Deformation Behaviors Of Sn At Cryogenic Temperatures And Modification Of Mechanical Properties

The ability of space exploration is one of the most important symbols of national high-tech competitiveness.The development of space exploration technology can help advance the basic science and high technologies.Spacecrafts such as satellite and space station are the main platforms of space exploration.Some spacecraft instruments are required to operate directly under the space environment.Moreover,there will be more and more instruments that have to directly work in the space environments.Interconnection is the weakest part in the electrical instruments.The reliability of the interconnection part becomes severer in the extreme and harsh space environments.The extremely low temperature（<173 K）is the important service conditions of electrical instruments working directly in the space environments.As the widely used interconnection materials,Sn-based solder alloys have low melting point and electrical resistivity as well as great ductility.However,the phase transformation and deformation brittleness of Sn at cryogenic temperatures cause potential hazards to reliable service of these Sn-based solder alloys.Hence,to investigate the phase transformation and deformation brittleness mechanisms of Sn at cryogenic temperatures and present the methods to enhance the cryogenic ductility of Sn will enrich the knowledge about the cryogenic reliability of the electrical interconnection materials and provide key theoretical bases for design and fabrication of space electronics.Firstly,this paper designed a unique,readily and reliable buoyancy-force-based kinetics determination ofβ→αphase transformation in bulk tin plates.The approach shows a high sensitivity about the transformed fraction with detection limits of about2%.This paper investigated the influence of storage temperature,artificial nuclei and grain size on transformation kinetics and nucleation mechanisms.It is found that the nucleation is more critical toβ→αphase transformation in bulk Sn plates.The long nucleation period allows separation between the study of phase transformation and the study of cryogenic brittleness.Through the uniaxial tensile experiments on polycrystalline Sn at room temperature（RT）and the liquid nitrogen temperature（LNT）,it is found that as compared to those at RT,the ultimate tensile strength,σ_UTS rises from 20 MPa to 78MPa（by～290%）at LNT.Despite this large increase in strength,the tensile ductility（engineering strain to failure,ε_f）declines markedly from 90%to 8%.A premature brittle failure occurred at LNT with a main intergranular fracture pattern.Moreover,a linear strain hardening occurred at LNT but the brittle fracture amid the linear hardening stopped the hardening.The plastic deformation at LNT is determined by{301}deformation twins and intersecting of{301}twins.The strong impeding effects of intersected twins on dislocation slip led to the linear hardening.The mechanism of brittle fracture within the linear hardening process has been determined:It is found the premature fracture was due to the velocity mismatch between dislocation glide（～2.5μm/s）and twin thickening（～10μm/s）at LNT.The large velocity difference rendered the twin-thickening-induced shear strain at twin-grain boundary intersections unable to be relaxed totally,leading to stress concentrations;under the accumulated stress,cracks ultimately nucleated at these intersections then rapidly propagated along the grain boundaries.According to uniaxial tensile tests over a complete temperature range from 293 K to 77 K,the influences of temperature on deformation behaviors of Sn have been clarified:the critical resolved stress of dislocation slip increased with the declining temperature while the stress of twin nucleation was not sensitive to temperature.As a result,{30 1}deformation twins were activated during the deformation of Sn at cryogenic temperatures.The lower the temperature was,the more significant the contributions of deformation twins to plastic deformation were.As the temperature decreased,the deformatio n mechanisms of Sn transformed from dislocation slip-dependent to deformation twinning-dependent.At the strain rate of 0.01/s,the deformation features of Sn transformed from elastic-viscoplasticity to elastic-plasticity at 233 K,while a linear strain hardening occurred at the temperature below 123 K.There is a critical temperature between 113 K and 87 K,below which the dislocation slip velocity was smaller than twin thickening rate,leading to a brittle fracture amid the linear hardening.This paper has proposed a new idea to enhance the cryogenic ductility of Sn by deformation twinning-stimulated recrystallization within the cryogenic deformation process and designed a pre-treatment method of pre-twining-RT tempering.A model of enhancing ductility by pre-twinning-RT tempering has been built:Pre-deformation caused multiple twin-grain boundary and twin-twin intersections in Sn.RT tempering promoted the formation of recrystallization embryos at these intersections.These embryos could grow to recrystallized grains within the subsequent cryogenic deformation,which could relieve the stress concentration at these twin-grain boundary and twin-twin intersections,and thus delayed the fracture and enhanced the cryogenic ductility.In-situ tensile experiments unravel the deformation mechanisms of as-treated Sn at extremely low temperatures:recrystallization,poly slip,transformation of slip system and detwinning.As a result,the fracture elongation of Sn at LNT increased to～14%.Considering the engineering demand,this paper then investigated the the influences of microstructures on the cryogenic mechanical behaviors of Sn.Based on the model describing the relationship between Vickers indentation morphologies and strain hardening exponent,this paper firstly investigated the influence of grain orientation on the strain hardening of Sn at LNT and RT.It is found that the Sn grain becomes harder with a higher hardening coefficient when the indented direction deviates from the c axis.The micro-mechanisms behind this relationship was further revealed that with the deviation of the indented direction from the c axis,the resolved shear stress on（100）[001]slip system declined;dislocation slip on（100）[001]therefore became more difficult.Moreover,{301}deformation twins were observed around the LNT indentations.The occurrence of{301}twins impeded dislocation slip,leading to the appreciable strain hardening of the Sn grain which displaced early.As a result,LNT indentations exhibit the morphology of sink-in.This paper then investigated the influences of second phases on the cryogenic mechanical behaviors of Sn according to the Sn-Pb eutectic solder（63Sn37Pb）.It is found that when 63Sn37Pb deformed at cryogenic temperatures,the dislocation slip firstly occurred in the Pb phases,which led to the formation of 45oshear band.With the propagation of 45oshear band and the occurrence of multiple 45oshear bands,the plastic deformation was transferred to the Sn matrix.While the deformation mechanism of Sn matrix was controlled by deformation twinning under cryogenic environments.Furthermore,at temperatures ranging from 293 K to 123 K,Pb phases sufferred an in-plane shear stress and fractured in the sliding mode（Mode-Ⅱ）under the maximum shear stress.Consequently,a ductile shear fracture on the 45oshear planes with respect to the tensile axis occurred in the eutectic sloder and the fracture elongation was maintained at 25%～30%in this temperature range.However,at 77 K,the brittleness of Sn phases led to the occurrence of maximum normal stress-induced fracture in Pb phases.As a result,a brittle fracture on the 90oplanes occurred in63Sn37Pb solder at 77 K.