加州大學(xué)洛杉磯分校(UCLA) 亨利薩穆埃利工程與應(yīng)用科學(xué)學(xué)院的研究人員帶領(lǐng)團(tuán)隊成功研制出一種新型的超強(qiáng)輕質(zhì)結(jié)構(gòu)性金屬。這種金屬擁有超高的比強(qiáng)度和比模量，換言之，這種新型材料的剛重比（stiffness-to-weight）非常高。新材料是由金屬鎂和注入其中的密集且均勻分布的陶瓷硅碳化納米顆粒組成。用這種材料制造的飛機(jī)、航天器和汽車重量更輕，因此燃料經(jīng)濟(jì)性也更高。
為了研制這種超強(qiáng)但輕質(zhì)的金屬材料，UCLA團(tuán)隊找到了一種方法，可以在熔融金屬中分散納米顆粒，并使其保持穩(wěn)定。他們還研發(fā)了一種可擴(kuò)大規(guī)模的制造方法，用以制造更多的高性能輕質(zhì)金屬。
UCLA制造工程講席教授李曉春是Raytheon合作項(xiàng)目的主要負(fù)責(zé)人，他表示：“過去有人提出納米顆粒可以真正增強(qiáng)金屬的強(qiáng)度，而且不會影響其可塑性，特別是與鎂這種輕質(zhì)金屬結(jié)合時尤其有效。但在我們之前，還沒有哪個研究機(jī)構(gòu)可以在熔融金屬中注入分散的陶瓷硅納米顆粒。
通過物理注入和材料加工的方法，我們可以向多種不同的熔融金屬中均勻注入密集納米顆粒，開創(chuàng)了提高金屬性能的新途徑，有助于應(yīng)對當(dāng)下社會面臨的能源節(jié)約和可持續(xù)發(fā)展挑戰(zhàn)。”
結(jié)構(gòu)性金屬指的是承擔(dān)支撐任務(wù)的金屬，多用于建筑和汽車之中。金屬鎂的密度僅為鋁的三分之二，是最輕的結(jié)構(gòu)性金屬。碳化硅是一種超級堅固的陶瓷材料，常用于工業(yè)刀片的制造。研究人員在金屬鎂中注入大量直徑小于100納米的碳化硅顆粒，能夠顯著提升材料的強(qiáng)度、硬度和可塑性，以及高溫下的耐用性。
用研究人員的話來說，這種注入碳化硅的新型鎂材料具有“創(chuàng)紀(jì)錄”的比強(qiáng)度和比模量，前者指材料在破裂前可以承受的最大重量，后者指材料的剛重比。這種材料還在高溫下表現(xiàn)出優(yōu)異的穩(wěn)定性。
長久以來，陶瓷顆粒一直被視為提高金屬強(qiáng)度的可行方法。不過，在過去，由于顆粒僅能做到微米級，注入后會影響材料的可塑性，因而一直未得到有效利用。
與微米顆粒相比，納米顆粒不但可以增強(qiáng)金屬的硬度，而且還能保持、甚至提升材料的可塑性。但這里有個問題，由于微小粒子間會相互吸引，納米陶瓷顆粒很容易聚成一團(tuán)，而非均勻分布在新材料中。
為了解決這一問題，研究人員將陶瓷微粒注入熔融狀態(tài)下的鎂鋅合金之中。這種新發(fā)現(xiàn)的納米粒子主要依靠粒子運(yùn)動時的動能實(shí)現(xiàn)分散。這可以穩(wěn)定顆粒的分布，抑制聚團(tuán)現(xiàn)象。
為了進(jìn)一步提高新金融材料的強(qiáng)度，研究人員采用了高壓扭轉(zhuǎn)法(high-pressure torsion)對其進(jìn)行壓縮。
李教授表示，“對于這類具有革命性屬性和功能性的金屬材料，目前我們的研究成果仍僅涉及一點(diǎn)皮毛而已。”
這種新金屬材料，更準(zhǔn)確的說是這種新金屬納米復(fù)合材料，其成分為14%的碳化硅納米顆粒和86%的金屬鎂。研究人員指出，由于鎂資源相當(dāng)豐富，以此為原料擴(kuò)大生產(chǎn)并不會危害環(huán)境。
作者：Jean L. Broge
來源：SAE 《航空工程》雜志
翻譯：SAE 上海辦公室
UCLA researchers develop new strong and lightweight metal nanocomposite
A team led by researchers from the UCLA Henry Samueli School of Engineering and Applied Science has created a super-strong yet light structural metal with a very high specific strength and modulus, or stiffness-to-weight ratio. The new metal is composed of magnesium infused with a dense and even dispersal of ceramic silicon carbide nanoparticles. It could be used to make lighter airplanes, spacecraft, and cars, helping to improve fuel efficiency.
