No it's not. It is really bad.
Basically, this is a stream cipher, where your keystream is $key | key| key|key....$. This is really bad, similar to the level of "using OTP twice". As If you take a block of size keylength, and XOR two such blocks, then you get the XOR of the plaintexts. Depending on the nature of the plaintext, this can be really easy to guess.
If you want an easy encryption scheme, like XOR the plaintext with a keystream, use a stream cipher or a block cipher in OFB mode of operation. If you want a construction with SHA512, that is also possible, see e.g.
However, since hash functions have different security properties than stream ciphers, it might be better to use a construction which is actually intended to be used this way. In a similar context, Bruce Schneier wrote (about ciphers based on hash functions):
While these constructions can be secure, they depend on the choice of the underlying one-way hash function. A good one-way hash function does not necessarily make a secure encryption algorithm. Cryptographic requirements are different. For example, linear cryptanalysis is not a viable attack against one-way hash functions, but works against encryption algorithms. A one-way hash function such as SHA could have linear characteristics which, while not affecting its security as a one-way hash function, could make it insecure in an encryption algorithm such as MDC. I know of no cryptanalytic analysis of particular one-way hash functions as block ciphers; wait for such analysis before you trust any of them.