This article demonstrates how differential scanning calorimetry (DSC) determines the four key phase transformation temperatures of NiTi-based shape memory alloys — the martensite start (Ms), martensite finish (Mf), austenite start (As), and austenite finish (Af) — using the AMI DSC 600, part of AMI’s range of thermal analysis instruments. For a broader overview of AMI’s thermal characterization capabilities, see our thermal properties analysis overview.
Shape memory alloys (SMAs) are among the most functionally distinctive metallic materials in modern engineering. Their defining property — the ability to recover a pre-programmed shape after deformation simply by heating — makes them indispensable in applications where compact, reliable, thermally-driven actuation is required. Understanding DSC shape memory alloy characterization is essential for anyone designing, specifying, or quality-controlling NiTi components, because the transformation temperatures that govern this behavior cannot be reliably predicted from composition alone — they must be measured directly on each sample.
Shape memory alloys are metallic materials composed of more than two elements that exhibit the shape memory effect through a thermoelastic martensitic phase transformation — a reversible, non-diffusive solid-state phase change that occurs without altering chemical composition. This transformation involves cooperative, localized atomic movement forming a metastable phase via twinning. The reversibility of the transformation and the mechanical property changes it produces are what make SMAs functionally useful.
SMAs are broadly categorized into NiTi-based, Cu-based, and Fe-based alloys. Among these, NiTi-based alloys (Figure 1; alt text: NiTi-based shape memory alloy material) are by far the most commercially successful, offering superior shape recovery, superelasticity, corrosion resistance, and biocompatibility. Their applications span:
In NiTi alloys, the high-temperature phase is austenite (B2 cubic crystal structure) and the low-temperature phase is martensite (B19′ monoclinic structure). The transformation between them is temperature-dependent and fully reversible:
NiTi components are typically fixed into a pre-set shape at high temperature (austenitic phase), then cooled below Ms to become soft and deformable in the martensitic phase. Upon reheating above Af, the alloy returns to its original austenitic shape — the shape memory effect. The width between the heating and cooling transformation ranges is the thermal hysteresis, which is characteristic of the alloy composition and processing history.
| Why composition analysis is insufficient: Even small variations in Ni:Ti atomic ratio — as little as 0.1 at.% — can shift transformation temperatures by 10°C or more. Thermomechanical processing history further modifies transformation behavior independently of composition. DSC is therefore the preferred method for determining actual transformation behavior in samples with known thermal processing history, not for estimating it from composition. |
DSC measures the difference in heat flow between a sample and an inert reference during controlled heating and cooling. When the NiTi sample undergoes a phase transformation, it either releases heat (exothermic — austenite to martensite on cooling) or absorbs heat (endothermic — martensite to austenite on heating). These thermal events appear as distinct peaks on the DSC heat flow curve, and the characteristic temperatures of each peak directly correspond to the transformation temperatures of the alloy.
This makes DSC uniquely suited to DSC shape memory alloy characterization: it directly detects the latent heat associated with each transformation, providing not just the temperatures but also the transformation enthalpy, hysteresis width, and any multi-stage or shoulder behavior that indicates R-phase or partial transformations in more complex alloys. For a broader overview of DSC applications across pharmaceutical and materials characterization, see our article on differential scanning calorimetry application.
NiTi-based SMA samples were tested using the AMI DSC 600 differential scanning calorimeter. The sample was cut to match the crucible’s inner diameter and placed inside a solid crucible, then pressed flat to ensure optimal thermal contact between the sample and the crucible wall.
| Parameter | Condition |
|---|---|
| Instrument | AMI DSC 600 |
| Sample mass | 6.58 mg |
| Crucible | Solid (non-perforated) |
| Atmosphere | Nitrogen, 50 mL/min |
| Temperature range | −20°C to 50°C |
| Heating/cooling rate | 10°C/min |
Phase transformation temperatures were determined using the tangent method — the approach specified by ASTM F2004, the industry standard test method for NiTi transformation temperature measurement by thermal analysis. The method draws tangent lines to the baseline and to the steepest portion of the transformation peak. The intersection of these tangents defines the start temperature, while the return to baseline defines the finish temperature. Peak temperature is taken at the maximum of the exothermic or endothermic event. The AMI DSC 600 software provides built-in DSC Peak Area and DSC Onset analysis functions (Figure 3; alt text: AMI DSC 600 software showing tangent method functions for determining Ms, Mf, As, and Af) that apply this method directly.
