Interaction Mechanisms of Edge Dislocations with Obstacles in Fe and Metal Alloys

The need for energy and electricity in the modern society places large interest in producing power in more efficient and environmentally friendly ways. Most of the new concepts also need new or improved materials. One key component of the power plants is the structural materials. Both current and future power plants are and will be built from some kinds of steel, which has shown very good properties. Alloying and choice of manufacturing technique have been shown to change the properties of steel and even better materials have been obtained. Conventionally, these improvements have been studied by conducting experiments on the possible materials, but recently computer simulations have started to be an essential tool in the search for better materials. The mechanical properties of materials are known to be determined by the movement of dislocations. If the dislocation movement is hindered, the material will become harder, but also more brittle. Previous computational studies have investigated the effect of voids, bubbles and other dislocation structures on the movement of dislocations. However, the effect of non-coherent obstacles, man-made or irradiation induced, have received much less attention. To address this lack of data, we have investigated the effect of carbides, to represent non-coherent obstacles, on the dislocation movement. In this study we investigated different carbides and compared them with other defect structures, voids and fixed atoms. The effect of size and temperature were also determined. We obtained two parameters for each obstacle, the needed unpinning stress and the unpinning mechanism, that can be used in other simulation methods to be able to investigate larger systems. We found that the nanostructure of the obstacle can drastically change the needed unpinning stress. We also found that the obstacles with same unpinning mechanism showed similar unpinning stresses, but to obtain the subtle differences we need to consider the nanostructure. A comparison of spherical and rod shaped obstacles showed that the surface curvature will be an important factor in the unpinning event. Recently, a new class of metal alloys have been found, where the concentrations of the alloying elements are in large or equal fractions. We wanted to investigate the possibility to use these equiatomic multicomponent alloys in environments where radiation is present. These alloys have been shown to have very promising properties, but there were no data on the irradiation response of these alloys. We studied two equiatomic multicomponent alloys, two-element and three-element alloys, and compared them with the single element specimen. We found a major reduction of defect build-up in the alloys compared to the single element sample, in agreement with experiments. We found one of the key mechanisms of this reduction to be the reduced dislocation mobility in the alloys.För att möta det växande energibehovet måste vi hitta nya och bättre produktionsmetoder, för att kunna göra produktionen på ett effektivt och miljövänligt sätt. Det finns många nya metoder och koncept på hur man kunde producera energin, men flesta av dem kräver nya eller förbättrade material för att fungera korrekt. En av nyckelfaktorerna i produktionen är de strukturella materialen, vilka utsätts för t.ex. frätande ämnen eller strålning. Strålning förekommer förutom i kärnkraftverk också i rymden, vilket gör att de nya materialen kan användas i andra tillämpningar än energiproduktion. En av de viktigaste egenskaperna hos strukturella material är de mekaniska egenskaperna och dessa beror på hur vissa fel i materialen, sk. dislokationer, beter sig. Man vet att ifall dislokationerna kan röra sig fritt, så är materialet för det mesta mjukt, men ifall dislokationerna hindras av andra fel i materialet blir det hårdare. Att materialet blir hårdare är i vissa fall en bra egenskap, men det kan även leda till att materialet blir skört, vilket är en icke önskad bieffekt. I den här avhandlingen studerade vi m.h.a. datorsimuleringar hur dislokationernas rörelse hindras av olika sorters fel i materialen. Vi undersökte effekten av fel som finns i material samt sådana fel eller orenheter som man endera tillsatt för att uppnå bättre egenskaper eller sådana som uppkommer p.g.a. strålning. I den här avhandlingen studerades hur olika sorters fel påverkar hur mycket dislokationernas rörelse hindras och hur det här sker på atomnivå. Förutom att vi bestämde styrkan av olika fel, ville vi undersöka hur en ny klass av material beter sig ifall det utsätts för strålning. Den här nya klassen av material skiljer sig från normala legeringar, genom att alla olika grundämnen finns i stora andelar, s.k. högentropi legeringar (eng. High Entropy Alloys, HEA). Vi undersökte en underklass till dessa som bestod av två eller tre grundämnen i lika stora andelar (eng. Equiatomic multicomponent alloys) och jämförde dem med det rena grundämnet. I den här avhandlingen fann vi att de olika felen i material vill hindra dislokationer med olika styrka och att olika hinder har olika mekanismer för att passera hindret. Vi fann även att temperatur, struktur, storlek och grundämnesinnehåll kan ändra på styrkan och mekanismen. Vi undersökte även icke sfäriska hinder och visade att formen på hindret också påverkar styrkan, även om hindret var lika stort. Undersökningen av de nya multikomponent legeringarna visade en drastisk minskning i antalet uppkomna defekter i legeringarna om man jämför dem med det rena materialet. Vi fann att orsaken till minskningen var relaterad till dislokationernas rörelse. Vi såg även att legeringen med tre grundämnen visade mindre antal fel än den med två, vilket indikerar att ytterligare förbättringar kan möjligtvis uppnås genom att tillsätta flera grundämnen eller genom att inte ha lika koncentrationer av alla grundämnen

