The optimisation of compressor stage aerofoil and fan blade design remains an important area of titanium alloy research and development for aerospace gas turbines. Such research has important implications for critical and sensitive component integrity and efficiency. In particular, a better understanding of how deformation twinning interacts with microstructural features in titanium alloys is required, because such twinning facilitates plastic deformation at a higher strain rate than dislocations. To investigate this behaviour, commercial purity titanium and the titanium alloy Ti–6Al–4V were subjected to ballistic impact testing at room temperature with a high strain rate of 10³s⁻¹. In addition, a detailed analysis was conducted of three manufacturing processes of Ti–6Al–4V (wt. %) that are likely to cause deformation twinning: metallic shot peening, laser shock peening and deep cold rolling. The results presented in this thesis have furthered the understanding of the role of deformation twinning in the plastic deformation of α-phase titanium. Key findings of the research include the characterisation of deformation twinning types and the conditions that favour certain deformation twinning types. From the analysis of the ballistic testing of commercial purity titanium, the first definitive evidence for the existence of {112‾4} twinning as a rare deformation twinning mode at room temperature in coarse-grained commercial purity titanium is presented. In addition, the ballistic testing results of the Ti–6Al–4V alloy highlighted very different deformation twinning characteristics. Commercial purity titanium deformed plastically by a combination of {101‾2} and {112‾1} tensilve twinning and {112‾4} and {112‾2} compression twinning modes. By contrast, the deformation twinning of Ti–6Al–4V was limited to only the {101‾2} and {112‾1} tensile twinning modes. The two tensile deformation twinning types have very different morphologies in equiaxed fine grained Ti–6Al–4V. {112‾1} deformation twins span multiple grain boundaries and {101‾2} deformation twins reorient entire grains to a twinned orientation. This observation provides evidence for whole grain twinning of equiaxed fine grained Ti–6Al–4V by {101‾2} twinning. Grain boundary interactions between various deformation twinning types and alpha phase grain boundaries in commercial purity titanium and Ti–6Al–4V are reported and analysed. In commercial purity titanium {101‾2} as well as other deformation twinning types were observed interacting across alpha phase boundaries and higher angle alpha phase grain boundaries. The analyses of the manufacturing processes of Ti–6Al–4V highlight the very different dislocation and deformation twinning structures in surfaces processed by these techniques. A notable feature of material processed by laser shock peening is the almost complete absence of deformation twinning, contrasting with the frequent observation of extensive deformation twinning observed in the material processed by metallic shot peening and deep cold rolling. Therefore, the findings suggest that there is a strain rate limit above which deformation twinning is suppressed. The implications of this research are that a better understanding of the conditions that that favour certain deformation twinning types or propagation behaviours will enable more accurate plasticity modelling and better alloy design. This is important for the design and the manufacturing of titanium components and the high strain rate deformation to which titanium components in aerospace gas turbines can be subjected because of bird strike, foreign object debris ingestion or fan blade failures.

Title: Investigation of titanium alloys using neutron diffraction Author: Gergely Németh Department / Institute: Department of Physics of Materials Supervisor of the master thesis: prof. RNDr. Kristián Mathis, Ph.D., DrSc., Department of Physics of Materials Abstract: Titanium grade 2 was treated by multiple passes of the continuous equal- channel angular pressing technique (CONFORM ECAP) and, after each pass, additionally by rotary swaging. The residual strain field in samples processed by only CONFORM ECAP was studied by neutron diffraction strain scanning. In order to elucidate the microscopic background and calculate the related residual stress field, the local microstructure was thoroughly investigated by various experimental techniques. The microstructure and the deformation behavior of the rotary swaged samples was studied by transmission electron microscopy and by in-situ neutron diffraction during compression. The results of the analyses indicated that microstructural gradients were present in the material as the result of the inhomogeneous deformation during the CONFORM ECAP treatment. These gradients were identified as the main reason of the presence of residual stress fields. The distributions of stress fields calculated based on microstructural parameters were in correlation with simulation...

