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Aspects of Precipitation in Alloy Inconel 718
Malmö högskola, School of Technology (TS).
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A study was made of the microstructure of the Ni-base alloy Inconel 718 with emphasis on the precipitation and stability of intermetallic phases as affected by heat treatments. In addition the effect of the precipitation on selected mechanical properties namely hardness, creep notch sensitivity and hot ductlity were investigated. The materials studied were a spray-formed version and three wrought versions of the alloy. The spray-formed version of the alloy was of interest since it exhibited a superior compositional homogeneity to more conventional forms of the alloy. To characterise the precipitation kinetics and to determine the associated hardening, a series of isothermal ageing treatments were carried out on both spray-formed and ring rolled material at selected temperatures between 600°C and 1025°C for up to 100h. The results were presented in the form of TTH diagrams for the two alloys. The heat treated specimens were then studied using optical microscopy, scanning electron microscopy and transmission electron microscopy. Where possible these were supplemented with quantitative metallographic measurements of volume fraction and precipitates size.The three main intermetallic precipitation phases in IN 718 are (i) gamma’ having a composition Ni3(Al,Ti), a cubic crystal structure and cubic or spherical particle shape (ii) gamma” having a composition Ni3Nb, a bct crystal structure and a lens-like disc shape (iii)delta having composition Ni3Nb, an orthorhombic crystal structure and forming as grain boundary particles and films as well as thin plates extending long distances into the grains. In thermomechanically processed material the delta-phase can also occur as shorter platelets and particles resulting from fragmentation of the original plates. In this work the rates of precipitation and the temperature ranges of their existence were determined qualitatively. Moreover the coarsening of the ganmma" precipitates was determined quantitatively. The kinetics of precipitation and dimensional coarsening of the delta-phase were also established quantitatively. Similarly, the dissolution kinetics of the delta-phase were measured in the spray-formed and three wrought versions of the alloy leading to a determination of the effect of Nb-content on the delta solvus temperature. After a standard heat treatment, the spray-formed IN 718 was found to be creep notch sensitive according to an SAE-AMS standard rupture test. It was found that notch ductility could be restored to the alloy by inserting a delta-phase precipitation treatment at 875°C-900°C prior to the standard heat treatment. This could be attributed to the formation of a more favourable delta-phase morphology than that formed during the solution treatment at 954°C in the standard heat treatment cycle. A study was made of high temperature tensile properties of a wrought version of IN 718 in particular in relation to the problem of hot cracking in repair welding. Measurements were made of the strength loss temperature on heating and the strength and ductility return temperatures on cooling. These properties provide an indication of the sensitivity of the alloy to hot cracking due to liquation. The tests were made on the alloy after various heat treatments designed to produce microstructures with two different grain sizes with and without delta-phase precipitation. Hot tensile tests were also performed on the in-situ solidified alloy in order to describe the weld metal ductility and properties. The grain size and the presence of large fractions of delta-phase precipitated at 900°C had only small effects on the strength loss temperature and brittle-ductile transition of the alloy. On the other hand, a small fraction of delta-phase precipitated in the grain boundaries at 960°C reduced the brittle-ductile transition temperature significantly. It is proposed that the reduction was associated with interaction between the grain boundary delta-phase and grain boundary boron segregation that was observed in all the heat treated materials.

Place, publisher, year, edition, pages
Luleå tekniska universitet, 2004.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:mau:diva-7777Local ID: 4919OAI: oai:DiVA.org:mau-7777DiVA, id: diva2:1404718
Note

Paper III and IV in dissertation as accepted manuscripts.

