One solution is X-ray diffraction

Sorry, what is a martensitic or a austinitic phase of a heusler alloy? I thought Heusler alloys were fcc based.

There are several ways to identify the change in phase of the material. The most commonly used is Differential Scanning Calorimetry (DSC) which measures the change in heat flow through the sample and onset of phases can be identified by either endothermic or exothermic behavior.

Another method in Heusler alloys is to use Vibrating Scanning Magnetometry (VSM) with a heat stage attached. The change in the magnetism of the sample at certain temperatures can give information on change of phase, structure and can also give curie temperatures foir each phase.

Also, X-Ray and Neutron diffraction can be used, but this will not give you any magnetic information, this would give purely structural changes within the sample, obviously neutron diffraction would be prferred here as the penetration depth is much greater and the atomic scattering factors of materials close to each other on the periodic table have similar scattering lengths, therefore neutron data is much more useful here.

Martensitic and austenitic phases can be determined by XRD, DSC, Magnetisation, Ac-chi, neutron diffraction etc as a function of temperature.

Temperature dependent XRD gives only the information of structural change across the transition temperature.

DSC gives the signature of both the first order martensitic transition and the second order curie transition. Magnetization and ac-chi can give the signature of both martensitic and curie transition temperature if the system is having martensitic transition below the curie transition i.e if the system is showing a ferromagnetic martensitic transition.

But if the martensitic transition is in the paramagnetic phase then only DSC can give you the information of both the martensitic and curie transition and XRD and Magnetisation can be used as a complimentry technique to justify the paramagnetic martensitic transition.

Find a reference of mine showing the experimental results on the Ni-Mn-Ga shape memory alloys have the ferromagnetic martensitic transition and the paramagnetic martensitic transition for different compositions. [Banik et al., PHYSICAL REVIEW B 74, 085110 (2006)].

A simple microetch, follow astm standard ASTM E407 - 07e1 Standard Practice for Microetching Metals and Alloys

For Fe containing alloys Mossbauer is the best technique to identify Martensite and Austenite phase

57 Fe Mössbauer spectroscopy will provide the best and ACCURATE results.

Answers by Soma Banik and JL Sánchez Llamazares are clear and complementary, but I do not agree with that by C Quinn, neutron diffraction is not useful from the magnetic point of view? By the way, this technique is the most powerful to determine the magnetic structure of a compound (Shull was awarded with the Nobel Prize in 1994 for his contributions to this topic). Therefore, neutron thermo-diffraction ahould be very helpful to follow "in situ" a martensite-austenite transition together with the change in the magnetic moment associated with such transition.

Yes Pedro, you have a valid point regarding Neutron diffraction, but there are other methods I would select first. Refining neutron data can be very time consuming and if was just finding the temperature of an austinitic-martensitic transition I would opt for VSM or SQUID measurements or DSC as these can normally be achieved in most research departments; getting beam time at ISIS or the ILL can prove difficult.

By XRD technique!!!

3D-neutron depolarisation is also a good technique to probe the transformation. It probes directly the magnetic structure. For more information: doi:10.4028/www.scientific.net/MSF.500-501.339