A Role of α′ Martensite Introduced by Thermo‐Mechanical Treatment in Improving Shape Memory Effect of an Fe‐Mn‐Si‐Cr‐Ni Alloy

Abstract

The evolution of α′ martensite with different thermo‐mechanical treatment and its effect on the shape memory effect were studied in an Fe‐14Mn‐5Si‐8Cr‐4Ni alloy. The α′ martensite was introduced by only 5% pre‐deformation, and its amount increased with increasing pre‐deformation up to 20%. The α′ martensite started to transform into austenite when the annealing temperature was 773 K. As the annealing temperature increased to 1 073 K, the α′ martensite almost transformed fully into austenite. The α′ martensite introduced by the thermo‐mechanical treatment could prevent collisions between different ε martensite bands during deformation. The intrusion of α′ martensite was another key reason that the stress‐induced ε martensite bands in Fe‐Mn‐Si based shape memory alloys formed in a domain‐specific manner in addition to that of uniformly distributed stacking faults after thermo‐mechanical treatment.

Key Factors Achieving Large Recovery Strains in Polycrystalline Fe–Mn–Si‐Based Shape Memory Alloys: A Review

“…One is the transformation of face‐centered cubic (FCC) γ austenite to hexagonal close‐packed (HCP) ϵ martensite, and the other is the ϵ martensite to body‐centered cubic (BCC) α ′ martensite . Studies showed that only stress‐induced FCC → HCP martensite and its reverse transformation are responsible for the SME of Fe – Mn – Si‐based SMAs, the occurrence of γ (FCC) → ϵ (HCP) → α ′ (BCC) deteriorates the SME, which will be discussed in another section. Note that FCC → HCP martensitic transformation in Fe – Mn – Si‐based SMAs is non‐thermoelastic, in which a much bigger thermal‐hysteresis of above 100 K exists .…”

“…First, the occurrence of γ (FCC) → ϵ (HCP) → α ' (BCC) martensitic transformation depends on the Mn content and deformation strain in Fe – Mn – Si‐based SMAs . The α ' martensite is generally formed at the intersections between different orientation HCP martensite . Hoshino et al observed that the α ' martensite at the intersections of HCP martensite resulted in the increase of HCP → FCC reverse transformation temperature by in situ TEM .…”

“…In addition, the α ' martensite can skip the α ' (BCC) → ϵ (HCP) transformation step and revert directly to FCC parent phase upon heating . Therefore, the intrusion of α ' martensite must deteriorate the SME of Fe – Mn – Si‐based SMAs . As a result, suppressing the formation of α ' martensite is important for obtaining a good SME.…”

Thermodynamic Explanation for the Large Difference in Improving Shape Memory Effect of Fe–Mn Alloys by Co and Si Addition

“…[2] In 1986, Murakami and coworkers developed polycrystalline Fe- (28)(29)(30)(31)(32)Mn-(4-6.5)Si alloys showing almost complete SME as well. [3] In 1991, Otsuka et al developed Fe- (14)(15)(16)(17)(18)(19)(20)(21)(22)Mn-(5-6)Si- (8)(9)(10)(11)(12)Cr-(5-7)Ni alloys exhibiting good SME along with good corrosion resistance through alloying with Ni and Cr. [4] Thereafter, much research has been carried out on these Fe-Mn-Si-based alloys to further improve their recovery strain, including training, [5][6][7][8] thermo-mechanical treatment, [9][10][11] ausforming, [12,13] and precipitation of second-phase particles.…”

Effect of Pre‐Deformation and Subsequent Aging on the Damping Capacity of Mn‐20 at.%Cu‐5 at.%Ni‐2 at.%Fe alloy

“…The crystal defects and internal stress field could be introduced by cold pre‐deformation. Actually, a great number of recent studies have been focused on the effect of pre‐deformation on the properties of various materials, including aluminum alloy, Zirconium alloy, shape memory alloys, FeMn damping alloy and Mn‐15 at% damping alloy . In all, the defects and internal stress distribution aroused by the pre‐deformation play a critical role in affecting the properties of the above alloys.…”