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Dr Sergey Belyakov – Optimising Sn-Cu-Ni solders for reflow and micro-joints

Investigator: Dr Sergey Belyakov

Supervisor: Dr Chris Gourlay

Duration: 30/06/2016 - 01/07/2013 (PDRA)

Collaborators: Nihon Superior Co., Ltd.

Description:Sn-0.7Cu-0.05Ni has been used as a Pb-free solder since 1999 and has become a popular choice for wave-soldering. This project seeks to understand microstructure formation during reflow soldering and microstructure evolution in service when any Sn-Cu-Ni alloy is soldered to Cu or Ni substrates. The research aims to control and predict solder joint reliability in this system and to understand the how joint miniaturisation affects microstructure formation and stability. This project involves controlled solidification experiments, reflow soldering and analytical electron microscopy.

Read More:

  • S.A. Belyakov, C.M. Gourlay, NiSn4 formation during solidification of Sn-Ni alloys. Intermetallics. 25 pp. 48-59, 2012
  • S.A. Belyakov, C.M. Gourlay, NiSn4 formation in Ni-Sn and ENIG-Sn couples. Journal of Electronic Materials. 41(12) pp. 3331- 3341, 2012
  • S.A. Belyakov, C.M. Gourlay, Role of Fe impurities in the nucleation of metastable NiSn4, Intermetallics. 37, pp. 32-41, 2013

Armin Daszki - Nucleation kinetics of Lead-free solder joints

TITLE:  Armin Daszki - Nucleation kinetics of Lead-free solder joints

 

Project title: Nucleation kinetics of Lead-free solder joints

Investigator: Armin Daszki

Supervisors: Dr. Christopher Gourlay

Duration: 01/10/2016 – 30/09/2020

 

Project Description:

Lead-free solders, which have been developed to substitute lead based alloys, suffer greatly from anisotropy compared to the original tin-lead alloys due to the tetragonal crystal structure of the majority component, βSn. Additionally βSn suffers from nucleating difficulties usually leading to a single nucleation event which results in a single βSn grain, cyclic-twinned βSn grains, and/or interlaced-twinned βSn grain morphology. This nucleation-difficulty phenomena is seen clearly when repeatedly melting and solidifying Sn-based alloys and measuring the nucleation undercooling. The stochastic value of the nucleation undercooling can vary (depending on sample purity and size) from 45K to 80K, this is in stark comparison to the melting which is seen to always happen at a fixed repeatable value. Nucleation kinetics will be studied in lead-free solders, with and without soldering substrates, in order to understand fundamental nucleation laws with experimental data, as well as the implication of nucleation onset on the resulting microstructure. By understanding the nucleation phenomena of βSn in lead free solders we seek to control the microstructure sufficiently to alter the mechanical, and electrical properties of solder joints to improve reliability of electrical devices. Finally, the gained knowledge of nucleation in the studied system will be generalised and extended to any system. 

 

 

 

Cheng-Jung Lin - New approaches to the grain refinement of Mg-Al based alloys

New approaches to the grain refinement of Mg-Al based alloys

Investigator: Cheng-Jung Lin

Supervisor: Dr. Christopher Gourlay

Co-supervisor: Dr. Qianqian Li

Duration: 05/11/2018 - 04/11/2022

Abstract: Grain refinement is vital for enhancing magnesium alloys in terms of castability, formability, strength and ductility. The four main known methods of grain refining Mg-Al based alloys are superheating, the Elfinal process, native grain refinement and carbon inoculation, but these refining processes have not found widespread industrial use. Part of the reason for this is that the mechanisms of grain refinement in these methods is not fully understood. For instance, in the carbon inoculation method, the active Nucleant has been reported as Al4C3, Al2Mg2C2 and Al2CO, but this has not been confirmed. In this study, analytical electron microscopy approaches are being used to identify nucleants and investigate orientation relationships with the Mg-matrix. This new understanding is then being applied to develop an improved grain refiner for Mg-Al based alloys.

Cheng-Jung Lin
Figure 1. The colour etching of as-cast AZ91 with optical microscope using cross-polarised light. It shows the colour contrast from the grains.