Cascade overlap with vacancy-type defects in Fe

In order to understand the effect of irradiation on the material properties, we need to look into the atomistic evolution of the system during the recoil event. The nanoscale features formed due to irradiation will ultimately affect the macroscopic properties of the material. The defect production in pristine materials have been subject to investigation previously, but as the dose increases, overlap will start to happen. This effect of cascades overlapping with pre-existing debris has only recently been touched, and mainly been investigated for interstitial-type defects. We focus on vacancy-type defect clusters in BCC Fe and start a recoil event in their near vicinity. The final defect number as well as the transformation of the defect clusters are investigated, and their behaviour is related to the distance between the defect and the cascade centre. We found that for vacancy-type defects, the suppression of defect production is not as strong as previously observed for interstitial-type defects. The cascade-induced transformation, such as change in Burgers vector or creation of dislocations, was determined for all initial defect structures.Peer reviewe

Atomistic Study of Irradiation-Induced Plastic and Lattice Strain in Tungsten

We demonstrate a practical way to perform decomposition of the elasto-plastic deformation directly from atomistic simulation snapshots. Through molecular dynamics simulations on a large single crystal, we elucidate the intricate process of converting plastic strain, atomic strain, and rigid rotation during irradiation. Our study highlights how prismatic dislocation loops act as initiators of plastic strain effects in heavily irradiated metals, resulting in experimentally measurable alterations in lattice strain. We show the onset of plastic strain starts to emerge at high dose, leading to the spontaneous emergence of dislocation creep and irradiation-induced lattice swelling. This phenomenon arises from the agglomeration of dislocation loops into a dislocation network. Furthermore, our numerical framework enables us to categorize the plastic transformation into two distinct types: pure slip events and slip events accompanied by lattice swelling. The latter type is particularly responsible for the observed divergence in interstitial and vacancy counts, and also impacts the behavior of dislocations, potentially activating non-conventional slip systems

Temperature effect on irradiation damage in equiatomic multi-component alloys

Multiprincipally designed concentrated solid solution alloys, such as high entropy alloys (HEA) and equiatomic multi-component alloys (EAMC-alloys) have shown much promise for use as structural components in future nuclear energy production concepts. The irradiation tolerance in these novel alloys has been shown to be superior to that in more conventional metals used in current nuclear reactors. However, studies involving irradiation of HEAs and EAMC-alloys have usually been performed at room temperature. Hence, in this study the irradiation damage is investigated computationally in two different Ni-based EAMC-alloys and pure Ni at four different temperatures, ranging from 138 K to 800 K. The irradiation damage was studied by analyzing point defects, defect cluster sizes and dislocation networks in the materials. Dislocation loop mobility calculations were performed to help understanding the formation of different dislocation networks in the irradiated materials. Utilizing the knowledge of the depth distribution of damage, and using simulations of Rutherford backscattering in channeling conditions (RBS/c), we can relate our results to experimental data. The main findings are that the alloys have superior irradiation tolerance at all temperatures as compared to pure Ni, and that the damage is reduced in all materials with an increase in temperature.Peer reviewe

