[sandy42011 (OP)]

Is there any alloy wire which tends to straighten itself on heating say about 70-80 degrees??

[CliffordK]

What is your goal?
Bent wire?  Or simply straightening a curve?
Dial thermometers, or mercury thermostats, and perhaps other devices typically have a coil in the middle.  The coil is made with an annealed bi-metal, with two different thermal expansion coefficients. 

Presumably you could make the same thing so that it would be straight at your target temperature, but bend at other temperatures. 

I'm not sure if you could make a wire that would automatically remove an arbitrary bend.  However, if it was to exist, I'd try something with a low thermal expansion core surrounded by a high thermal expansion outer sheath. 

[RD]

I think the OP is looking for "memory metal" ... https://en.wikipedia.org/wiki/Memory_metal

[sandy42011 (OP)]

 actually I m trying to make a generator by joining two pulleys by using a nitinol wire,by converting the auto straightening  into work done against a generator.

[sandy42011 (OP)]

I am trying to generate electricity by joining two pulleys of different diameter by nitinol wire which shows auto straightening property when heated. So water will get heated by sun and then used to heat wire around larger pulley. So the straightening action will be converted into work done against a generator.
Comments plz about its possibility and if sombdy knows about similar model.

[RD]

... water will get heated by sun and then used to heat wire around larger pulley ...

If you have flowing water I suspect you'd get more energy from a low-tech water-wheel than from memory-metal springs being heated by water. 

[CliffordK]

Since you wish something to expand in one direction with heating, and in another direction with cooling, then the thermostat coil should do quite well.  However, the power from the simple coil is quite minimal.  Potentially it could be connected to a piezoelectric device though.

The simple thermostat coil may be too limiting though.

You might be able to expand the concept of two materials with different thermal expansion characteristics as one finds in the thermostat with larger, thicker materials.  However, the constant bending may be a problem.

One could potentially make a teeter-totter using two significantly different materials such as a metal, and some plastics.

If you alternated heat/cold sources such as sun/shade, you might be able to even do it with a single material. 

The reaction would be slow though.