Editing Inconel (section)

== Strengthening mechanisms ==
The most prevalent hardening mechanisms for Inconel alloys are [[Precipitation hardening|precipitate strengthening]] and [[solid solution strengthening]]. In Inconel alloys, one of the two often dominates. For alloys like Inconel 718, precipitate strengthening is the main strengthening mechanism. The majority of strengthening comes from the presence of gamma double prime (γ″) precipitates.<ref name="Mignanelli-2017">{{Cite journal |last1=Mignanelli |first1=P. M. |last2=Jones |first2=N. G. |last3=Pickering |first3=E. J. |last4=Messé |first4=O. M. D. M. |last5=Rae |first5=C. M. F. |last6=Hardy |first6=M. C. |last7=Stone |first7=H. J. |date=2017-07-15 |title=Gamma-gamma prime-gamma double prime dual-superlattice superalloys |journal=Scripta Materialia |language=en |volume=136 |pages=136–140 |doi=10.1016/j.scriptamat.2017.04.029 |issn=1359-6462|doi-access=free }}</ref><ref name="Devaux-2008">{{Cite journal |last1=Devaux |first1=A. |last2=Nazé |first2=L. |last3=Molins |first3=R. |last4=Pineau |first4=A. |last5=Organista |first5=A. |last6=Guédou |first6=J. Y. |last7=Uginet |first7=J. F. |last8=Héritier |first8=P. |date=2008-07-15 |title=Gamma double prime precipitation kinetic in Alloy 718 |url=https://www.sciencedirect.com/science/article/pii/S0921509307016073 |journal=Materials Science and Engineering: A |language=en |volume=486 |issue=1 |pages=117–122 |doi=10.1016/j.msea.2007.08.046 |issn=0921-5093|url-access=subscription }}</ref><ref name="Hosseini-2019">{{Cite journal |last1=Hosseini |first1=E. |last2=Popovich |first2=V. A. |date=2019-12-01 |title=A review of mechanical properties of additively manufactured Inconel 718 |url=https://www.sciencedirect.com/science/article/pii/S221486041930226X |journal=Additive Manufacturing |language=en |volume=30 |pages=100877 |doi=10.1016/j.addma.2019.100877 |issn=2214-8604|url-access=subscription }}</ref><ref name="Shankar-2001">{{Cite journal |last1=Shankar |first1=Vani |last2=Bhanu Sankara Rao |first2=K |last3=Mannan |first3=S. L |date=2001-02-01 |title=Microstructure and mechanical properties of Inconel 625 superalloy |url=https://www.sciencedirect.com/science/article/pii/S0022311500007236 |journal=Journal of Nuclear Materials |language=en |volume=288 |issue=2 |pages=222–232 |doi=10.1016/S0022-3115(00)00723-6 |bibcode=2001JNuM..288..222S |issn=0022-3115|url-access=subscription }}</ref> Inconel alloys have a γ matrix phase with an [[Face Centered Cubic | FCC]] structure.<ref name="Hosseini-2019" /><ref name="Tucho-2017">{{Cite journal |last1=Tucho |first1=Wakshum M. |last2=Cuvillier |first2=Priscille |last3=Sjolyst-Kverneland |first3=Atle |last4=Hansen |first4=Vidar |date=2017-03-24 |title=Microstructure and hardness studies of Inconel 718 manufactured by selective laser melting before and after solution heat treatment |url=https://www.sciencedirect.com/science/article/pii/S092150931730223X |journal=Materials Science and Engineering: A |language=en |volume=689 |pages=220–232 |doi=10.1016/j.msea.2017.02.062 |issn=0921-5093|url-access=subscription }}</ref><ref name="Yu-2021">{{Cite journal |last1=Yu |first1=Xiaobin |last2=Lin |first2=Xin |last3=Tan |first3=Hua |last4=Hu |first4=Yunlong |last5=Zhang |first5=Shuya |last6=Liu |first6=Fencheng |last7=Yang |first7=Haiou |last8=Huang |first8=Weidong |date=2021-02-01 |title=Microstructure and fatigue crack growth behavior of Inconel 718 superalloy manufactured by laser directed energy deposition |url=https://www.sciencedirect.com/science/article/pii/S0142112320305375 |journal=International Journal of Fatigue |language=en |volume=143 |pages=106005 |doi=10.1016/j.ijfatigue.2020.106005 |issn=0142-1123|url-access=subscription }}</ref><ref name="Jambor-2017">{{Cite journal |last1=Jambor |first1=Michal |last2=Bokůvka |first2=Otakar |last3=Nový |first3=František |last4=Trško |first4=Libor |last5=Belan |first5=Juraj |date=2017-06-01 |title=Phase Transformations in Nickel base Superalloy Inconel 718 during Cyclic Loading at High Temperature |journal=Production Engineering Archives |language=en |volume=15 |issue=15 |pages=15–18 |doi=10.30657/pea.2017.15.04|doi-access=free }}</ref> γ″ precipitates are made of Ni and Nb, specifically with a Ni<sub>3</sub>Nb composition. These precipitates are fine, coherent, disk-shaped, intermetallic particles with a tetragonal structure.<ref name="Devaux-2008" /><ref name="Hosseini-2019" /><ref name="Shankar-2001" /><ref name="Tucho-2017" /><ref name="Bennett-2021">{{Cite journal |last1=Bennett |first1=Jennifer |last2=Glerum |first2=Jennifer |last3=Cao |first3=Jian |date=2021-01-01 |title=Relating additively manufactured part tensile properties to thermal metrics |url=https://www.sciencedirect.com/science/article/pii/S0007850621000779 |journal=CIRP Annals |language=en |volume=70 |issue=1 |pages=187–190 |doi=10.1016/j.cirp.2021.04.053 |issn=0007-8506|url-access=subscription }}</ref><ref name="Li-2020">{{Cite journal |last1=Li |first1=Zuo |last2=Chen |first2=Jing |last3=Sui |first3=Shang |last4=Zhong |first4=Chongliang |last5=Lu |first5=Xufei |last6=Lin |first6=Xin |date=2020-01-01 |title=The microstructure evolution and tensile properties of Inconel 718 fabricated by high-deposition-rate laser directed energy deposition |url=https://www.sciencedirect.com/science/article/pii/S2214860419308450 |journal=Additive Manufacturing |language=en |volume=31 |pages=100941 |doi=10.1016/j.addma.2019.100941 |issn=2214-8604|url-access=subscription }}</ref><ref name="Glerum-2021">{{Cite journal |last1=Glerum |first1=Jennifer |last2=Bennett |first2=Jennifer |last3=Ehmann |first3=Kornel |last4=Cao |first4=Jian |date=2021-05-01 |title=Mechanical properties of hybrid additively manufactured Inconel 718 parts created via thermal control after secondary treatment processes |url=https://www.sciencedirect.com/science/article/pii/S0924013621000078 |journal=Journal of Materials Processing Technology |language=en |volume=291 |pages=117047 |doi=10.1016/j.jmatprotec.2021.117047 |issn=0924-0136|url-access=subscription }}</ref><ref name="Deng-2018">{{Cite journal |last1=Deng |first1=Dunyong |last2=Peng |first2=Ru Lin |last3=Brodin |first3=Håkan |last4=Moverare |first4=Johan |date=2018-01-24 |title=Microstructure and mechanical properties of Inconel 718 produced by selective laser melting: Sample orientation dependence and effects of post heat treatments |url=https://www.sciencedirect.com/science/article/pii/S0921509317316416 |journal=Materials Science and Engineering: A |language=en |volume=713 |pages=294–306 |doi=10.1016/j.msea.2017.12.043 |issn=0921-5093|url-access=subscription }}</ref>

