Nickel-Titanium Alloy Microstructure Control via Atomization
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Solution Overview
Problem
Existing nickel-titanium alloys used in medical devices and other applications face challenges due to large second phases that reduce fatigue life and surface quality, particularly in small-diameter or thickness applications, where these phases can be of similar size to the article dimensions, affecting their performance and reliability.
Innovation Solution
A process involving melting and atomizing pre-alloyed near-equiatomic nickel-titanium alloys to form powders, which are then consolidated and hot worked, resulting in articles with second phases of less than 10 micrometers in size, thereby improving microstructure and reducing the area fraction of these phases.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional nickel-titanium alloys are used, then the alloys exhibit shape memory and superelastic properties, but large second phases are present that reduce fatigue life and surface quality
Solution Approach 1:
The alloy is divided into fine particles through atomization, creating numerous small segments instead of large homogeneous structures. This segmentation prevents the formation of large second phases by limiting the growth distance during solidification, thereby improving fatigue life while maintaining shape memory and superelastic properties
Solution Approach 2:
The patent changes the cooling rate parameter from conventional slow cooling to rapid cooling during atomization. This parameter change transforms the solidification process, preventing large second phase formation by rapidly freezing the microstructure before significant phase separation can occur, thus improving both fatigue life and surface quality
2Manufacturing precision
If conventional processing methods are used, then production is simpler, but second phases are large in size affecting performance in small-diameter applications
Solution Approach 1:
The alloy is pre-alloyed before atomization, ensuring homogeneous composition is established prior to the rapid cooling process. This preliminary action prevents compositional segregation during atomization, enabling the subsequent formation of fine second phases without requiring complex post-processing steps
Solution Approach 2:
The patent utilizes controlled phase transitions during rapid cooling, where the austenite phase transforms to martensite at controlled rates. This phase transition control, combined with the atomization process, produces fine second phases by limiting the time and space available for phase growth, achieving precise microstructure control
3Reliability
If rapid cooling is applied to reduce second phase size, then fatigue resistance improves, but energy consumption increases
Solution Approach 1:
The patent replaces conventional mechanical cooling systems with a spray atomization system that utilizes fluid dynamics and surface tension. This substitution achieves rapid cooling through the formation and evaporation of fine droplets, reducing second phase size and improving fatigue resistance while consuming less energy than traditional quenching methods
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 process produces nickel-titanium alloys with significantly smaller second phases, enhancing fatigue resistance and reducing surface defects, particularly in fatigue-critical applications like implantable stents and other medical devices, while maintaining the alloys' shape memory and superelastic properties.
Implementation Method 1
The molten near-equiatomic nickel-titanium alloy particles are cooled to form a near-equiatomic nickel-titanium alloy powder
Implementation Method 2
these alloys, which are commonly referred to as 'Nitinol' alloys, are known to undergo a martensitic transformation from a parent phase (commonly referred to as the austenite phase) to at least one martensite phase on cooling to a temperature below the martensite start temperature ('Ms') of the alloy
Implementation Method 3
When a shape-memory nickel-titanium alloy is strained at a temperature above the Af of the alloy but below the so-called martensite deformation temperature ('Md'), the alloy can undergo a stress-induced transformation from the austenite phase to the martensite phase
Data Source
AI summary
Processes for producing a nickel-titanium alloy are disclosed. The processes are characterized by the production of nickel-titanium alloy articles having improved microstructure. A pre-alloyed nickel-titanium alloy is melted and atomized to form molten nickel-titanium alloy particles. The molten nickel-titanium alloy particles are cooled to form nickel-titanium alloy powder. The nickel-titanium alloy powder is consolidated to form a fully-densified nickel-titanium alloy preform that is hot worked to form a nickel-titanium alloy article. Any second phases present in the nickel-titanium alloy article have a mean size of less than 10 micrometers measured according to ASTM E1245-03 (2008) or an equivalent method.