Generally, Phases with the same chemical composition Can have big different diffraction. The atomic arrangement of the same phase can be different for each state. Thus different XRD patterns are produced.

Even material with same composition can have different phases which results in different XRD patterns. The phase formation depends on processing conditions, growth direction and xrd measurement conditions.

In the case of alloys of metals X-ray diffraction patterns of powder form and bulk form may be different due to following reasons:

1) In the bulk form of the alloys at surface there may/are present inhomogeneous composition of the constituent components which caused different XRD pattern.

2) In powder form the constituent components become homogeneous and produce different from bulk and more reliable XRD pattern.

There may be difference in intensity.