Mid-cap stocks are commonly defined as having a market cap between $2 billion and $10 billion. Companies in this valuation range tend to already be somewhat established, but they often still have lots of room for expansion -- and are generally more likely to enjoy big stock-price gains than their large-cap and megacap peers.

In this roundtable, three Motley Fool contributors weigh in with their picks for top mid-cap stocks to buy right now. Read on to see why they think Allegheny Technologies (NYSE:ATI), BlackBerry (NYSE:BB), and Baozun (NASDAQ:BZUN) are worth adding to your portfolio.

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An under-the-radar play on aerospace and metal 3D printing

Maxx Chatsko (Allegheny Technologies): It may not be on the radar of most investors, but Allegheny Technologies has quietly executed against its turnaround strategy in recent years and now finds itself well-positioned to capitalize on several huge trends in engineered metals. That includes novel alloys for aerospace applications, new nickel and titanium powders for metal 3D-printing applications, and a new manufacturing process (that the company invented) for flat-rolled steel products. The stock is up 48% in the last year as a result.

While a year ago there may have been some reason to be doubtful of the company's restructuring plan, a strong first half of 2018 has all but neutralized any lingering concerns. Allegheny Technologies' revenue increased 13.9%, segment operating profit was up 56.5%, and net income jumped 294% compared to the first half of last year. The business also delivered a $120 million improvement in operating cash flow in that time span.

Those are remarkable numbers for any company, but especially so in the relatively boring specialty metals industry. It might be just the beginning, too. While there's still some uncertainty regarding the impact trade tensions and tariffs could have on the business, it's actually possible that leading aerospace companies -- which are customers of Allegheny Technologies -- will have to divert international business to the American alloy producer. That may happen in any case, as it appears the company's past (and current) investments in research and development and new products are paying off in spades. Considering the stock trades at a PEG ratio of just 0.20 today, this $3.4 billion company is worth a closer look

A misunderstood tech stock

Leo Sun (BlackBerry): BlackBerry is often thought of as a former smartphone leader that lost the market to iPhones and Android devices. However, today the company no longer produces new smartphones -- it merely licenses its brand and designs to Chinese tech company TCL to generate higher-margin licensing revenues.

Moreover, BlackBerry has expanded its enterprise software portfolio with products like BES (BlackBerry Enterprise Service), which helps companies monitor their employees' mobile devices; the enterprise messaging app BBM (BlackBerry Messenger); QNX, the world's top embedded operating system for connected vehicles; and various cybersecurity solutions.

BlackBerry's software and services revenue accounted for 89% of its non-GAAP revenue last quarter, up from 79% in the prior-year quarter. BlackBerry's transition to recurring software and services revenue isn't complete yet -- but it's resulting in shallower sales declines. BlackBerry's revenue fell just 11% year over year during the first quarter of fiscal 2019, compared to a 20% decline in the fourth quarter and a 22% drop in the third quarter.

Wall Street expects BlackBerry's sales to fall just 8% this year, followed by 6% growth next year. Its non-GAAP profit is expected to fall 36% this year (partly due to changes in accounting standards), but rebound 67% next year. Those estimates indicate that BlackBerry is approaching an inflection point that could precede a big comeback.

That comeback isn't guaranteed, since BlackBerry still faces plenty of big competitors in the enterprise software space. But if it can fend off those rivals and keep growing, its stock could climb much higher.

Tap into Chinese e-commerce momentum

Keith Noonan (Baozun): Baozun has already seen its stock climb roughly 600% over the last three years to reach its current $3.3 billion market capitalization. While that's enough to give a potential investor pause -- how much gas is still left in the tank? -- the concern is even more pressing in light of the possibility that the e-commerce company, which specializes in providing online stores for major Western brands, could experience a halt in growth as a result of heightened trade tensions between the United States and China.

Baozun trades at roughly 50 times this year's expected earnings, so it's fair to say it's not a low-risk stock. However, it's one that I think will be a big winner over the long term.  

China is already the world's largest e-commerce market, and its roughly $1.15 trillion in sales conducted through online platforms spending last year accounted for about half of the world's online retail spending. Perhaps that incredible share makes you question whether the market is already in a relatively mature, slow-growth stage, but you can lay that concern to rest.

Chinese e-commerce spending increased 34% year over year in the first quarter of 2018 -- impressive growth, especially considering that total spend in the category had already risen 32% in 2017. And Baozun is well-positioned to take advantage of this growth.

The stores that it provides for its brand partners are featured on China's leading e-commerce and social media platforms. The company has big avenues to growth as it attracts new partners, consumer spending continues to migrate online, and the purchasing power of the Chinese middle class keeps increasing. Baozun's business is also becoming more profitable as the company moves away from distributing merchandise while prioritizing its sales portal, marketing, and customer management business.

With a strong position in its e-commerce niche, Baozun has the potential to be a huge winner for long-term shareholders.