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Hoch manganhaltige Eisenbasislegierungen sind bei Raumtemperatur austenitisch und antiferromagnetisch (afm). Dabei besteht die Besonderheit, dass sich durch Legierung die afm Übergangstemperatur (Néeltemperatur) so einstellen lässt, dass sie nahe Raumtemperatur liegt. FeMn-Basislegierungen zeigen in Abhängigkeit von der Zusammensetzung Transformation Induced Plasticity (TRIP) und/oder Twinning Induced Plasticity (TWIP), d.h. die niedrige Stapelfehlerenergie dieser Legierungen führt zu verformungsinduzierter, metastabiler Phasenbildung (TRIP) bzw. zur Bildung von Verformungszwillingen (TWIP) und dadurch zu außerordentlich hoher Duktilität bei gleichzeitig hoher Verfestigung. Darüber hinaus haben FeMn-Basislegierungen einen ausgeprägten Magnetovolumeneffekt und magnetoelastischen Effekt durch magnetische Ordnung. Daher sind die untersuchten FeMnNiCr-Basislegierungen auch prototypisch für afm Élinvarlegierungen. Da Élinvar jedoch für invariable Elastizität steht, bedingt eine Anwendung als temperaturkompensierte Konstantmodullegierungen die Glättung der ausgeprägten magnetischen Anomalien, die industriell noch in keiner Anwendung realisiert wurde. Der Vorteil dies für eine Anwendung zu erreichen, läge in der Unempfindlichkeit feinmechanischer Bauelemente, gegenüber magnetischen Feldern, die bei den industriell verfügbaren ferromagnetischen Élinvarlegierungen nicht gewährleistet ist. Mit Bezug zu feinmechanischen Schwingsystemen spielen dabei neben der Einstellung der magnetoelastischen Eigenschaften die Prozessierbarkeit, Kaltumformbarkeit und Festigkeit sowie deren wechselseitige Beeinflussung eine große Rolle. Die vorliegende Arbeit befasst sich daher mit der Anwendbarkeit der untersuchten FeMnNiCr-Legierungen. Dabei wurden grundlegende Untersuchungen zur Plastizität durchgeführt, die die mechanische Zwillingsbildung in diesen Legierungen charakterisiert und ein Modell der mechanischen Zwillingsbildung bei kleinen plastischen Dehnungen vorschlägt, das eine Abschätzung der Stapelfehlerenergie erlaubt. Die Untersuchung des Antiferromagnetismus umgeformter Proben zeigt das Auftreten thermoremanenter Magnetisierung (TRM), deren Größe mit dem Umformgrad der untersuchten Proben skaliert. Sie wird den durch Umformdefekte erzeugten unkompensierten Momenten in der afm Spinstruktur zugeschrieben. Diese werden durch eine magnetische Feldkühlung magnetisiert und koppeln durch Austauschwechselwirkung an die umgebende antiferromagnetische Matrix unterhalb der Néeltemperatur. Das komplexe thermomagnetische Verhalten der unkompensierten Momente wird experimentell beschrieben und phänomenologisch gedeutet. Die Weiterentwicklung und Bewertung technischer, ausscheidbarer FeMnNiCrBe- und FeMnNiCr(Ti, Al)-Legierungen wird mit Bezug zu den grundlegenden Untersuchungen dargestellt. Es wird gezeigt, dass die neu entwickelten ausscheidbaren FeMnNiCr(Ti, Al)-Legierungen eine vielversprechende Ausgangsbasis darstellen, afm Élinvarlegierungen technisch umzusetzen. / High manganese iron-base alloys are austenitic and antiferromagnetic (afm) at room temperature. By further alloying it is possible to tune the afm transition temperature (Néel temperature) near room temperature. FeMn-base alloys show extraordinary strain hardening as well as ductility because of Transformation Induced Plasticity (TRIP) and/or Twinning Induced Plasticty (TWIP), i.e. in dependence on composition the generally low stacking fault energy in these alloys allows for the mechanically induced formation of metastable phases (TRIP) or deformation twinning (TWIP). Furthermore, magnetic order causes distinct magnetovolume and magnetoelastic effects in these afm FeMn-base alloys. The investigated FeMnNiCr-base alloys are therefore prototypic for afm Élinvar alloys. However, as Élinvar is meant for