The cooling curve (Figure 2, black curve; alt text: NiTi DSC profile showing exothermic cooling peak and endothermic heating peak between −20°C and 50°C) shows a clear exothermic peak corresponding to the austenite-to-martensite transformation:
| Temperature Parameter | Value | Physical Meaning |
|---|---|---|
| Ms — Martensite start | 14.00°C | Temperature at which austenite begins converting to martensite on cooling |
| Mp — Martensite peak | 11.18°C | Temperature of maximum transformation rate on cooling |
| Mf — Martensite finish | 8.18°C | Temperature at which the transformation to martensite is complete |
The heating curve (Figure 2, red curve) shows an endothermic peak corresponding to the reverse martensite-to-austenite transformation:
| Temperature Parameter | Value | Physical Meaning |
|---|---|---|
| As — Austenite start | 10.18°C | Temperature at which martensite begins converting back to austenite on heating |
| Ap — Austenite peak | 13.64°C | Temperature of maximum transformation rate on heating |
| Af — Austenite finish | 18.51°C | Temperature at which the alloy has fully returned to the austenitic phase |
| Key findings: The thermal hysteresis — the gap between the average cooling transformation temperature and the average heating transformation temperature — is approximately 4–5°C for this sample. The narrow transformation range (Mf to Ms span of ~5.8°C on cooling, As to Af span of ~8.3°C on heating) confirms a high-quality, well-ordered NiTi microstructure with uniform composition. The precision and clean resolution of both peaks demonstrates the DSC 600’s sensitivity to the subtle thermal events characteristic of NiTi transformations near room temperature. |
The DSC 600 (Figure 4; alt text: AMI DSC 600 differential scanning calorimeter for NiTi shape memory alloy phase transformation analysis) is well-suited to DSC shape memory alloy characterization for several specific reasons:
DSC is the most direct and accurate method for characterizing DSC shape memory alloy phase transformation behavior. Applied to NiTi alloys, it provides precise, repeatable determination of Ms, Mf, As, and Af — the four temperatures that govern actuation, fatigue resistance, and service stability. Because these transformation temperatures are sensitive to composition, processing history, and heat treatment, direct DSC measurement on each sample (rather than estimation from nominal composition) is essential for ensuring performance consistency in functional applications.
The AMI DSC 600 delivers the sensitivity, baseline stability, and precise temperature control needed for reliable NiTi SMA characterization — from the detection of subtle single-stage transformations to the resolution of multi-stage R-phase and martensitic behavior in complex heat-treated alloys. Explore AMI’s full range of thermal analysis instruments, or visit the AMI Technical Library for further application notes on DSC and thermal characterization methodology.
Differential scanning calorimetry is preferred because it directly and precisely measures the heat flow associated with the martensitic and austenitic phase transformations during controlled heating and cooling. Chemical composition analysis alone cannot predict transformation temperatures with sufficient accuracy — even 0.1 at.% variation in Ni:Ti ratio can shift temperatures by 10°C or more, and thermomechanical processing history further modifies transformation behavior independently of composition. DSC provides a direct, material-state measurement of the actual sample in its current condition, making it more accurate and reproducible than any compositional or structural method for this purpose. It is the basis of ASTM F2004, the industry standard test method for NiTi transformation temperature measurement.
Ms (martensite start) — the temperature at which austenite begins converting to martensite on cooling. Mf (martensite finish) — the temperature at which the martensitic transformation is complete. As (austenite start) — the temperature at which martensite begins converting back to austenite on heating. Af (austenite finish) — the temperature at which the alloy has fully returned to the austenitic phase. Together these four temperatures define the functional temperature window of the alloy — the range over which shape recovery occurs — and directly govern actuation behavior, thermal hysteresis, fatigue resistance, and service stability in all SMA applications.
On cooling, austenite transforms to martensite and releases latent heat — detected by DSC as an exothermic peak on the cooling curve. The onset and completion of this peak correspond to Ms and Mf. On heating, the reverse transformation occurs: martensite converts back to austenite, absorbing heat from the surroundings and producing an endothermic peak on the heating curve, with onset and completion at As and Af. The area under each peak represents the transformation enthalpy, and the separation between the cooling and heating peak temperatures is the thermal hysteresis — a characteristic of the specific alloy composition and thermomechanical processing.
Transformation temperatures are determined using the tangent method, as specified by ASTM F2004. Tangent lines are drawn to the pre-transition baseline and to the steepest portion of the transformation peak. Their intersection defines the start temperature (Ms or As), while the return to baseline defines the finish temperature (Mf or Af). Peak temperature (Mp or Ap) is taken at the maximum of the exothermic or endothermic event. The AMI DSC 600’s built-in DSC Peak Area and DSC Onset software functions apply this method directly to the measured data, eliminating the need for manual graphical construction.
The NiTi sample should be cut to match the inner diameter of the DSC crucible and pressed flat to ensure optimal thermal contact between the sample and the crucible wall — maximizing heat transfer and minimizing thermal lag. A solid (non-perforated) crucible is used to contain the sample. Measurements are conducted under an inert nitrogen atmosphere to prevent surface oxidation during the test cycle. The temperature range should bracket the expected transformation temperatures with sufficient margin to establish clear baselines above Af and below Mf.
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