Liuqing Peng - Reactions between liquid AZ91 and mild steel crucibles

Investigator: Liuqing Peng

Supervisors: Dr. Christopher Gourlay

Title: Reactions between liquid AZ91 and mild steel crucibles

Duration: 01/12/2016 – 30/11/2020

 

Project Description:

In the Mg industry, liquid Mg-Al alloys are commonly melted, ladled and/or poured from steel containers because the ceramics used by the Al industry would react with liquid Mg and because there is a low solubility for Fe in liquid Mg. However, Fe reacts with the Al and Mn in AZ91 when the molten alloy is held above the liquidus temperature, e.g. for many hours before handling in high pressure die casting (HPDC). This can lead to the formation of Al-Mn-(Fe) intermetallic particles that build up as die-casting sludge, block filters in DC casting, and affect the corrosion performance of Mg components. In this study, electron backscatter diffraction (EBSD) and energy dispersive X-ray spectrometry (EDX) are utilized to understand the nucleation and growth mechanisms of Al-Mn-(Fe) intermetallics in liquid AZ91 in contact with mild steel, at a range of temperatures and holding times. The quantitative results are used to discuss (i) the build-up of die-casting sludge and methods to minimize this and (ii) the impurity pick-up from the steel crucibles.

LP
SEM image of a typical microstructure of AZ91 and mild steel crucible after isothermal holding

Te-Cheng Su - In-situ synchrotron radiography and tomography studies of semi-solid deformation in Al alloys and steels

Investigator: Te-Cheng Su

Supervisor : Dr Chris Gourlay

Duration: 06/10/2014 - 31/03/2018 (Imperial College PhD Scholarship)

Collaborators: Tomoya Nagira, Osaka University; Hideyuki Yasuda, Kyoto University; Japan Synchrotron Radiation Research Institute (JASRI)

Description: The investigation of deformation of partially solidified alloys by in-situ X-ray synchrotron radiography has been widely reported in the last five years. It develops from the 2-D direct observation and a series of image analysis techniques on semi-solid alloys in isothermal holding. Further, some breakthroughs have been made such as tomographic reconstruction to observe the three-dimensional granular behaviour of partially solidified alloys during casting. There are several unexplored combinations of semi-solid microstructure, stress and accumulated strain which may affect the mechanical response of partially solidified alloys under loading. Therefore, the project is investigating the effect of grain size, shear strain rate and solid fraction on deformation mechanisms of Al-Cu alloys and steels in the semi-solid state by the application of in-situ X-ray synchrotron radiography and tomography. We are adapting ideas and methods from soil mechanics to track the microstructural evolution including the liquid flow field, strain field and grain motion during loading in order to develop new perspectives for filling, feeding and defect formation in alloy casting.

Jingwei Xian - The nucleation and growth of Cu6Sn5 in solders

Investigator: Jingwei Xian

Supervisor:  Dr Chris Gourlay

Duration: 5/11/2012 - 4/11/2015 (PhD Studentship)

Description: Cu6Sn5 crystals are commonly found in Pb-free solder joints and their size, shape and volume fraction influence the reliability of joints (e.g. during thermomechnical fatigue). This project aims to control the Cu6Sn5 size and morphology by building a fundamental understanding of the nucleation and growth of Cu6Sn5 in both the bulk solder and the interfacial layer. The project uses analytical electron microscopy to investigate the 3D growth morphology, and stable isotope SIMS to study diffusion mechanisms in interfacial layers.

Yi Cui - Nucleation in intermetallic solidification

In the casting of Al alloys, Al-Ti-B master alloys have been commonly used as grain refiners. A large number of studies have focused on the α-Al grain refinement effect of this master alloy and it was found that the two compounds, Al3Ti and TiB2, are mainly responsible for the nucleation of α-Al matrix. The detailed nucleation mechanism in this alloy system is still a controversial question and one of the widely accepted explanations suggests that the TiB2 is a highly potent substrate for Al3Ti to nucleate, promoting the subsequent nucleation of the Al matrix on Al3Ti. In this study, by investigating the location, growth morphology and orientation relationship between Al3Ti and TiB2, we aim to gain a better understanding on the nucleation and growth mechanism of Al3Ti. 

Yi Cui
TiB2 on Al3Ti surface extracted from a Al-3Ti-1B master alloy