Low-Expansion Fe-Ni-Co Alloy for Cryogenic Dimensional Stability
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Solution Overview
Problem
Current low thermal expansion alloys, such as Super Invar and Invar, face limitations in achieving zero thermal expansion and low temperature stability at temperatures between 100° C. and −70° C., with Super Invar's martensite structure forming at low temperatures increasing thermal expansion and Invar's coefficient being too high, while existing methods like additive manufacturing do not consistently achieve zero expansion or Invar's stability.
Innovation Solution
A low thermal expansion alloy with a composition of C: 0.015% or less, Si: 0.10% or less, Mn: 0.15% or less, Ni: 35.0% to 37.0%, Co: less than 2.0%, and Ni+0.8Co: 35.0% to 37.0%, with a secondary dendrite arm spacing of 5 μm or less, produced through laser or electron beam additive manufacturing, achieving an average thermal expansion coefficient of 0±0.2 ppm/° C. and an Ms point of −196° C. or lower.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If Ni is replaced by Co to reduce thermal expansion coefficient, then thermal expansion is reduced, but austenite is destabilized and martensite structure is generated at higher temperatures, impairing low thermal expansibility
Solution Approach 1:
The invention changes the chemical composition parameters by strictly limiting Co content to less than 2.0% (conventional SI has 5% Co) and controlling Ni content at 35.0-37.0%, along with controlling impurities (C: 0.015% or less, Si: 0.10% or less, Mn: 0.15% or less). This parameter optimization prevents excessive destabilization of austenite while achieving low thermal expansion, maintaining Ms point below -196°C for low temperature stability.
Solution Approach 2:
The invention creates an optimized composite alloy system by combining Fe, Ni, and Co in specific proportions (Fe balance, Ni: 35.0-37.0%, Co: less than 2.0%) with controlled impurity levels. This composite composition achieves synergistic effects where the limited Co provides thermal expansion reduction while the high Ni content and pure Fe base maintain austenite stability at low temperatures.
2Reliability
If Invar alloy is used to maintain low temperature stability, then structure does not change at low temperatures, but thermal expansion coefficient is too high (1 to 2 ppm/° C.) to fulfill high level demands
Solution Approach 1:
The invention changes the composition parameters by optimizing Ni content (35.0-37.0%, higher than conventional Invar) and strictly limiting Co content (less than 2.0%). This parameter optimization achieves thermal expansion coefficient of 0±0.2 ppm/°C. in the range of 100°C to -70°C., significantly improving upon Invar's 1 to 2 ppm/°C. while maintaining low temperature stability through Ms point below -196°C.
3Manufacturing precision
If additive manufacturing is used to produce alloy with fine structure, then secondary dendrite arm spacing is reduced, but consistent zero expansion and Invar-level stability are not achieved
Solution Approach 1:
The invention changes the composition parameters (C: 0.015% or less, Si: 0.10% or less, Mn: 0.15% or less, Ni: 35.0-37.0%, Co: less than 2.0%) to achieve consistent zero thermal expansion (0±0.2 ppm/°C.) when processed by additive manufacturing. The controlled impurity levels and optimized alloying elements ensure reproducible Ms point below -196°C. and uniform austenite structure, achieving Invar-level stability that was not consistently obtained with previous compositions.
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 alloy achieves practically zero thermal expansion and low temperature stability equivalent to Invar, suitable for precision devices in aerospace and measurement instruments, with reduced material costs and minimal management requirements due to lower Co content.
Implementation Method 1
melting and solidifying a low thermal expansion alloy material having a specific composition by means of a laser or electron beam to carry out additive manufacturing, thereby producing a low thermal expansion alloy having a solidification structure with a secondary dendrite arm spacing of 5 μm or less
Data Source
AI summary
Provided is a low thermal expansion alloy that contains, in mass %, not more than 0.015% of C, not more than 0.10% of Si, not more than 0.15% of Mn, 35.0-37.0% of Ni, and less than 2.0% of Co. Ni+0.8Co is 35.0-37.0%, and the remaining portion is Fe and unavoidable impurities. The low thermal expansion alloy has a solidification structure in which the secondary dendrite-arm spacing is 5 μm or less, has an average thermal expansion coefficient in a range of 0±0.2 ppm/° C. at 100° C. to −70° C., and has an Ms point of −196° C. or less.