Secondary precipitate strengthening comes from gamma prime (γ') precipitates. The γ' phase can appear in multiple compositions such as Ni<sub>3</sub>(Al, Ti).<ref name="Devaux-2008" /><ref name="Hosseini-2019" /><ref name="Shankar-2001" /> The precipitate phase is coherent and has an FCC structure, like the γ matrix;<ref name="Deng-2018" /><ref name="Tucho-2017" /><ref name="Bennett-2021" /><ref name="Li-2020" /><ref name="Glerum-2021" /> The γ' phase is much less prevalent than γ″. The volume fraction of the γ″ and γ' phases are approximately 15% and 4% after precipitation, respectively.<ref name="Devaux-2008" /><ref name="Hosseini-2019" /> Because of the coherency between the γ matrix and the γ' and γ″ precipitates, strain fields exist that obstruct the motion of dislocations. The prevalence of carbides with MX(Nb, Ti)(C, N) compositions also helps to strengthen the material.<ref name="Hosseini-2019" /> For precipitate strengthening, elements like niobium, titanium, and tantalum play a crucial role.<ref name="Aeether">{{Cite web |author=Aeether Co Limited |title=What is Solid Solution? Why do Nickel Alloy / Superalloy need Solution Treatment? |url=https://www.aeether.com/AEETHER/media/media-29/media.html |access-date=2023-05-08 |website=aeether.com |language=en}}</ref> 

