The Effect of Concurrent Straining on Phase Transformations in NiAl Bronze During the Friction Stir Processing Thermomechanical Cycle

Abstract

Equivalent strains up to a value of %2.7 were determined by evaluation of the shape changes of the phases in a duplex a(fcc)/b(bcc) microstructure formed ahead of the pin tool extraction site during the friction stir processing (FSP) thermomechanical cycle in a cast NiAl bronze alloy. Correlation of the local strains with volume fractions of the various microstructure constituents in this alloy shows that the concurrent straining of FSP results in acceleration of the a + b fi b reaction in the thermomechanically affected zone (TMAZ) ahead of the pin extraction site. The resulting volume fraction of b (as determined by the volume fraction of its transformation products formed during post-FSP cooling) corresponds closely to the volume fraction expected for the peak stir zone temperature measured separately by means of thermocouples embedded within the tool pin profile along the tool path. The stir zone (SZ) in this material exhibits nearequilibrium microstructures despite brief dwells near the peak temperature (T peak % 0.95T melt ), reflecting large local strains and strain rates associated with this process.

Effect of Post Heat Treatment on the Microstructure and Microhardness of Friction Stir Processed NiAl Bronze (NAB) Alloy

“…At "low" temperature, NAB undergoes intense plastic deformation during FSP, leading to grain refinement and microstructure uniformity. According to the previous literature, the refined grains and homogeneous microstructure can improve the mechanical properties of the NAB alloy [13,14]. The corrosion resistance of FSP NAB alloy is also enhanced by the refined grains [15].…”

Strengthening Mechanisms in NiAl Bronze: Hot Deformation by Rolling and Friction-Stir Processing

“…[6,[8][9][10]23,24] Briefly, multipass FSP was conducted using tools designed with a pin in the shape of a truncated cone and fabricated in Densimet 176. The tool shoulder diameter was 28.6 mm, the pin base diameter was 15 mm, the length was 12.7 mm, and tip diameter was 6.3 mm.…”

“…[8,9] Also, strains in the thermomechanically affected zone (TMAZ) surrounding the SZ can be estimated by evaluating the shape change of microstructure constituents. [10] Nevertheless, the rapid transients and steep gradients within the SZ, within the TMAZ, and into the base metal preclude direct correlation of the FSP thermomechanical cycle with local details of the microstructure, e.g., composition as well as grain size. However, annealing alone or uniform deformation under isothermal conditions will enable separate characterization of temperature and strain effects on the details of microstructure of the NiAl bronze material of the current investigation.…”

“…[6,8,9,23] Essentially, peak SZ temperatures are in the range 1073 K to 1273 K (800°C to 1000°C), and corresponding microstructures consist of various mixtures of deformed primary a that has not transformed, as well as transformation products of the b formed during heating and deformation. [10,24] The j iv particles precipitated during slow cooling of the as-cast materials are highly refined in the SZ primary a, suggesting that the j iv dissolves when local temperatures exceeds 1133 K (860°C) during FSP and then reprecipitated during subsequent cooling. The primary a grains typically are equiaxed but vary in size from <1 to >10 lm in size depending on SZ location and processing conditions.…”

Investigation of microstructure and mechanical properties of hot worked NiAl bronze alloy with different deformation degree

“…In addition, the strengthening mechanisms of deformed NAB alloy have been investigated in many literatures [15,16]. Ni et al [17] found that dynamic recrystallization occurred in the stir zone during FSP.…”