invariant elasticity, an application as temperature compensated alloy with constant elastic modulus requires the smoothing of the pronounced magnetic anomalies, that is not industrially available yet. The advantage of afm Élinvar alloys in precision mechanics applications, would be their impassiveness with respect to magnetic fields that is not achievable by their ferromagnetic counterparts. For precision components like mechanic oscillators not only the tuning of the magnetoelastic properties but also the processing, cold formability and mechanical properties as well as their interplay have strong influence. Therefore this work addresses the applicability of the studied FeMnNiCr alloys. Elementary investigations on plasticity characterise the occurrence of TWIP in these alloys and propose a modell for deformation twinning at low plastic strains that allows for an estimation of the stacking fault energy. The investigations on the antiferromagnetism of deformed samples show the appearance of thermoremanent magnetisation (TRM). Its magnitude scales with the degree of deformation. The TRM is therefore attributed to uncompensated moments in the afm spin structure due to deformation induced defects. These are magnetised by a magnetic field cooling and couple to the afm matrix by exchange interaction below the Néel temperature. The complex thermomagnetic behaviour of the uncompensated moments is experimentally described and phenomenologically explained. The further development and assessment of engineering-grade pecipitable FeMnNiCrBe and FeMnNiCr(Ti, Al) alloys is presented in relation to the aforementioned elementary investigations. It is shown that the newly developped precipitable FeMnNiCr(Ti, Al) alloys are good candidates for afm Élinvar alloys in application.

Die vorliegende Arbeit zeigt einen Weg, Kupfer und einphasige Kupferlegierungen mit stark verzwillingten Gefügen durch ein technisch relevantes Umformverfahren herzustellen. Der Drahtzug bildet dabei aufgrund seines Spannungszustands und der entsprechenden Texturentwicklung in kubischflächenzentrierten Metallen ein ideales Umformverfahren, um einen Großteil des Gefüges durch mechanische Zwillingsbildung zu verfeinern. Für die Aktivierung der Zwillingsbildung in reinem Kupfer unter den untersuchten Werkstoffvarianten sind Temperaturen nahe der Temperatur des flüssigen Stickstoffs notwendig. Um den Drahtzug in flüssigem Stickstoff umzusetzen, wurden verschiedene Feststoffschmiermittel auf ihre Eignung hin getestet. Die Textur der mit Stickstoffkühlung hergestellten Halbzeuge ist durch eine dreifache Fasertextur bestehend aus <111>-, <001>- und <115>-Fasertexturkomponente charakterisiert. Anhand der strengen Orientierungsverhältnisse konnte der Volumenanteil von verzwillingtem Material bestehend aus Matrixkörnern und Verformungszwillingen auf 71 vol% durch röntgenografische Globaltexturmessungen abgeschätzt werden, wobei das Volumenverhältnis von Zwillingen zu Matrix bei knapp 0,7:1 liegt. Die Zwillinge zeigen eine breite Zwillingslamellenweitenverteilung von wenigen Nanometern bis einige 100 nm im höchstverformten Stadium. Durch die Absenkung der Umformtemperatur und die daraus resultierende Aktivierung der Zwillingsbildung kann die Zugfestigkeit von reinem Kupfer um 140 MPa im Vergleich zu einem ohne Kühlung hergestellten Draht auf 582 MPa erhöht werden. Dabei reduziert sich die elektrische Leitfähigkeit um 6,5% gegenüber einem grobkorngeglühten Kupfer. Eine Absenkung der Stapelfehlerenergie auf 30 mJ/m² in CuAl2 führt zur Aktivierung der mechanischen Zwillingsbildung beim Drahtzug ohne Kühlung. Durch diese Aktivierung der Zwillingsbildung kann bei fortschreitender Verringerung der Stapelfehlerenergie wie in CuAl7 die Zugfestigkeit des umgeformten Drahtes auf weit über 1 GPa erhöht werden. Das entsprechende Gefüge ist dabei ultrafeinkörnig.