Available from: 2020-02-28 Created: 2020-02-28 Last updated: 2024-03-16Bibliographically approved
List of papers
1. Precipitation in spray-formed IN718
Open this publication in new window or tab >>Precipitation in spray-formed IN718
2001 (English)In: Proceedings of Fifth International Special Emphasis Symposium on Sureralloys, The Minerals, Metals, and Materials Society, 2001, p. 617-626Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
The Minerals, Metals, and Materials Society, 2001
National Category
Materials Engineering
Identifiers
urn:nbn:se:mau:diva-12656 (URN)10.7449/2001/Superalloys_2001_617_626 (DOI)2-s2.0-0034877554 (Scopus ID)4858 (Local ID)4858 (Archive number)4858 (OAI)
Conference
International Special Emphasis Symposium on Sureralloys
Available from: 2020-02-29 Created: 2020-02-29 Last updated: 2024-05-21Bibliographically approved
2. Precipitation and age hardening in the nickel base superalloy IN718
Open this publication in new window or tab >>Precipitation and age hardening in the nickel base superalloy IN718
2001 (English)In: Proc. SCANDEM 2001, 2001, p. 208-209Conference paper, Oral presentation with published abstract (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:mau:diva-12358 (URN)4854 (Local ID)4854 (Archive number)4854 (OAI)
Conference
SCANDEM 2001
Available from: 2020-02-29 Created: 2020-02-29 Last updated: 2024-03-16Bibliographically approved
3. Aging response of spray-formed Inconel 718 between 620C and 1000C
Open this publication in new window or tab >>Aging response of spray-formed Inconel 718 between 620C and 1000C
2005 (English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:mau:diva-2667 (URN)7556 (Local ID)7556 (Archive number)7556 (OAI)
Available from: 2020-02-27 Created: 2020-02-27 Last updated: 2024-04-12Bibliographically approved
4. Delta Phase Precipitation in Inconel 718
Open this publication in new window or tab >>Delta Phase Precipitation in Inconel 718
2004 (English)In: Materials Characterization, ISSN 1044-5803, E-ISSN 1873-4189, no 52, p. 7-16Article in journal (Refereed) Published
Abstract [en]

The precipitation and dissolution kinetics of the δ-phase were studied in three wrought versions and one spray-formed (SF) version of the nickel alloy, Inconel 718. The precipitation in the spray-formed version and one wrought version was followed during isothermal ageing for up to 100 h between 700 and 1000 °C. The δ-phase precipitates in the form of nonuniformly distributed thin platelets and an alternative method of measuring volume fraction of this precipitate morphology is proposed. At and above 800 °C, the precipitation was measured quantitatively in terms of the volume fraction and platelet thickness distribution. The maximum rate of precipitation occurred at approximately 900 °C. The solvus temperature lay between 1005 and 1015 °C for Nb contents of 5.06 and 5.41 wt.%, respectively. A study was made of the dissolution of the δ-phase in all four alloys using isochronal treatments and this confirmed the above solvus temperature range. The effect of δ-phase and its dissolution on the grain growth of the alloys was also determined.

Place, publisher, year, edition, pages
Elsevier, 2004
Keywords
Alloy 718, Inconel 718, Spray-formed IN 718, Precipitation, Delta phase, Volume fraction
National Category
Materials Engineering
Identifiers
urn:nbn:se:mau:diva-2337 (URN)10.1016/j.matchar.2004.07.004 (DOI)000225127100002 ()2-s2.0-7544226213 (Scopus ID)4816 (Local ID)4816 (Archive number)4816 (OAI)
Available from: 2020-02-27 Created: 2020-02-27 Last updated: 2024-09-05Bibliographically approved
5. Effect of Delta Phase Precipitation on the Creep Notch Sensitivity of Spray-Formed IN 718
Open this publication in new window or tab >>Effect of Delta Phase Precipitation on the Creep Notch Sensitivity of Spray-Formed IN 718
2004 (English)Manuscript (preprint) (Other academic)
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:mau:diva-66347 (URN)
Available from: 2024-03-16 Created: 2024-03-16 Last updated: 2024-03-16Bibliographically approved
6. The High Temperature Tensile Behaviour of IN 718 in Relation to Liquation Cracking
Open this publication in new window or tab >>The High Temperature Tensile Behaviour of IN 718 in Relation to Liquation Cracking
Show others...
2004 (English)Manuscript (preprint) (Other academic)
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:mau:diva-66348 (URN)
Available from: 2024-03-16 Created: 2024-03-16 Last updated: 2024-03-16Bibliographically approved

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