Cobalt Alloy Nitridation for High-Temperature Strength
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Solution Overview
Problem
Current wrought cobalt alloys used in gas turbine engines face challenges with welding, strength at high temperatures, and difficulty in cold working due to phase transformations, and existing nitridation methods are limited to impractically thin components.
Innovation Solution
A new composition of cobalt alloys with specific ranges of chromium, iron, nickel, titanium, niobium, and other elements, allowing for through-thickness nitridation and strengthening, enabling the production of sheets up to 2 mm thickness that maintain high strength at elevated temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If precipitation hardening is used to strengthen nickel alloys, then strength is improved, but weldability deteriorates due to hardening precipitates forming in heat-affected zones
Solution Approach 1:
The invention changes the strengthening mechanism from precipitation hardening to solid solution strengthening by adjusting the alloy composition parameters. The cobalt-based alloy uses a different strengthening approach that does not involve temperature-sensitive precipitates, thereby maintaining weldability while achieving high strength through controlled solution strengthening elements.
Solution Approach 2:
The invention uses a different strengthening mechanism that is less sensitive to thermal processing history. By relying on solid solution strengthening rather than precipitation hardening, the material becomes more tolerant to welding thermal cycles, effectively making the strengthening mechanism more robust to process variations and thermal exposure.
2Strength
If gamma-prime and gamma-double prime precipitates are used for strengthening, then strength at certain temperatures is improved, but strength deteriorates at higher temperatures when precipitates coarsen
Solution Approach 1:
The invention changes the strengthening mechanism to solid solution strengthening, which maintains effectiveness at higher temperatures unlike precipitation hardening. The alloy composition is designed with specific solid solution strengthening elements that provide continuous strength retention through the temperature range, avoiding the coarsening issue that plagues precipitation-hardened alloys at elevated temperatures.
Solution Approach 2:
The invention creates a composite strengthening effect by combining multiple solid solution strengthening elements in the cobalt-based alloy. This multi-element approach provides synergistic strengthening that maintains effectiveness across a broad temperature range, replacing the temperature-limited precipitation hardening mechanism.
3Strength
If chromium is added to stabilize hcp structure in cobalt alloys, then high-temperature strength is improved, but cold workability deteriorates due to high work hardening rates
Solution Approach 1:
The invention optimizes the chromium content parameter within a specific range (15-30 wt.%) to balance the competing requirements. This controlled parameter adjustment allows sufficient hcp structure stabilization for high-temperature strength while preventing excessive work hardening that would impair cold workability. The precise parameter control resolves the contradiction between these two properties.
Solution Approach 2:
The invention applies local quality control by carefully managing the chromium distribution and concentration to achieve the desired balance. The alloy composition is designed with specific local concentrations of chromium and other elements that provide hcp stabilization where needed for high-temperature performance while maintaining overall ductility and cold workability through controlled heterogeneity in the microstructure.
4Ease of manufacture
If nickel is added to reduce transformation temperature for easier cold working, then cold workability is improved, but nitrogen absorption decreases reducing nitridation effectiveness
Solution Approach 1:
The invention optimizes the nickel content parameter within a specific range (0-25 wt.%) to balance cold workability and nitrogen absorption. By controlling the nickel concentration, the alloy achieves sufficient fcc structure stabilization for easy cold working while maintaining adequate nitrogen solubility and absorption capacity for effective nitridation strengthening. This parameter optimization resolves the contradiction between the two competing requirements.
Solution Approach 2:
The invention utilizes the nitriding process to create a nitrogen-enriched layer or dispersion of nitride particles within the alloy structure. This approach compensates for the reduced bulk nitrogen absorption by creating concentrated nitrogen regions that provide strengthening, effectively working around the limitation imposed by nickel addition.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The new alloy composition achieves stress rupture lives exceeding 150 hours at 980°C and 55 MPa, and 250 hours at 52 MPa, meeting the requirements for practical thickness and high-temperature strength, while being wroughtable and resistant to phase transformations.
Implementation Method 1
a procedure for absorbing and diffusing nitrogen into cobalt alloys, to induce the formation of a fine dispersion of nitride particles
Implementation Method 2
absorbing and diffusing nitrogen into cobalt alloys
Implementation Method 3
cobalt exists in two forms. At temperatures up to about 420° C., the stable structure is hexagonal close-packed (hcp). Beyond this temperature, up to the melting point, the structure is fcc
Implementation Method 4
solid solution-strengthened cobalt alloys
Data Source
AI summary
A wroughtable, cobalt alloy capable of through thickness nitridation and strengthening using practical treatments and practical sheet thicknesses contains in weight percent about 23 to about 30% chromium, about 15 to about 25% iron, up to about 27.3% nickel, about 0.75 to about 1.7% titanium, about 0.85 to about 1.9% niobium or zirconium, up to 0.2% carbon, up to 0.015% boron, up to 0.015% rare earth elements, up to 0.5% aluminum, up to 1% manganese, up to 1% silicon, up to 1% tungsten, up to 1% molybdenum, and the balance cobalt plus impurities and the total weight percent of titanium plus niobium or equivalents is from about 1.6 to about 3.6.