Self-Healing Aluminum Metal Matrix Composite (MMC)

Shape memory alloys (SMAs) are metals that can return to their original shape following thermal input. They are commonly used as functional materials in sensors, actuators, clamping fixtures and release mechanisms across industries. SMAs can suffer from dimensional/thermal instability, creep, and/or low hardness, resulting in alloys with little to no work output in the long term. To combat these deficiencies, NASA has developed a process of incorporating nanoparticles of refractory materials (i.e., carbide, oxide, and nitride materials with high temperature resistance) into the alloys. Using various processing methods, the nanoparticles can be effectively mixed and dispersed into the metal alloys as shown in the figure below. In these processes the SMA and refractory material powder is mixed and the refractory nanoparticles incorporated through extrusions, melting, or directly used in additive manufacturing to create parts for applications across the aerospace, automotive, marine, or biomedical sectors. The nanoparticle dispersion is a controllable method to strengthen the SMAs, increasing the hardness of the alloys, reducing the impact of creep, and improving the overall dimensional and thermal stability of the alloys. The related patent is now available to license. Please note that NASA does not manufacture products itself for commercial sale.

This innovation allows the incorporation of metal matrix composite reinforcing material into a metallic structure as part of the structures fabrication process. It does not require secondary processing that may affect the structures mechanical properties. It does not require bonding agents that would limit the benefits of the reinforcing material. It adds reinforcement to only the specific regions of the structure that need enhanced strength, stiffness, and/or damage tolerance, thereby allowing for more efficient design and reduction in structural weight.

A composite fabrication process cycle was developed from composite precursor materials developed at LaRC to fabricate composite laminates. The precursor material is a pre-impregnated unidirectional carbon fiber preform, or prepreg. In the pre-pregging process, the high strength, structural reinforcing carbon fiber is wetted by a solution containing a self-healing polymer. The resulting material is of aerospace quality and exhibits a significant decrease of internal damage following impacts tests (using ASTM D 7137 standard).

Glenn's revolutionary SMARS device is fabricated from nickel-titanium-halfnium (NiTiHf), nickel-titanium-zirconium compositions, or a combination. These compositions contain a secondary, nanometer-sized precipitate phase, which is produced through processes of compositional control and ageing heat treatments. Glenn's novel materials and processes have yielded a SMA composition that produces much higher stresses than other SMAs on the commercial market. The SMARS device is composed of 1) SMA material as the actuating member; 2) a casing heater placed around the SMA member; 3) a DC or AC power source to provide current through the heater; 4) pointed tips for acute penetration into rock formations; and 5) a hand-press to reset the SMA element after each use. In the rock-splitting process, a hole equal to the diameter of the SMA element is drilled in the portion of the rock where the fracture is desired. Next, the pre-compressed SMA is inserted into the hole, and AC or DC current is applied to energize the devices heaters. Once the heater achieves the critical transformation temperature, the SMA will begin to expand within seconds. Since its expansion is constrained by the rock walls, the SMA will eventually exert up to 1500 MPa of stress, splitting the rock apart. When the current is removed and the heater cools, the SMA material returns to its pre-compressed state. At this point, the material can be recovered, so the process is repeatable after reshaping. The SMA actuating members were also designed to achieve displacement greater than the materials strain output. Glenns SMARS device provides high-powered rock fracturing that is controllable, reliable, and comparatively simple without the use of explosives, hydraulics, or chemicals.

Glenn researchers have optimized how shape memory alloys (SMAs) are trained by reconceptualizing the entire stabilization process. Whereas prior techniques stabilize SMAs during thermal cycling, under conditions of fixed stress (known as the isobaric response), what Glenn's innovators have done instead is to use mechanical cycling under conditions of fixed temperature (the isothermal response) to achieve stabilization rapidly and efficiently. This novel method uses the isobaric response to establish the stabilization point under conditions identical to those that will be used during service. Once the stabilization point is known, a set of isothermal mechanical cycling experiments is then performed using different levels of applied stress. Each of these mechanical cycling experiments is left to run until the strain response has stabilized. When the stress levels required to achieve stabilization under isothermal conditions are known, they can be used to train the material in a fraction of the time that would be required to train the material using only thermal cycling. As the strain state has been achieved isothermally, the material can be switched back under isobaric conditions, and will remain stabilized during service. In short, Glenn's method of training can be completed in a matter of minutes rather than in days or even weeks, and so SMAs become much more practical to use in a wide range of applications.