Magnesium alloys, owing to their low density and high specific strength, are potential candidates for structural applications in automotive and aerospace industry. While considerable research effort has been devoted in recent years to understand deformation twinning in these alloys and Mg single crystals, only few studies have been conducted on their fracture behavior. This issue assumes importance since some investigations have shown that Mg alloys may possess low fracture toughness (less than Al alloys). Therefore, a combined experimental and numerical study of fracture in Mg single crystals under mode-I loading is performed in this work. The fracture experiments are conducted using three point bend(TPB) specimens inside a scanning electron microscope(SEM) stage equipped with specially designed fixtures. Three crystallographic orientations are considered where c-axis [0001] is along the normal to the flat surface of the notch in the first two orientations, while in the third it is aligned with the notch front. In-situ electron back scattered diffraction (EBSD) observations are made in the region around the notch root to monitor the evolution of tensile twinning on the specimen free surface. Along with EBSD, optical metallography, fractography and surface profilometry are also performed on the specimens to obtain a comprehensive understanding on the micromechanics of fracture in Mg single crystals. From the EBSD data, it is noticed that all the orientations show profuse tensile twinning of {1012}-type. Further, in the first two orientations, basal and prismatic slip traces are identified along with secondary basal slip inside the twins. The growth of the most prominent twin is monitored as a function of load and it is found that its width saturates at around 120 -150 μm, while twins continue to nucleate farther away to accommodate plastic deformation. The 3D nature of twinning is examined by comparing distribution of twin traces and the average twin volume fraction at the free surface and the mid-plane. It is noted that in all the orientations crack initiation occurs before the attainment of peak load and the crack grows stably along twin-matrix interface. Further, zigzaging of the crack path occurs due to deflection of the crack at the twin-twin intersections. It is found that profuse tensile twinning is an important energy dissipating mechanism that enhances the toughness of the material. Indeed, the experimental results show that the energy release rate J versus load histories corroborate with evolution of average twin volume fraction around the notch root. In order to gain further insights on the mechanics of fracture in Mg single crystals, 3D finite element simulations are carried out using a crystal plasticity framework, which includes crystallographic slip and twinning. The predicted load-displacement curves, slip traces and tensile twinning activity from finite element analysis are in good corroboration with the experimental observations. The numerical results are used to understand the 3D nature of the crack tip stress, plastic slip and twin volume fraction distributions near the notch root. The occurrence of tensile twinning in all three orientations is rationalized from the distribution of nor-mal stress ahead of the notch tip. In particular, compressive normal stress beyond the plastic hinge point causes out-of-plane bulging that is accompanied by tensile twinning for the third orientation in which the c-axis is aligned along the specimen thickness. The above behavior emphasizes the importance of tensile twinning since this orientation has relevance to polycrystalline Mg alloys that have a basal texture.