Because the γ″ phase is metastable, over-aging can result in the transformation of γ″ phase precipitates to delta (δ) phase precipitates, their stable counterparts.<ref name="Hosseini-2019" /><ref name="Tucho-2017" /> The δ phase has an orthorhombic structure, a Ni<sub>3</sub>(Nb, Mo, Ti) composition, and is incoherent.<ref>{{Cite journal |last1=Wang |first1=Yachao |last2=Shi |first2=Jing |date=2019-12-01 |title=Microstructure and Properties of Inconel 718 Fabricated by Directed Energy Deposition with In-Situ Ultrasonic Impact Peening |url=https://doi.org/10.1007/s11663-019-01672-3 |journal=Metallurgical and Materials Transactions B |language=en |volume=50 |issue=6 |pages=2815–2827 |doi=10.1007/s11663-019-01672-3 |bibcode=2019MMTB...50.2815W |issn=1543-1916|url-access=subscription }}</ref><ref name="Jambor-2017" /> As a result, the transformation of γ″ to δ in Inconel alloys leads to the loss of coherency strengthening, making for a weaker material. That being said,  in appropriate quantities, the δ phase is responsible for [[Grain boundary strengthening|grain boundary pinning]] and strengthening.<ref name="Deng-2018" /><ref name="Glerum-2021" /><ref name="Jambor-2017" />

Another common phase in Inconel alloys is the Laves intermetallic phase. Its compositions are (Ni, Cr, Fe)<sub>x</sub>(Nb, Mo, Ti)<sub>y</sub> and Ni<sub>y</sub>Nb, it is brittle, and its presence can be detrimental to the mechanical behavior of Inconel alloys.<ref name="Tucho-2017" /><ref name="Deng-2018" /><ref name="Sohrabi-2018">{{Cite journal |last1=Sohrabi |first1=Mohammad Javad |last2=Mirzadeh |first2=Hamed |last3=Rafiei |first3=Mohsen |date=2018-08-01 |title=Solidification behavior and Laves phase dissolution during homogenization heat treatment of Inconel 718 superalloy |url=https://www.sciencedirect.com/science/article/pii/S0042207X18305700 |journal=Vacuum |language=en |volume=154 |pages=235–243 |doi=10.1016/j.vacuum.2018.05.019 |bibcode=2018Vacuu.154..235S |issn=0042-207X|url-access=subscription }}</ref> Sites with large amounts of Laves phase are prone to crack propagation because of their higher potential for stress concentration.<ref name="Li-2020" /> Additionally, due to its high Nb, Mo, and Ti content, the Laves phase can exhaust the matrix of these elements, ultimately making precipitate and solid-solution strengthening more difficult.<ref name="Glerum-2021" /><ref name="Sohrabi-2018" /><ref name="Yu-2021" /> 

For alloys like Inconel 625, solid-solution hardening is the main strengthening mechanism. Elements like Mo {{clarification needed | date= July 2024}} are important in this process. Nb and Ta can also contribute to solid solution strengthening to a lesser extent.<ref name="Aeether" /> In solid solution strengthening, Mo atoms are substituted into the γ matrix of Inconel alloys. Because Mo atoms have a significantly larger radius than those of Ni (209 pm and 163 pm, respectively), the substitution creates strain fields in the crystal lattice, which hinder the motion of dislocations, ultimately strengthening the material. 

The combination of elemental composition and strengthening mechanisms is why Inconel alloys can maintain their favorable mechanical and physical properties, such as high strength and fatigue resistance, at elevated temperatures, specifically those up to 650°C.<ref name="Mignanelli-2017" />