To create the super-strong but lightweight metal, the team found a new way to disperse and stabilize nanoparticles in molten metals. They also developed a scalable manufacturing method that could pave the way for more high-performance lightweight metals.
“It’s been proposed that nanoparticles could really enhance the strength of metals without damaging their plasticity, especially light metals like magnesium, but no groups have been able to disperse ceramic nanoparticles in molten metals until now,” said Xiaochun Li, the principal investigator on the research and RaytheonChair in Manufacturing Engineering at UCLA.
“With an infusion of physics and materials processing, our method paves a new way to enhance the performance of many different kinds of metals by evenly infusing dense nanoparticles to enhance the performance of metals to meet energy and sustainability challenges in today’s society,” he said.
Structural metals are load-bearing metals, and are used in buildings and vehicles. Magnesium, at just two-thirds the density of aluminum, is the lightest structural metal. Silicon carbide is an ultra-hard ceramic commonly used in industrial cutting blades. The researchers’ technique of infusing a large number of silicon carbide particles smaller than 100 nanometers into magnesium added significant strength, stiffness, plasticity, and durability under high temperatures.
The researchers’ new silicon carbide-infused magnesium demonstrated what they have described as "record levels" of specific strength—how much weight a material can withstand before breaking—and specific modulus—the material’s stiffness-to-weight ratio. It also showed superior stability at high temperatures.
Ceramic particles have long been considered as a potential way to make metals stronger. However, with microscale ceramic particles, the infusion process results in a loss of plasticity.
Nanoscale particles, by contrast, can enhance strength while maintaining or even improving metals’ plasticity. But nanoscale ceramic particles tend to clump together rather than dispersing evenly, due to the tendency of small particles to attract one other.
To counteract this issue, researchers dispersed the particles into a molten magnesium zinc alloy. The newly discovered nanoparticle dispersion relies on the kinetic energy in the particles’ movement. This stabilizes the particles’ dispersion and prevents clumping.
To further enhance the new metal’s strength, the researchers used a technique called high-pressure torsion to compress it.
“The results we obtained so far are just scratching the surface of the hidden treasure for a new class of metals with revolutionary properties and functionalities,” Li said.
The new metal (more accurately called a metal nanocomposite) is about 14% silicon carbide nanoparticles and 86% magnesium. The researchers noted that magnesium is an abundant resource and that scaling up its use would not cause environmental damage.
Author: Jean L. Broge
Source: SAE Aerospace Engineering Magazine
純金屬的形變樣本（左）；金屬鎂和碳化硅納米顆粒組成的新型超強(qiáng)金屬材料（右）。中央微柱的直徑在4微米左右。
項(xiàng)目主要研究員、UCLA制造工程講席教授李曉春表示：“過去有人提出納米顆?？梢哉嬲鰪?qiáng)金屬的強(qiáng)度，而且不會影響其可塑性，特別是與鎂這種輕質(zhì)金屬結(jié)合尤其有效。但在我們之前，還沒有哪個研究機(jī)構(gòu)可以在熔融金屬中注入分散的陶瓷硅納米顆粒。”