Additive manufacturing (AM), also known as 3D printing, is a concept and method of a manufacturing process that builds a three-dimensional object layer-by-layer. Opposite to the conventional subtractive manufacturing, it conquers various limitations on component design freedom and raises interest in various fields, including aerospace, automotive and medical applications. This thesis studies the mechanical behavior of thin-walled component manufactured by a common AM technique, laser powder bed fusion (LPBF). The studied material is Hastelloy X, which is a Ni-based superalloy, and it is in connection to a component repair application in gas turbines. The influence of microstructure on the deformation mechanisms at elevated temperatures is systematically investigated. This study aims for a fundamental and universal study that can apply to different material grades with FCC crystallographic structure. It is common to find elongated grain and subgrain structure caused by the directional laser energy input in the LPBF process, which is related to the different printing parameters and brands of equipment. This thesis will start with the study of scan rotation effect on stainless steel 316L in an EOS M290 equipment. The statistic texture analysis by using neutron diffraction reveals a clear transition when different level of scan rotation is applied. Scan rotation of 67° is a standard printing parameter with intention to lower anisotropy, yet, the elongated grain and cell structure is still found in the as-built microstructure. Therefore, the anisotropic mechanical behavior study is carried out on the sample printed with scan rotation of 67° in this thesis. Thin-walled effects in LPBF are investigated by studying a group of plate-like HX specimens, with different nominal thicknesses from 4mm down to 1mm, and a reference group of rod-like sample with a diameter of 18mm. A texture similar to Goss texture is found in rod-like sample, and it becomes &lt;011&gt;//BD fiber texture in the 4mm specimen, then it turns to be &lt;001&gt; fiber texture along the transverse direction (TD) in the 1mm specimen. Tensile tests with the strain rate of 10−3 s−1 have been applied to the plate-like specimens from room temperature up to 700 ℃. A degradation of strength is shown when the sample becomes thinner, which is assumed to be due to the overestimated load bearing cross-section since the as-built surface is rough. A cross-section calibration method is proposed by reducing the surface roughness, and a selection of proper roughness parameters is demonstrated with the consideration of the calculated Taylor’s factor and the residual stress. The large thermal gradient during the LPBF process induces high dislocation density and strengthens the material, hence, the LPBF HX exhibits better yield strength than conventionally manufactured, wrought HX, but the work hardening capacity and ductility are sacrificed at the same time. Two types of loading condition reveal the anisotropic mechanical behavior, where the vertical and horizontal tests refer to the loading direction being on the BD and TD respectively. The vertical tests exhibit lower strength but better ductility that is related to the larger lattice rotation observed from the samples with different deformation level. Meanwhile, the elongated grain structure and grain boundary embrittlement are responsible for the low horizontal ductility. A ductile to brittle transition is traced at 700 ℃, so a further study with two different slow strain rates, 10−5 s−1 and 10−6 s−1, are carried out at 700 ℃. Creep damage is shown in the slow strain rates testing. Deformation twinning is found only in the vertical tests where it forms mostly in the twin favorable &lt;111&gt; oriented grain along the LD. The large lattice rotation and the deformation twinning make the vertical ductility remain high level under the slow strain rates. The slow strain rate tensile testing lightens the understanding of creep behavior in LPBF Ni-based superalloys. In summary, this thesis uncovers the tensile behavior of LPBF HX with different variations, including geometry-dependence, temperature-dependence, crystallographic texture-dependence and strain rate-dependence. The generated knowledge will be beneficial to the future study of different mechanical behavior such as fatigue and creep, and it will also enable a more robust design for LPBF applications. / Additiv tillverkning, eller 3D-utskrifter, är tillverkningsmetoder där man skapar ett tredimensionellt objekt genom att tillföra material lager for lager. Till skillnad från konventionella avverkande tillverkningsmetoder elimineras många geometriska begränsningar vilket ger större designfrihet och metoderna har därför väckt stort intresse inom en rad olika områden, inklusive flyg-, fordons- och medicinska tillämpningar. I denna avhandling studeras mekaniska egenskaper hos tunnväggiga komponenter tillverkade med en