Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some materials-focused stocks to your portfolio, the First Trust Materials AlphaDEX ETF (NYSEMKT:FXZ) could save you a lot of trouble. Instead of trying to figure out which companies will perform best, you can use this ETF to invest in lots of them simultaneously. It's an "enhanced" index fund, favoring stocks with appealing growth and value attributes.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The First Trust ETF's expense ratio -- its annual fee -- is 0.70%. That's a bit higher than many ETFs, but it's also considerably lower than the typical managed stock mutual fund. The fund is fairly small, too, so if you're thinking of buying, beware of possibly large spreads between its bid and ask prices. Consider using a limit order if you want to buy in.

This ETF has performed well, handily beating the world market over the past three and five years. As with most investments, of course, we can't expect outstanding performances in every quarter or year. Investors with conviction need to wait for their holdings to deliver.

Why materials?
Our global economic slump won't last forever, and there are already signs of life here and there. Thus, the materials industry is poised to prosper as construction and infrastructure projects get under way and manufacturing kicks into a higher gear.

More than a handful of materials-related companies  had strong performances over the past year. Paint giant Sherwin-Williams (NYSE:SHW) soared 80% as the housing market finally seems like it's turning around. The company recently bought global paint giant Comex, based in Mexico, for $2.3 billion. Some don't like that the deal will add to the company's debt, but others see it as a smart strategic move. Sherwin-Williams is growing  in developing economies such as Brazil. Some see the stock as pricey now.

Timken (NYSE:TKR), gained 10% and sports an attractive valuation, with a current and forward P/E of about  8. Many like that the company is committed to manufacturing in America and is not outsourcing its work. The company recently posted some disappointing quarterly results, and some would like to see it spin off  its steel unit. Its fans like its 2% dividend, significant insider ownership, strong return on equity, and solid growth prospects.

Other companies didn't do as well last year, but could see their fortunes change in the coming years. Cliffs Natural Resources (NYSE:CLF), for example, sank 56%. It's yielding more than 8%, but that's mainly due to the stock-price drop. Cliffs has struggled along with the coal industry. Still, bulls like its prospects, especially once the auto industry and others are recovering more strongly, as Cliffs' metallurgical coal is used in making steel. In the meantime, though, the company is cutting back on production. For long-term investors, Cliffs seems attractively priced, and will pay you well to wait.

Commercial Metals (NYSE:CMC), recently yielding 3.5%, is about flat over the past year. It's a Texas-based company that manufactures and recycles items made of steel and other metals. Among other things, it processes scrap metal into raw materials for other manufacturers. In its last quarter, the company posted  an earnings-per-share increase of 13% on revenue down a smidge. It has been pressured  by weak scrap metal markets and Europe's troubles but points to a strong backlog of orders. The company is planning  to get out of its less profitable business lines and has been strengthening  its balance sheet.

The big picture
Demand for materials isn't going away anytime soon. A well-chosen ETF can grant you instant diversification across any industry or group of companies -- and make investing in and profiting from it that much easier.

Longtime Fool contributor Selena Maranjian, whom you can follow on Twitter, owns shares of Cliffs Natural Resources. The Motley Fool has no positions in the stocks mentioned above. Motley Fool newsletter services recommend Sherwin-Williams. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.