Damage buildup and edge dislocation mobility in equiatomic multicomponent alloys

Volume: 393A new class of single phase metal alloys of equal atomic concentrations has shown very promising mechanical properties and good corrosion resistance. Moreover, a significant reduction in damage accumulation during prolonged irradiation has also been observed in these equiatomic multicomponent alloys. A comparison of elemental Ni with the two component NiFe- and the three component NiCoCr-alloy showed a substantial reduction in damage in both alloys, and an even larger difference was seen if only larger clusters were considered. One of the factors limiting the damage build-up in the alloys compared to the elemental material was seen to be dislocation mobility (Granberg et al., 2016). In this Article, we focus on a more thorough investigation of the mobility of edge dislocations in different cases of the Ni-, NiFe- and NiCoCr-samples. We find that even though the saturated amount of defects in the alloys is lower than in elemental Ni, the defect buildup in the early stages is faster in the alloys. We also find that the dislocation mobility in NiFe is lower than in Ni, at low stresses, and that the onset stress in NiFe is higher than in Ni. The same phenomenon was seen in comparison between NiFe and NiCoCr, since the three component alloy had lower dislocation mobility and higher onset stress. The dislocation velocity in elemental Ni plateaued out just under the forbidden velocity, whereas the alloys showed a more complex behaviour. (C) 2016 Published by Elsevier B.V.Peer reviewe

Estimate for thermal diffusivity in highly irradiated tungsten using molecular dynamics simulation

The changing thermal conductivity of an irradiated material is among the principal design considerations for any nuclear reactor, but at present few models are capable of predicting these changes starting from an arbitrary atomistic model. Here we present a simple model for computing the thermal diffusivity of tungsten, based on the conductivity of the perfect crystal and resistivity per Frenkel pair, and dividing a simulation into perfect and athermal regions statistically. This is applied to highly irradiated microstructures simulated with molecular dynamics. A comparison to experiments shows that simulations closely track observed thermal diffusivity over a range of doses from the dilute limit of a few Frenkel pairs to the high-dose saturation limit at three displacements per atom (dpa).Peer reviewe

Effect of surface morphology on Tungsten sputtering yields

Nuclear fusion is one of the most promising concepts for future energy production, due to the almost endless source of fuel and the lack of greenhouse effects during operation. However, to successfully build a fusion reactor, the development of new materials and knowledge of their behavior are needed. One important structural part of the reactor and the reactor vessel is the wall facing the plasma. The wall will be bombarded by the products of the nuclear reaction, which will erode and degrade its performance. In this work, we study the sputtering of different tungsten surfaces under various conditions, obtaining a deeper understanding of the process using molecular dynamics simulations. Additionally, we present the evolution of W fuzz cells and the effect of surface feature height on the erosion and sputtering of the surfaces under ion irradiation.Peer reviewe

Radiation damage buildup by athermal defect reactions in nickel and concentrated nickel alloys

Peer reviewe

Radiation stability of nanocrystalline single-phase multicomponent alloys

In search of materials with better properties, polycrystalline materials are often found to be superior to their respective single crystalline counterparts. Reduction of grain size in polycrystalline materials can drastically alter the properties of materials. When the grain sizes reach the nanometer scale, the improved mechanical response of the materials make them attractive in many applications. Multicomponent solid-solution alloys have shown to have a higher radiation tolerance compared with pure materials. Combining these advantages, we investigate the radiation tolerance of nanocrystalline multicomponent alloys. We find that these alloys withstand a much higher irradiation dose, compared with nanocrystalline Ni, before the nanocrystallinity is lost. Some of the investigated alloys managed to keep their nanocrystallinity for twice the irradiation dose as pure Ni.Peer reviewe

Punching of arbitrary face prismatic loops from hydrogen nanobubbles in copper

When a metal surface is exposed to prolonged irradiation with energetic H-, the ions are expected to penetrate into bulk and dissolve in the matrix. However, the irradiated surfaces exhibit dramatic morphological changes in the form of "blisters " covering the surface exposed to irradiation. Blistering is usually explained by accumulation of implanted gas in the bubbles near surface. However, the exact mechanism of continuous growth of a bubble after it reaches the measurable size is still not fully clear. Commonly such growth is related to prismatic loop punching, which is a short time scale process not easily accessible by experimental techniques. Even atomistic modelling of loop punching in FCC metals is somewhat cumbersome. Since the void surfaces in these metals yield easily through shear loops, these were debatably suggested to explain the plastic growth of a bubble in copper, without demonstrating the detachment of these loops from the void. We address the mechanisms of fast bubble growth in Cu which is associated with blistering of Cu surface exposed to H- irradiation. We observe the emission of a complete prismatic loop enclosed within the number of shear loops with the Burgers vectors aligned with the gliding direction of the prismatic loop. We show that the prismatic loops punched from the bubble surface do not need to be smaller than the bubble cross-section. These simulations capture the general trend of dislocation emission in the condition of hydrostatic pressure exerted by the accumulated gas on the wall of the bubble. (c) 2021 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc.Peer reviewe