mm, using a IMMASH machine at an initial strain rate of 2 10s. The surface relief was investigated by means of the LEO 1530 FEG-SEM at an accelerating voltage of 1 kV . Experimental results Electron microscopy. The cold rolling of ECAP samples caused intense refining of the s tructure (Fig. 1a). Elongated grains 0.3 μm diameter and 0.5 μm length extended in the direction of rolling were observed. Dislocations inside the grains were observed, arranged in walls and cells. A peculiarity of the structure obtained is the absence of extinction ontours on boundaries of the majority of grains. Such a grain boundary state corresponds to a high degre e of nonequilibrium of Solid State Phenomena Online: 2003-06-20 ISSN: 1662-9779, Vol. 94, pp 51-54 doi:10.4028/www.scientific.net/SSP.94.51

Fatigue Crack Growth Rates and Tensile Strength of Titanium Produced by Means of Selective Laser Melting

Mini-samples technique was utilized to determine mechanical properties of technically pure titanium produced by means of selective laser melting (SLM). Full-field digital image correlation (DIC) measurements and inverse method were applied for crack tip position and stress intensity factors calculations in the case of fatigue crack growth rate tests. DIC was also used for strain measurement during tensile tests on sub sized samples. There was studied the influence of samples orientation on the mechanical properties of mini-samples. Samples were cut out from rectangular cubes and were oriented with 0°, 45° or 90° angle to the direction of laser beam travel. There were also tested samples directly produced via SLM. Additionally microstructure observations were performed to verify the quality of SLM processed materials and explain mechanical properties variations.

Mechanical properties and deformation behaviour of ultra-fine grained nickel

The mechanical properties and deformation behaviour of ultra-fine grained (UFG) Ni subjected to severe plastic deformation (SPD) were investigated. The UFG Ni characterized possessed an homogeneous structure with a grain size of 120 nm and high angle nonequilibrium grain boundaries. This nickel possessed an ultimate tensile stress of 1270 MPa. Investigation of the deformation relief on the polished surface of the UFG Ni sample by HRSEM revealed grain boundaries (GBs) steps already at small degrees of tension that testifying to the involvement of grain boundaries in the deformation process at room temperature. After significant tensile strain the deformation relief revealed a network of crossed shear bands oriented at an angle in the range 35°- 45° to the axis of tensile deformation. Shear bands propagated along GBs parallel to the plane of maximal shear stress. The formation of shear bands occurred due to a strong shift and rotation of grain groups. This led to a deformation mechanism involving collective relative displacement of grain groups, with extensive grain boundary sliding at room temperature. Over a length of a few micrometers the material can be regarded as uniform and therefore the local strain distribution becomes more uniform than in coarse-grained materials. It is plausible that this mode of deformation may contribute to the enhanced ductility. The deformation behaviour of Ni having different grain sizes and various grain boundary states are also considered. The Opportunity to achieve a combination high strength and good ductility by control of the microstructure of in metals and alloys opens perspectives industrial applications, particularly, for mirco-systems and for items of complex geometry to be produced by superplastic forming.

TEM in situ annealing of severely deformed Ni3Al intermetallic compound

Abstract Severe plastic deformation by torsion was applied to refine the structure of Ni3Al intermetallic compound. Transmission electron microscopy (TEM) observations revealed that in as-deformed state the intermetallic crystal structure was disordered. In order to investigate the dynamics of the transition from the disordered to the L12 lattice structure, in situ TEM annealing experiments were performed. These experiments and studies of the bulk material have revealed details of the processes taking place during annealing of severely deformed Ni3Al.

Effect of grain boundary phenomena on the softening of 316L austenitic steel at high temperature

Abstract In this paper we present results of tests concerning changes in the fraction of grain boundaries (GBs) free from trapped lattice dislocations (TLDs) as a function of the heating temperature. 316L austenitic stainless steel was used. The tests were performed both on thin foils and on bulk samples. The hardening coefficient for uniaxial tension and low cycle fatigue (LCF) was tested. The relationship between the hardening of the tested material and the presence of TLDs on boundaries was investigated. It was found that the kinetics of trapped lattice dislocation spreading in GBs in thin foils, measured in in situ experiments, is controlled by boundary diffusion. In bulk samples, in ex-post investigations the kinetics of dislocation trapping by GBs can control whether or not the boundaries are free from dislocations, after heating the sample at a given temperature. During uniaxial tension at a strain rate ϵ dot = 2.8 × 10 −5 s −1 an increase in the fraction of GBs free from dislocations is directly proportional to the decrease in the value of the hardening coefficient as a function of temperature. This means that, under the described conditions of straining, GB migration and GB sliding play a decisive role in the softening of the tested steel. When the strain rate ϵ dot is 2.8 × 10−4 s−1, GB sliding and GB migration play a smaller role in the softening of the tested steel. During LCF, changes in the hardening coefficient are strongly affected by strain aging. The effect of strain aging of the tested steel on the changes in the hardening coefficient as a function of strain temperature is much more intensive during LCF than during uniaxial tension.

Influence of Polivinylalcohol Cryogel Material Model in FEM Simulations on Deformation of LV Phantom

One of the available tools for validation of strain imaging methods are physical phantoms, most frequently produced of polivinylalcohol cryogel (PVA). This material was often assumed to exhibit elastic properties, but it has more complex nature. In this work we examine the influence of the applied material model – elastic vs hyperelastic – on the strains within the numerical model of the phantom obtained from FEM. This influence appeared significant – hyperelastic model provides lower strain contrasts and also the ratios between radial, circumferential and longitudinal strains differ for both models.

Fatigue Crack Growth in Fe Mini-Samples Consolidated by Means of Impact Sintering

The paper presents the results of fatigue crack growth rate test of iron sinters. The samples were produced by means of the impact sintering. Applied production method allowed to obtain dense sinters with fine grain size resulted from large shear strains. Due to the limited size of the final products the mechanical tests were carried out in mini-samples. Optical, non-contact method of displacement measurement, namely Digital Image Correlation (DIC), was applied for determination of the displacement fields near the crack tip at the maximal force of selected loading cycles. The results of DIC measurement were utilized in the calculations of stress intensity factors and crack tip coordinates by means of the iterative procedure based on inverse method. These parameters were used for measuring crack development rate. There were investigated two types of materials produced by the consolidation of two different kinds of Fe powders and sintered in different temperatures. The results of crack growth rate tests were correlated with the microstructure changes, as well as yield and ultimate strength of the materials.