vanligt förekommande laserbaserad pulverbädds-teknik, laser powder bed fusion (LPBF). Det studerade materialet är Hastelloy X, en Ni-baserad superlegering som är vanligt förekommande for både nytillverkning och reparation av komponenter för gasturbiner. Inverkan av mikrostruktur på deformationsmekanismerna vid förhöjda temperaturer undersöks systematiskt. Detta arbete syftar till att ge grundläggande och generisk kunskap som kan tillämpas på olika materialtyper med en kubiskt tätpackad (FCC) kristallstruktur. Det är vanligt att man hittar en utdragen kornstruktur orsakad av den riktade tillförseln av laserenergi i LPBF-processen, vilket kan relateras till olika processparametrar och kan variera mellan utrustningar frän olika leverantörer. Denna avhandling inleds med studien av effekten av scanningsstrategi vid tillverkning av rostfritt stål 316L i en EOS M290-utrustning. En statistisk texturanalys med hjälp av neutrondiffraktion påvisar en tydlig övergång mellan olika mikrostrukturer när olika scanningsstrategier tillämpas. En scanningsrotation på 67 mellan varje lager är en typisk standardinställning med avsikt att sanka anisotropin i materialet, dock finns den utdragna kornstrukturen oftast kvar. I denna avhandling studeras därför de anisotropa egenskaperna hos material tillverkade med 67 scanningsrotation. Effekten av tunnväggiga strukturer i LPBF undersöks genom att studera en uppsättning platta HX-prover, med olika nominella tjocklekar från 4 mm ner till 1 mm, samt en referensgrupp med cylindriska prov med en diameter på 18 mm. Kristallografisk textur som liknar den av Goss-typ återfinns i de cylindriska proverna vilket gradvis övergår från en fibertextur med &lt;011&gt; i byggriktningen for 4mm-proven till en fibertextur med &lt;001&gt; i tvärriktningen for 1mm-proven. Dragprovning med en töjningshastighet på 10−3 s−1 har utförts på de platta provstavarna från rumstemperatur upp till 700 ℃. En sänkning av styrkan uppvisas när proven blir tunnare, vilket kan antas bero på att det lastbarande tvärsnittet överskattas på grund av den grova ytan. En metod för tvärsnittskalibrering föreslås genom att kompensera for ytråheten, och valet av lämplig ytfinhetsparameter motiveras med hänsyn till den beräknade Taylor-faktorn och förekomsten av restspänningar. Den stora termiska gradienten som uppstår for LPBF-processen inducerar en hög dislokationstäthet vilket höjer materialets styrka och följaktligen uppvisar LPBF HX högre sträckgräns an konventionellt tillverkad, smidda HX, men förmågan till deformationshårdnande samt duktiliteten i materialet sänks samtidigt. Tester utförda i två olika belastningsriktningar, vertikalt respektive horisontellt mot byggriktningen, demonstrerar det anisotropiska mekaniska beteendet. De vertikala testerna uppvisar lägre hållfasthet men bättre duktilitet vilket kan relateras till en större benägenhet for kristallstukturen att rotera när deformationsgraden ökar. Samtidigt är den utdragna kronstukturen ansvarig for den lägre duktiliteten for de horisontella proverna. En övergång från ett duktilt till ett mer sprött beteende noterades vid 700 ℃, och därför initierades ytterligare en studie där tester med två lägre töjningshastigheter, 10−5 s−1 och 10−6 s−1, utfördes vid 700 ℃. Det kan noteras att krypskador återfinns i tester med en långsam deformationshastighet och deformationstvillingar uppstår endast i de vertikala provstavarna där det främst bildas tvillingar i korn orienterade med &lt;111&gt; riktningen längs belastningsriktningen. Den stora förmågan till rotation i kristallstrukturen och deformationstvillingarna bidrar till att den vertikala duktiliteten förblir hög även i testerna med en låg deformationshastighet. Testerna med en långsam draghastighet bidrar därför till en bättre förståelse av krypbeteendet i LPBF Nibaserade superlegeringar. Sammanfattningsvis så bidrar denna avhandling till bättre förståelse av de mekaniska egenskaperna hos LPBF HX i olika utföranden och förhållanden, inklusive geometriberoende, temperaturberoende, deformationshastighetsberoende samt inverkan av kristallografisk textur. Den genererade kunskapen kommer att vara till stor nytta vid fortsatta studier av olika mekaniska egenskaper som utmattning och kryp, samt bidrar till att möjliggöra en mer robust design for LPBF-tillämpningar.

