I started my career as a graduate student at the Department of Solid State Physics at the University of Uppsala, Sweden in 1964. In those first years I developed a dislocation model for the plastic deformation of ferritic steel and I also spent some time carrying out TEM-studies of dislocation substructures in fatigued iron. In 1966 I received licentiate degree in solid state physics and continued my work in Uppsala for another two years as a lecturer in physics. I mixed this work with further studies in the field of work hardening of metals.
In 1968 I moved to the Swedish Institute for Metals Research, Stockholm where I worked with single-phase materials as aluminium, copper, ferritic steel and stainless steel. The objective of this work was to continue the development of simple, physically based theories for the plastic deformation behaviour of metals and to adapt these theories to experimental data, mostly uniaxial tensile test data. 1971 I got my Ph.D. and also the title docent in physics.
During this period we also derived a dislocation theory for single-phase fcc metals and carried out intensive studies, related to static and dynamic strain ageing, resulting in new theories in both these disciplines. We also conducted a lot of structure studies based on light optical microscopy LOM, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM was particularly used to analyse dislocation structures, measure dislocation densities and study the development of dislocation cell structures. Later on we had access to a 1 MV transmission electron microscope, which also could be used for in-situ studies of the plastic deformation process. In the latter work we were assisted by a skilled operator - Börje Lehtinen. Since it was possible to plastically deform specimens inside the microscope and to follow the dislocation movements and the formation of dislocation tangles and dislocation cells in-situ, we learned a lot about the non-homogeneity of the plastic deformation process in single-phase metals. During these years I had a stimulating co-operation with Dr William Roberts and we published a number of papers together.
Later on I learned that the non-homogeneity situation is much more complicated in the advanced high strength steels which at the moment are the subject of intensive studies around the world.
1976 I was employed as professor in Mechanical Metallurgy at the Royal Institute of Technology, KTH, in Stockholm. During this period, which lasted for nine years, I became engaged in studies of the formability of sheet metals and investigations concerning for instance fracture mechanisms, hardness testing and impact testing. At the end of this period I became deeply engaged in material problems in the off-shore industry and took a leave from my work at KTH for three years working as a consultant.
During this time I also wrote an extensive summary of most of my work in the field of plastic deformation and this was published 1983.
1985 I left the Royal Institute of Technology and began working in the industry. At the same time I was appointed the title adjunct professor at the Uppsala University on part-time.
Now I started a career as an expert on high strength offshore chains and travelled around the world solving problems and selling offshore chains. After a while I became a third-part owner of an offshore chain producing company in Sweden. After around 5 years this factory was sold to a Norwegian group.
At this time I was asked by some steel works in Sweden if I would be interested in helping them to start a master-education in material science in mid Sweden, where most of the steelworks are located. They thought that this would be a good idea and I got their full support. At the same time I became a full professor in material science at the University of Uppsala and I started my new job. This was a stimulating job which turned out well since we had a strong support from the Swedish government. However, during this time I had no possibility to carry out research and it turned out to take almost 25 years before I could return to research again. One reason for the delay was that I took part in a project related to investments in small-size companies and the start of a financing company dealing with this type of business.
Now I am a retired professor, but a Swedish steelwork has engaged me on part-time to carry out research in the area of advanced high strength steels. I even have a Dr student. This suits me well because I can now focus entirely on the research. This project has now been going on for almost three years and I feel that I am “back in business” again. We have managed to formulate a new theory for DP steel which seems to work nicely. This simple theory, based on non-traditional ideas, has made it possible to understand the inhomogeneous deformation process in this type of steel, and also to understand the detailed mechanisms behind the excellent work hardening, the good ductility and the high initial rates of energy absorption. The figure on the front page of this homepage is an example of an analysis of a DP 600 steel. Similarly good results are obtained for DP steels containing up to around 50% martensite. At higher martensite contents the situation becomes more complicated.
List of publications
1. A dislocation model for the stress-strain behaviour of polycrystalline alpha-iron with special emphasis on the variation of the densities of mobile and immobile dislocations.
