Why do some alloys have much higher electrical resistance than their constituent elements? For instance, a typical Nichrome alloy has a resistance of $1.0 \times 10^{-6} \Omega \cdot m$, which is much higher than that of pure Nickel ($7 \times 10^{-8} \Omega \cdot m$) or Chrome ($1 \times 10^{-7} \Omega \cdot m$). 
 A: A perfect crystal lattice of ions at very low temperatures would offer very little impediment to the free electrons i.e. it  would have a very low resistance.
Add some imperfections like making the lattice ions vibrate as a result of them being at a higher temperature will result in more interaction between the free electrons and the ions - the resistance goes up. Dislocations in the crystal and impurities also increase the resistance. 
So in an alloy you have two species of ions and hence a situation which is far from a perfect crystal lattice. So there are lots of interactions between the lattice ions and the free electrons leading to a high resistance.
A free electron only "sees" imperfections and it is the interaction between the free electrons and the imperfection which is the origin of resistance. Free electrons are scattered off imperfections and alloys have many more of them than pure metals. 
A: The higher resistivity in alloys as compared to the constituents is caused by an additional scattering mechanism of the electrons called "alloy scattering". The reason for this is that the atomic ions of the different constituents in an alloy are, in general, not periodically arranged in the crystal lattice but their relative concentrations fluctuate statistically in the lattice. These fluctuations are additional "diffraction centers" for the electron waves leading to the additional scattering mechanism "alloy scattering" which is absent in the pure constituent materials.This explains the increased resistivity of alloys.  
A: In alloy Ni and Cr ions have different size and charge. They occupy random locations relative to each other.An electron passes through a very random medium and is very frequently deflected. So there is a small relaxation time and hence large resistivity.