Die vorliegende Arbeit zeigt einen Weg, Kupfer und einphasige Kupferlegierungen mit stark verzwillingten Gefügen durch ein technisch relevantes Umformverfahren herzustellen. Der Drahtzug bildet dabei aufgrund seines Spannungszustands und der entsprechenden Texturentwicklung in kubischflächenzentrierten Metallen ein ideales Umformverfahren, um einen Großteil des Gefüges durch mechanische Zwillingsbildung zu verfeinern. Für die Aktivierung der Zwillingsbildung in reinem Kupfer unter den untersuchten Werkstoffvarianten sind Temperaturen nahe der Temperatur des flüssigen Stickstoffs notwendig. Um den Drahtzug in flüssigem Stickstoff umzusetzen, wurden verschiedene Feststoffschmiermittel auf ihre Eignung hin getestet. Die Textur der mit Stickstoffkühlung hergestellten Halbzeuge ist durch eine dreifache Fasertextur bestehend aus <111>-, <001>- und <115>-Fasertexturkomponente charakterisiert. Anhand der strengen Orientierungsverhältnisse konnte der Volumenanteil von verzwillingtem Material bestehend aus Matrixkörnern und Verformungszwillingen auf 71 vol% durch röntgenografische Globaltexturmessungen abgeschätzt werden, wobei das Volumenverhältnis von Zwillingen zu Matrix bei knapp 0,7:1 liegt. Die Zwillinge zeigen eine breite Zwillingslamellenweitenverteilung von wenigen Nanometern bis einige 100 nm im höchstverformten Stadium. Durch die Absenkung der Umformtemperatur und die daraus resultierende Aktivierung der Zwillingsbildung kann die Zugfestigkeit von reinem Kupfer um 140 MPa im Vergleich zu einem ohne Kühlung hergestellten Draht auf 582 MPa erhöht werden. Dabei reduziert sich die elektrische Leitfähigkeit um 6,5% gegenüber einem grobkorngeglühten Kupfer. Eine Absenkung der Stapelfehlerenergie auf 30 mJ/m² in CuAl2 führt zur Aktivierung der mechanischen Zwillingsbildung beim Drahtzug ohne Kühlung. Durch diese Aktivierung der Zwillingsbildung kann bei fortschreitender Verringerung der Stapelfehlerenergie wie in CuAl7 die Zugfestigkeit des umgeformten Drahtes auf weit über 1 GPa erhöht werden. Das entsprechende Gefüge ist dabei ultrafeinkörnig.

Sanicro 28 and Alloy 625 are corrosion-resistant nickel alloys with a fully austenitic structure and a very low carbon content, which means they are both well suited for cold working. Since the millennium shift deformation twinning has been a live research issue as it enhances strength and ductility simultaneously. As nickel has been pointed out as a high stacking fault energy element and deformation twinning should be promoted by a low stacking fault energy level they have been considered as opposite poles. Nonetheless, it is known since long that deformation twins can emerge in high stacking fault face centred cubic elements at low temperatures. In this thesis, we have investigated deformation twinning behaviour in corrosion-resistant nickel alloys. The objective is trying to distinguish between deformation twinning in TWIP steel and corrosion resistant nickel alloys regarding for instance size and bundles. Interrupted uniaxial tensile tests have been performed at several cold working temperatures for the alloys: Sanicro 28 (31% nickel) and Alloy 625 (61% nickel). The microstructure has been characterized in homogeneous deformation volume, by scanning electron microscopy electron backscattering diffraction and electron channelling contrast imaging, transmission electron microscopy and X-ray diffraction. In one investigation fracture behaviour has also been studied with secondary electrons. Ab initio calculations, crystal plasticity modelling and DAMASK simulations have been performed to support emphasizing active deformation mechanisms. It has been revealed that deformation twinning can occur in high Ni alloys. With increasing deformation twinning levels, the diffuse necking decreases. Ab initio calculations indicates that the initiation of deformation twins cannot be determined solely by the stacking fault energy. Distinct features were discovered at low strains that could be rejected from being neither deformation twins nor stacking faults. Level of texture increases with increasing strain and decreasing temperature and the texture modes are changed with decreasing temperature. / Sanicro 28 och Alloy 625 är två legeringar med ett imponerande korrosionsmotstånd, ett lågt kolinnehåll och en helaustenitisk struktur. Det gör dem väl lämpade för kallbearbetning. Sedan millenieskiftet har aktivten varit mycket hög inom forskningsområdet: deformationstvillingar. TWIP (twinning induced plasticity)-effekten har den så eftertraktade egenskapen att både styrkan och duktiliteten förbättras på samma gång. Eftersom nickel har en hög staplingsfelsenergi och TWIP-effekten har uppmätts/beräknats till att aktiveras vid ett snävt och lågt värde, för densamma, har ett ökande nickelinnehåll och TWIP-effekten setts som direkta motpoler. Trots