Low Thermal Expansion Alloy Composition for Low Temperature Stability
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Solution Overview
Problem
Low thermal expansion alloys face challenges in maintaining stability and preventing martensite transformation at extremely low temperatures, such as −120° C., which affects the dimensional precision and thermal expansion characteristics of precision instruments.
Innovation Solution
A low thermal expansion alloy with a specific composition of Co and Ni, balanced to satisfy the condition [Co]≥−4×[Ni]+136 and [Co]≤−4×[Ni]+139, along with additional elements like C, Si, Mn, Al, Mg, Ca, Ce, and La, is developed to lower the martensite transformation temperature and maintain a stable austenitic structure even at −120° C., resulting in a mean thermal expansion coefficient of 0.5×10−6/° C. or less.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional low thermal expansion alloys (e.g., 32% Ni-5% Co—Fe) are used, then the mean thermal expansion coefficient is extremely low (1×10−6/° C. or less), but martensite transformation occurs at low temperatures causing remarkable expansion and deterioration of dimensional precision
Solution Approach 1:
The invention changes the compositional parameters by precisely controlling Ni content (30.0-34.0%) and Co content (4.5-6.5%) within specific ranges, and by adjusting the relationship parameter XT = (% Co) + 2.8(% Ni) to satisfy 93≤XT≤99. This parameter optimization shifts the martensite transformation temperature to −120° C. or lower, preventing transformation at operational low temperatures while maintaining the low thermal expansion coefficient of 1.0×10−6/° C. or less
Solution Approach 2:
The invention creates a composite alloy system by combining Fe, Ni, and Co in specific proportions to form a low thermal expansion alloy with stable austenitic structure. The synergistic combination of these three elements produces an alloy that maintains both low thermal expansion characteristics and structural stability at low temperatures, where neither element alone could achieve both properties
2Reliability
If Ni and Co contents are increased to achieve low thermal expansion coefficient, then the thermal expansion coefficient decreases, but martensite transformation temperature increases causing transformation at low temperatures
Solution Approach 1:
The invention optimizes the parameters of Ni content (30.0-34.0%) and Co content (4.5-6.5%) within specific ranges, and controls the relationship parameter XT = (% Co) + 2.8(% Ni) to satisfy 93≤XT≤99. This precise parameter control achieves the optimal balance where the thermal expansion coefficient is 1.0×10−6/° C. or less while the martensite transformation temperature is suppressed to −120° C. or lower, preventing transformation at operational temperatures
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 stable austenitic structure and low thermal expansion characteristics down to −120° C., enhancing the dimensional precision and thermal stability of components used in low-temperature regions.
Implementation Method 1
Steel undergoes martensite transformation if the temperature becomes lower. In a low thermal expansion alloy, the martensite transformation temperature is usually 0° C. or less, but if martensite transformation occurs, remarkable expansion occurs
Data Source
AI summary
Provided is a low thermal expansion alloy wherein martensitic transformation does not occur even at −120° C. This low thermal expansion alloy contains, in mass %, 1.50-5.00% of Co, while containing Ni in such an amount that if [Ni] (mass %) is the content of Ni and [Co] (mass %) is the content of Co, [Co]≥−4×[Ni]+136 and [Co]≤−4×[Ni]+139 are satisfied, with the balance being made up of Fe and unavoidable impurities. This low thermal expansion alloy has an average thermal expansion coefficient of 0.5×10−6/° C. or less for the range of 0-30° C., while having a martensitic transformation temperature of −120° C. or less.