Ni-based superalloys play a crucial role in various high-temperature applications, where their exceptional mechanical properties and resistance to corrosion are highly desirable. However, their response to low temperatures, especially concerning strain hardening, microstructural evolution, and deformation mechanisms, requires further scrutiny. In this study, we investigate the influence of temperature on the stacking fault energy (SFE) and its implications on deformation twinning in Alloy 625. Uniaxial tensile tests are performed at 298 K, 173 K and 77 K. The study reveals a notable increase in strain hardening at intermediate strain levels, suggesting the activation of a secondary deformation mechanism. To gain deeper insights, crystal plasticity-based simulations using the DAMASK framework are employed, complementing the experimental outcomes. Deformation twins are consistently observed at all temperatures, albeit with a small volume fraction and thickness. The critical strain for twinning decreased with decreasing temperature. Based on the numerous literature studies, experimental and computational observations, the SFE of the material is estimated to be constant over the studied temperature range.

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.

Deformation behaviour in the Nickel-base superalloy 625 has been studied by tensile testing at four temperatures: 295, 223, 173 and 77 K. The microstructure has been investigated using TEM, FIB-SEM, EBSD and ECCI techniques. Deformation in the alloy turns out to be a competitive course of events between at least two deformation mechanisms, namely dislocation slip and deformation twinning. Slip is the predominant deformation mechanism at higher temperatures. While at 77 K, deformation induced twinning gives an extra degree of freedom as one of the main deformation mechanisms, i.e., the material shows a twin induced plasticity, TWIP, behaviour. Ab initio calculations indicate that the influence of cryogenic/sub-zero temperatures on the stacking fault energy of this alloy can be limited and that the formation of deformation twins cannot be determined solely by the stacking fault energy. The results implies that it is the critical strain and strain hardening rate that influences the deformation twinning onset and twinning rate.

The influence of nitrogen on the mechanical properties of two high Ni containing advanced austenitic stainless steels with low stacking fault energies is investigated. The results show that increase of nitrogen content greatly increases both strength and elongation of the steel at the same time. At the cryogenic temperature, the steels show a twin induced plasticity behavior. Ab initio calculations indicate that the increase of nitrogen slightly increases the stacking fault energy and consequently the critical shear stress for twin initiation in the steel. However, addition of nitrogen significantly increases the flow stress. This leads to a smaller critical strain for twin initiation and promotes deformation twinning in the high nitrogen steel. This is confirmed by the microstructure investigation. Deformation in steels is a competitive process between slip and twinning. Dislocation slip is dominant at low strain range, but formation of stacking fault and twinning become important in the later stages of deformation. At cryogenic temperature, it is mainly deformation twinning. The influence of nitrogen addition on magnetic property and its effect on deformation twinning are also discussed. The present study increases the understanding for the development of high-performance and low-cost advanced austenitic stainless steels.