det, har man länge vetat om att deformationstvillingar också framträder, om än, vid låga temperaturer, i legeringar med kubiskt ytcentrerat gitter och hög staplingsfelsenergi. I den här avhandlingen har vi undersökt hur deformationstvillingar utvecklas, om de ens kan bildas i korrosionsbeständiga legeringar med ett högt nickelinnehåll. Målet är att se om det finns några större skillnader i tvillingbeteendet i TWIP-stål i jämförelse med korrosionsbeständiga legeringar med ett högt nickelinnehåll. Några egenskaper vi har tänkt att undersöka är: tjocklek på tvillingarna och om tvillingarna bildas i grupper. Vi hoppas på så sätt kunna svara på den övergripande forskningsfrågan: är det möjligt att designa ett rostfritt TWIP-stål, baserat på det vanligaste legeringssystemet för rostfria austenitiska stål, nämligen: järn-krom-nickel? Enaxliga dragprov har genomförts vid flera kallbearbetningstemperatuer; de har utförts både till brott och till förutbestämda töjningsnivåer. Legeringarna som har testats är: Sanicro 28 (31% nickel) och Alloy 625 (61% nickel). Mikrostrukturen har framför allt karakteriserats i material uttaget från volym där deformationen har varit homogen. De analysmetoder som har använts är: svepelektronmikroskopi, mer specifikt: ”electron backscatter diffraction” och ”electron channelling contrast imaging”. Transmissionselektronmikroskop och röntgendiffraktion har också använts. I en undersökning har också brottbeteende studerats med hjälp av "secondary electrons". Ab initioberäkningar, modellering av kristallplasticitet och materialbeteende med hjälp av DAMASK har också utförts för att kunna se vilka deformationsmekanismer som är aktiva. Vi upptäckte att deformationstvillingar faktiskt kan bildas i korrosionsbeständiga legeringar med ett högt nickelinnehåll. Den diffusa midjebildningen minskar på samma gång som andelelen deformationstvillingar ökar. Ab initioberäkningarnas resultat indikerar också på att deformationstvillingarnas inträde inte enbart kan bestämmas med staplingsfelsenergin. Tydliga mikrostrukturmönster upptäcktes med hjälp av transmissionsmiroskop och vid låga töjningsnivåer. De mikrostrukturmönstren kunde avfärdas från att vara både deformationstvillingar och staplingsfel. Texturnivån ökar med ökande töjningsnivå och sjunkande temperatur. Typen av textur förändras också med sjunkande temperatur. / <p>Funding agency: Tube division, Alleima AB</p>

In the present work, deformation behavior, texture and microstructure evolution of commercially pure titanium (CP Ti) are investigated by electron backscattered diffraction (EBSD) after compression tests at elevated temperatures. By analysing work hardening rate vs. flow stress, the deformation behaviour can be divided into three groups, viz. three-stage work hardening, two-stage work hardening and flow softening. A new deformation condition map is presented, dividing the deformation behavior of CP Ti into three distinct zones which can be separated by two distinct values of the Zener-Hollomon parameter. The deformed microstructures reveal that dynamic recovery is the dominant deformation mechanism for CP Ti during hot working. It is the first time that the Schmid factor and pole figures are used to analyse how the individual slip systems activate and how their activities evolve under various deformation conditions. Two constitutive equations are proposed in this work, one is for single peak dynamic recrystallization (DRX), the other is specially for CP Ti deformed during hot working. After the hot compression tests, some stress-strain curves show a single peak, leading to the motivation of setting up a DRX model. However, the examinations of EBSD maps and metallography evidently show that the deformation mechanism is dynamic recovery rather than DRX. Then, the second model is set up. The influence of the deformation conditions on grain size, texture and deformation twinning is systematically investigated. The results show that {10-12} twinning only occurs at the early stage of deformation. As the strain increases, the {10-12} twinning is suppressed while {10- 11} twinning appears. Three peaks are found in the misorientation frequency-distribution corresponding to basal fiber texture, {10-11} and {10-12} twinning, respectively. A logZ-value of 13 is found to be critical for both the onset of {10-11} compressive twinning and the break point for the subgrain size. The presence of {10-11} twinning is the key factor for effectively reducing the deformed grain size. The percentage of low angle grain boundaries decreases with increasing Z-parameter, falling into a region separated by two parallel lines with a common slope and 10% displacement. After deformation, three texture components can be found, one close to the compression direction, CD, one 10~30° to CD and another 45° to CD. / QC 20100819