Bergström Y., Mat.Sci.Eng. 5(1969-70)193
2. Effects of changes in temperature and strain rate on the double-n behaviour in alpha-iron.
Bergström Y. and Aronsson B., Met.Trans 3(1970)1029
3. Studies of strain hardening, strain ageing and fatigue in ferritic steel in terms of dislocation mechanisms (Dissertation)
Bergström Y., Acta Univ. Upps. 177(1971)
4. A relationship for the elongation to necking of mild steel tensile specimens. Bergström Y and Roberts W., Scr.Met. 5(1971)459
5. The application of a dislocation model to dynamical strain ageing in alpha-iron containing interstitial atoms.
Bergström Y. and Roberts W., Acta.Met. 19(1971)815
6. A dislocation model for dynamical strain ageing of alpha-iron in the jerky-flow region.
Bergström Y. and Roberts W., Acta.Met. 19(1971)1243
7. A HVEM study of “grown-in” dislocations in iron and their influence on the rate of strain hardening.
Karlsson S., Roberts W. and Bergström Y., Jernkont. Ann. 155(1971)
8. The grain size dependence of the yield and flow stresses of alpha-iron.
Roberts W. and Bergström Y., Z.Metallk. 10(1971)752
9. The effect of temperature on the average slip distance of mobile dislocations in alpha-iron.
Bergström Y., EMCON(1972)
10. A dislocation model for the strain-ageing behaviour of steel.
Bergström Y., Mat.Sci.Eng. 9(1972)101
11. The application of a dislocation model to the strain and temperature dependence of the strain hardening exponent n in the Hollomon Relation between stress and strain in mild steel.
Bergström Y and Aronsson B., Met.Trans. 3(1972)1951
12. The friction stress in ferritic and austenitic steels, with particular emphasis on Interstitial solid-solution hardening.
Roberts W. and Bergström Y., Scand.J.Met. 1(1972)265
13. The dynamical strain ageing of alpha-iron - effects of strain rate and nitrogen content in the jerky-flow region.
Bergström Y. and Roberts W., Acta.Met. 6(1973)741
14. The stress-strain behaviour of single crystals and polycrystals of face-centered cubic metals - a new dislocation treatment.
Roberts W and Bergström Y., Acta.Met. 21(1973)457
15. A reassessment of the critical strain associated with dynamic strain ageing (Portevin-le Chatelier effect) in fcc substitutional alloys.
Roberts W, Lenasson C.G. and Bergström Y., Scr.Met. 7(1973)395
16. The rate of dislocation multiplication in polycrystalline iron.
Roberts W., Karlsson S. and Bergström Y., Mat.Sci.Eng. 5(1973)247
17.´The effect of temperature on the work hardening behaviour of alpha-iron in terms of dislocation mechanisms.
Bergström Y., Annales des mines (1974)
18. On the strain hardening exponent n and the true strain to necking in iron and steel.
Bergström Y., IDDRG-8th biennial congress (1974)
19. An analysis of the low-temperature plastic deformation process in tempered martensite steels with o,5 and 9% Ni.
Bergström Y. and Josefsson Å., Scand.J.Met. 3(1974)259
20. Kinetics and hardening mechanism of the 475C-embrittlement in 18Cr-2Mo steels.
Jacobsson P., Bergström Y. and Aronsson B., Met.Trans (1974)
21. Influence of prestrain and strain ageing on the cleavage fracture in C-Mn steel.
Sandström R, Engberg G. and Bergström Y., Metal Science, 15(1981)409
22. An improved dislocation model for the stress-strain behaviour in polycrystalline alpha-iron.
Bergström Y. and Hallen H., Mat.Sci.Eng 55(1982)49
23. On the indentation hardness of metals.
Nylen M. and Bergström Y., Z.Metallkunde 74(1983)447
24. The plastic deformation of metals - a dislocation model and its applicability. Bergström Y., Reviews on powder metallurgy and physical ceramics 2(1983)84
25. A dislocation model for the stress-strain behaviour of dual-phase steel.
Bergström Y. and Granbom Y., Published in: IDDRG-2008, Olofström, Sweden, 2008, pp.173-184.
26. A dislocation based theory for the deformation hardening behaviour of DP-steels - Impact of martensite content and ferrite grain size.
Yngve Bergström, Ylva Granbom and Dirk Sterkenburg - To be published
© copyright 2010 Yngve Bergström